diff --git a/.gitignore b/.gitignore
index 3bef9a5b1..91928691b 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,4 @@
 /.phpcs-cache
 /.phpunit.result.cache
 /phpcs.xml
-/vendor
+./vendor
diff --git a/composer.json b/composer.json
index 1f68e07bb..c269be8cb 100644
--- a/composer.json
+++ b/composer.json
@@ -4,6 +4,7 @@
     "license": "MIT",
     "require": {
         "php": ">=7.2.0,<7.4.0",
+        "ext-json": "*",
         "jetbrains/phpstorm-stubs": "2019.1",
         "nikic/php-parser": "^4.0.4",
         "phpdocumentor/reflection-docblock": "^4.1.1",
diff --git a/composer.lock b/composer.lock
index f1317e851..0fdca5097 100644
--- a/composer.lock
+++ b/composer.lock
@@ -4,7 +4,7 @@
         "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#installing-dependencies",
         "This file is @generated automatically"
     ],
-    "content-hash": "03bf49a514a13fde0a6e5ab048048915",
+    "content-hash": "2227819d7c5d9b4044210f18c1ed1866",
     "packages": [
         {
             "name": "jetbrains/phpstorm-stubs",
@@ -1875,7 +1875,8 @@
     "prefer-stable": true,
     "prefer-lowest": false,
     "platform": {
-        "php": ">=7.1.0,<7.4.0"
+        "php": ">=7.2.0,<7.4.0",
+        "ext-json": "*"
     },
     "platform-dev": []
 }
diff --git a/docs/usage.md b/docs/usage.md
index 1190cadd3..b8b238a0e 100644
--- a/docs/usage.md
+++ b/docs/usage.md
@@ -118,6 +118,39 @@ $classInfo = ReflectionClass::createFromName('MyClass');
 $functionInfo = ReflectionFunction::createFromName('foo');
 ```
 
+### Inspecting code and dependencies of a composer-based project
+
+If you need to inspect code from a project that has a `composer.json` and
+its associated `vendor/` directory populated, this package offers some
+factories that ease the setup of the source locator. These are:
+
+ * `Roave\BetterReflection\SourceLocator\Type\Composer\Factory\MakeLocatorForComposerJsonAndInstalledJson` - if
+   you need to inspect project and dependencies
+ * `Roave\BetterReflection\SourceLocator\Type\Composer\Factory\MakeLocatorForComposerJson` - if you only want to
+   inspect project sources
+ * `Roave\BetterReflection\SourceLocator\Type\Composer\Factory\MakeLocatorForInstalledJson` - if you only want
+   to inspect project dependencies
+
+Here's an example of `MakeLocatorForComposerJsonAndInstalledJson` usage:
+
+```php
+<?php
+
+use Roave\BetterReflection\BetterReflection;
+use Roave\BetterReflection\Reflector\ClassReflector;
+use Roave\BetterReflection\SourceLocator\Type\AggregateSourceLocator;
+use Roave\BetterReflection\SourceLocator\Type\PhpInternalSourceLocator;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Factory\MakeLocatorForComposerJsonAndInstalledJson;
+
+$astLocator = (new BetterReflection())->astLocator();
+$reflector  = new ClassReflector(new AggregateSourceLocator([
+    (new MakeLocatorForComposerJsonAndInstalledJson)('path/to/the/project', $astLocator),
+    new PhpInternalSourceLocator($astLocator)
+]));
+
+$classes = $reflector->getAllClasses();
+```
+
 ### Using the Composer autoloader directly
 
 ```php
diff --git a/src/SourceLocator/Type/Composer/Factory/Exception/Exception.php b/src/SourceLocator/Type/Composer/Factory/Exception/Exception.php
new file mode 100644
index 000000000..ffc7e63bc
--- /dev/null
+++ b/src/SourceLocator/Type/Composer/Factory/Exception/Exception.php
@@ -0,0 +1,11 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Roave\BetterReflection\SourceLocator\Type\Composer\Factory\Exception;
+
+use Throwable;
+
+interface Exception extends Throwable
+{
+}
diff --git a/src/SourceLocator/Type/Composer/Factory/Exception/FailedToParseJson.php b/src/SourceLocator/Type/Composer/Factory/Exception/FailedToParseJson.php
new file mode 100644
index 000000000..09413beb7
--- /dev/null
+++ b/src/SourceLocator/Type/Composer/Factory/Exception/FailedToParseJson.php
@@ -0,0 +1,19 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Roave\BetterReflection\SourceLocator\Type\Composer\Factory\Exception;
+
+use UnexpectedValueException;
+use function sprintf;
+
+final class FailedToParseJson extends UnexpectedValueException implements Exception
+{
+    public static function inFile(string $file) : self
+    {
+        return new self(sprintf(
+            'Could not parse JSON file "%s"',
+            $file
+        ));
+    }
+}
diff --git a/src/SourceLocator/Type/Composer/Factory/Exception/InvalidProjectDirectory.php b/src/SourceLocator/Type/Composer/Factory/Exception/InvalidProjectDirectory.php
new file mode 100644
index 000000000..61e2d0cd4
--- /dev/null
+++ b/src/SourceLocator/Type/Composer/Factory/Exception/InvalidProjectDirectory.php
@@ -0,0 +1,19 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Roave\BetterReflection\SourceLocator\Type\Composer\Factory\Exception;
+
+use InvalidArgumentException;
+use function sprintf;
+
+final class InvalidProjectDirectory extends InvalidArgumentException implements Exception
+{
+    public static function atPath(string $path) : self
+    {
+        return new self(sprintf(
+            'Could not locate project directory "%s"',
+            $path
+        ));
+    }
+}
diff --git a/src/SourceLocator/Type/Composer/Factory/Exception/MissingComposerJson.php b/src/SourceLocator/Type/Composer/Factory/Exception/MissingComposerJson.php
new file mode 100644
index 000000000..1fb412ff9
--- /dev/null
+++ b/src/SourceLocator/Type/Composer/Factory/Exception/MissingComposerJson.php
@@ -0,0 +1,19 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Roave\BetterReflection\SourceLocator\Type\Composer\Factory\Exception;
+
+use UnexpectedValueException;
+use function sprintf;
+
+final class MissingComposerJson extends UnexpectedValueException implements Exception
+{
+    public static function inProjectPath(string $path) : self
+    {
+        return new self(sprintf(
+            'Could not locate a "composer.json" file in "%s"',
+            $path
+        ));
+    }
+}
diff --git a/src/SourceLocator/Type/Composer/Factory/Exception/MissingInstalledJson.php b/src/SourceLocator/Type/Composer/Factory/Exception/MissingInstalledJson.php
new file mode 100644
index 000000000..d812de2f5
--- /dev/null
+++ b/src/SourceLocator/Type/Composer/Factory/Exception/MissingInstalledJson.php
@@ -0,0 +1,19 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Roave\BetterReflection\SourceLocator\Type\Composer\Factory\Exception;
+
+use UnexpectedValueException;
+use function sprintf;
+
+final class MissingInstalledJson extends UnexpectedValueException implements Exception
+{
+    public static function inProjectPath(string $path) : self
+    {
+        return new self(sprintf(
+            'Could not locate a "vendor/composer/installed.json" file in "%s"',
+            $path
+        ));
+    }
+}
diff --git a/src/SourceLocator/Type/Composer/Factory/MakeLocatorForComposerJson.php b/src/SourceLocator/Type/Composer/Factory/MakeLocatorForComposerJson.php
new file mode 100644
index 000000000..f31b33cf5
--- /dev/null
+++ b/src/SourceLocator/Type/Composer/Factory/MakeLocatorForComposerJson.php
@@ -0,0 +1,145 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Roave\BetterReflection\SourceLocator\Type\Composer\Factory;
+
+use Roave\BetterReflection\SourceLocator\Ast\Locator;
+use Roave\BetterReflection\SourceLocator\Type\AggregateSourceLocator;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Factory\Exception\FailedToParseJson;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Factory\Exception\InvalidProjectDirectory;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Factory\Exception\MissingComposerJson;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Psr\Psr0Mapping;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Psr\Psr4Mapping;
+use Roave\BetterReflection\SourceLocator\Type\Composer\PsrAutoloaderLocator;
+use Roave\BetterReflection\SourceLocator\Type\DirectoriesSourceLocator;
+use Roave\BetterReflection\SourceLocator\Type\SingleFileSourceLocator;
+use Roave\BetterReflection\SourceLocator\Type\SourceLocator;
+use function array_filter;
+use function array_map;
+use function array_merge;
+use function file_exists;
+use function file_get_contents;
+use function is_array;
+use function is_dir;
+use function json_decode;
+use function realpath;
+
+final class MakeLocatorForComposerJson
+{
+    public function __invoke(string $installationPath, Locator $astLocator) : SourceLocator
+    {
+        $realInstallationPath = (string) realpath($installationPath);
+
+        if (! is_dir($realInstallationPath)) {
+            throw InvalidProjectDirectory::atPath($installationPath);
+        }
+
+        $composerJsonPath = $realInstallationPath . '/composer.json';
+
+        if (! file_exists($composerJsonPath)) {
+            throw MissingComposerJson::inProjectPath($installationPath);
+        }
+
+        $composer = json_decode((string) file_get_contents($composerJsonPath), true);
+
+        if (! is_array($composer)) {
+            throw FailedToParseJson::inFile($composerJsonPath);
+        }
+
+        $pathPrefix          = $realInstallationPath . '/';
+        $classMapPaths       = $this->prefixPaths($this->packageToClassMapPaths($composer), $pathPrefix);
+        $classMapFiles       = array_filter($classMapPaths, 'is_file');
+        $classMapDirectories = array_filter($classMapPaths, 'is_dir');
+        $filePaths           = $this->prefixPaths($this->packageToFilePaths($composer), $pathPrefix);
+
+        return new AggregateSourceLocator(array_merge(
+            [
+                new PsrAutoloaderLocator(
+                    Psr4Mapping::fromArrayMappings(
+                        $this->prefixWithInstallationPath($this->packageToPsr4AutoloadNamespaces($composer), $pathPrefix)
+                    ),
+                    $astLocator
+                ),
+                new PsrAutoloaderLocator(
+                    Psr0Mapping::fromArrayMappings(
+                        $this->prefixWithInstallationPath($this->packageToPsr0AutoloadNamespaces($composer), $pathPrefix)
+                    ),
+                    $astLocator
+                ),
+                new DirectoriesSourceLocator($classMapDirectories, $astLocator),
+            ],
+            ...array_map(static function (string $file) use ($astLocator) : array {
+                return [new SingleFileSourceLocator($file, $astLocator)];
+            }, array_merge($classMapFiles, $filePaths))
+        ));
+    }
+
+    /**
+     * @param mixed[] $package
+     *
+     * @return array<string, array<int, string>>
+     */
+    private function packageToPsr4AutoloadNamespaces(array $package) : array
+    {
+        return array_map(static function ($namespacePaths) : array {
+            return (array) $namespacePaths;
+        }, $package['autoload']['psr-4'] ?? []);
+    }
+
+    /**
+     * @param mixed[] $package
+     *
+     * @return array<string, array<int, string>>
+     */
+    private function packageToPsr0AutoloadNamespaces(array $package) : array
+    {
+        return array_map(static function ($namespacePaths) : array {
+            return (array) $namespacePaths;
+        }, $package['autoload']['psr-0'] ?? []);
+    }
+
+    /**
+     * @param mixed[] $package
+     *
+     * @return array<int, string>
+     */
+    private function packageToClassMapPaths(array $package) : array
+    {
+        return $package['autoload']['classmap'] ?? [];
+    }
+
+    /**
+     * @param mixed[] $package
+     *
+     * @return array<int, string>
+     */
+    private function packageToFilePaths(array $package) : array
+    {
+        return $package['autoload']['files'] ?? [];
+    }
+
+    /**
+     * @param array<string, array<int, string>> $paths
+     *
+     * @return array<string, array<int, string>>
+     */
+    private function prefixWithInstallationPath(array $paths, string $trimmedInstallationPath) : array
+    {
+        return array_map(function (array $paths) use ($trimmedInstallationPath) : array {
+            return $this->prefixPaths($paths, $trimmedInstallationPath);
+        }, $paths);
+    }
+
+    /**
+     * @param array<int, string> $paths
+     *
+     * @return array<int, string>
+     */
+    private function prefixPaths(array $paths, string $prefix) : array
+    {
+        return array_map(static function (string $path) use ($prefix) {
+            return $prefix . $path;
+        }, $paths);
+    }
+}
diff --git a/src/SourceLocator/Type/Composer/Factory/MakeLocatorForComposerJsonAndInstalledJson.php b/src/SourceLocator/Type/Composer/Factory/MakeLocatorForComposerJsonAndInstalledJson.php
new file mode 100644
index 000000000..4896b9ae8
--- /dev/null
+++ b/src/SourceLocator/Type/Composer/Factory/MakeLocatorForComposerJsonAndInstalledJson.php
@@ -0,0 +1,213 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Roave\BetterReflection\SourceLocator\Type\Composer\Factory;
+
+use Roave\BetterReflection\SourceLocator\Ast\Locator;
+use Roave\BetterReflection\SourceLocator\Type\AggregateSourceLocator;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Factory\Exception\FailedToParseJson;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Factory\Exception\InvalidProjectDirectory;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Factory\Exception\MissingComposerJson;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Factory\Exception\MissingInstalledJson;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Psr\Psr0Mapping;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Psr\Psr4Mapping;
+use Roave\BetterReflection\SourceLocator\Type\Composer\PsrAutoloaderLocator;
+use Roave\BetterReflection\SourceLocator\Type\DirectoriesSourceLocator;
+use Roave\BetterReflection\SourceLocator\Type\SingleFileSourceLocator;
+use Roave\BetterReflection\SourceLocator\Type\SourceLocator;
+use function array_filter;
+use function array_map;
+use function array_merge;
+use function array_merge_recursive;
+use function file_exists;
+use function file_get_contents;
+use function is_array;
+use function is_dir;
+use function json_decode;
+use function realpath;
+
+final class MakeLocatorForComposerJsonAndInstalledJson
+{
+    public function __invoke(string $installationPath, Locator $astLocator) : SourceLocator
+    {
+        $realInstallationPath = (string) realpath($installationPath);
+
+        if (! is_dir($realInstallationPath)) {
+            throw InvalidProjectDirectory::atPath($installationPath);
+        }
+
+        $composerJsonPath  = $realInstallationPath . '/composer.json';
+        $installedJsonPath = $realInstallationPath . '/vendor/composer/installed.json';
+
+        if (! file_exists($composerJsonPath)) {
+            throw MissingComposerJson::inProjectPath($installationPath);
+        }
+
+        if (! file_exists($installedJsonPath)) {
+            throw MissingInstalledJson::inProjectPath($installationPath);
+        }
+
+        $composer  = json_decode((string) file_get_contents($composerJsonPath), true);
+        $installed = json_decode((string) file_get_contents($installedJsonPath), true);
+
+        if (! is_array($composer)) {
+            throw FailedToParseJson::inFile($composerJsonPath);
+        }
+
+        if (! is_array($installed)) {
+            throw FailedToParseJson::inFile($installedJsonPath);
+        }
+
+        $classMapPaths       = array_merge(
+            $this->prefixPaths($this->packageToClassMapPaths($composer), $realInstallationPath . '/'),
+            ...array_map(function (array $package) use ($realInstallationPath) : array {
+                return $this->prefixPaths(
+                    $this->packageToClassMapPaths($package),
+                    $this->packagePrefixPath($realInstallationPath, $package)
+                );
+            }, $installed)
+        );
+        $classMapFiles       = array_filter($classMapPaths, 'is_file');
+        $classMapDirectories = array_filter($classMapPaths, 'is_dir');
+        $filePaths           = array_merge(
+            $this->prefixPaths($this->packageToFilePaths($composer), $realInstallationPath . '/'),
+            ...array_map(function (array $package) use ($realInstallationPath) : array {
+                return $this->prefixPaths(
+                    $this->packageToFilePaths($package),
+                    $this->packagePrefixPath($realInstallationPath, $package)
+                );
+            }, $installed)
+        );
+
+        return new AggregateSourceLocator(array_merge(
+            [
+                new PsrAutoloaderLocator(
+                    Psr4Mapping::fromArrayMappings(array_merge_recursive(
+                        $this->prefixWithInstallationPath($this->packageToPsr4AutoloadNamespaces($composer), $realInstallationPath),
+                        ...array_map(function (array $package) use ($realInstallationPath) : array {
+                            return $this->prefixWithPackagePath(
+                                $this->packageToPsr4AutoloadNamespaces($package),
+                                $realInstallationPath,
+                                $package
+                            );
+                        }, $installed)
+                    )),
+                    $astLocator
+                ),
+                new PsrAutoloaderLocator(
+                    Psr0Mapping::fromArrayMappings(array_merge_recursive(
+                        $this->prefixWithInstallationPath($this->packageToPsr0AutoloadNamespaces($composer), $realInstallationPath),
+                        ...array_map(function (array $package) use ($realInstallationPath) : array {
+                            return $this->prefixWithPackagePath(
+                                $this->packageToPsr0AutoloadNamespaces($package),
+                                $realInstallationPath,
+                                $package
+                            );
+                        }, $installed)
+                    )),
+                    $astLocator
+                ),
+                new DirectoriesSourceLocator($classMapDirectories, $astLocator),
+            ],
+            ...array_map(static function (string $file) use ($astLocator) : array {
+                return [new SingleFileSourceLocator($file, $astLocator)];
+            }, array_merge($classMapFiles, $filePaths))
+        ));
+    }
+
+    /**
+     * @param mixed[] $package
+     *
+     * @return array<string, array<int, string>>
+     */
+    private function packageToPsr4AutoloadNamespaces(array $package) : array
+    {
+        return array_map(static function ($namespacePaths) : array {
+            return (array) $namespacePaths;
+        }, $package['autoload']['psr-4'] ?? []);
+    }
+
+    /**
+     * @param mixed[] $package
+     *
+     * @return array<string, array<int, string>>
+     */
+    private function packageToPsr0AutoloadNamespaces(array $package) : array
+    {
+        return array_map(static function ($namespacePaths) : array {
+            return (array) $namespacePaths;
+        }, $package['autoload']['psr-0'] ?? []);
+    }
+
+    /**
+     * @param mixed[] $package
+     *
+     * @return array<int, string>
+     */
+    private function packageToClassMapPaths(array $package) : array
+    {
+        return $package['autoload']['classmap'] ?? [];
+    }
+
+    /**
+     * @param mixed[] $package
+     *
+     * @return array<int, string>
+     */
+    private function packageToFilePaths(array $package) : array
+    {
+        return $package['autoload']['files'] ?? [];
+    }
+
+    /**
+     * @param mixed[] $package
+     *
+     * @psalm-param array{name: string} $package
+     */
+    private function packagePrefixPath(string $trimmedInstallationPath, array $package) : string
+    {
+        return $trimmedInstallationPath . '/vendor/' . $package['name'] . '/';
+    }
+
+    /**
+     * @param array<string, array<int, string>> $paths
+     * @param array<string, array<int, string>> $package
+     *
+     * @return array<string, array<int, string>>
+     *
+     * @psalm-param array{name: string} $package
+     */
+    private function prefixWithPackagePath(array $paths, string $trimmedInstallationPath, array $package) : array
+    {
+        $prefix = $this->packagePrefixPath($trimmedInstallationPath, $package);
+
+        return array_map(function (array $paths) use ($prefix) : array {
+            return $this->prefixPaths($paths, $prefix);
+        }, $paths);
+    }
+
+    /**
+     * @param array<int|string, array<string>> $paths
+     *
+     * @return array<int|string, array<string>>
+     */
+    private function prefixWithInstallationPath(array $paths, string $trimmedInstallationPath) : array
+    {
+        return array_map(function (array $paths) use ($trimmedInstallationPath) : array {
+            return $this->prefixPaths($paths, $trimmedInstallationPath . '/');
+        }, $paths);
+    }
+
+    /**
+     * @param array<int, string> $paths
+     *
+     * @return array<int, string>
+     */
+    private function prefixPaths(array $paths, string $prefix) : array
+    {
+        return array_map(static function (string $path) use ($prefix) {
+            return $prefix . $path;
+        }, $paths);
+    }
+}
diff --git a/src/SourceLocator/Type/Composer/Factory/MakeLocatorForInstalledJson.php b/src/SourceLocator/Type/Composer/Factory/MakeLocatorForInstalledJson.php
new file mode 100644
index 000000000..aa36593ed
--- /dev/null
+++ b/src/SourceLocator/Type/Composer/Factory/MakeLocatorForInstalledJson.php
@@ -0,0 +1,190 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Roave\BetterReflection\SourceLocator\Type\Composer\Factory;
+
+use Roave\BetterReflection\SourceLocator\Ast\Locator;
+use Roave\BetterReflection\SourceLocator\Type\AggregateSourceLocator;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Factory\Exception\FailedToParseJson;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Factory\Exception\InvalidProjectDirectory;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Factory\Exception\MissingInstalledJson;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Psr\Psr0Mapping;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Psr\Psr4Mapping;
+use Roave\BetterReflection\SourceLocator\Type\Composer\PsrAutoloaderLocator;
+use Roave\BetterReflection\SourceLocator\Type\DirectoriesSourceLocator;
+use Roave\BetterReflection\SourceLocator\Type\SingleFileSourceLocator;
+use Roave\BetterReflection\SourceLocator\Type\SourceLocator;
+use function array_filter;
+use function array_map;
+use function array_merge;
+use function array_merge_recursive;
+use function file_exists;
+use function file_get_contents;
+use function is_array;
+use function is_dir;
+use function json_decode;
+use function realpath;
+
+final class MakeLocatorForInstalledJson
+{
+    public function __invoke(string $installationPath, Locator $astLocator) : SourceLocator
+    {
+        $realInstallationPath = (string) realpath($installationPath);
+
+        if (! is_dir($realInstallationPath)) {
+            throw InvalidProjectDirectory::atPath($installationPath);
+        }
+
+        $installedJsonPath = $realInstallationPath . '/vendor/composer/installed.json';
+
+        if (! file_exists($installedJsonPath)) {
+            throw MissingInstalledJson::inProjectPath($installationPath);
+        }
+
+        $installed = json_decode((string) file_get_contents($installedJsonPath), true);
+
+        if (! is_array($installed)) {
+            throw FailedToParseJson::inFile($installedJsonPath);
+        }
+
+        $classMapPaths       = array_merge(
+            [],
+            ...array_map(function (array $package) use ($realInstallationPath) : array {
+                return $this->prefixPaths(
+                    $this->packageToClassMapPaths($package),
+                    $this->packagePrefixPath($realInstallationPath, $package)
+                );
+            }, $installed)
+        );
+        $classMapFiles       = array_filter($classMapPaths, 'is_file');
+        $classMapDirectories = array_filter($classMapPaths, 'is_dir');
+        $filePaths           = array_merge(
+            [],
+            ...array_map(function (array $package) use ($realInstallationPath) : array {
+                return $this->prefixPaths(
+                    $this->packageToFilePaths($package),
+                    $this->packagePrefixPath($realInstallationPath, $package)
+                );
+            }, $installed)
+        );
+
+        return new AggregateSourceLocator(array_merge(
+            [
+                new PsrAutoloaderLocator(
+                    Psr4Mapping::fromArrayMappings(array_merge_recursive(
+                        [],
+                        ...array_map(function (array $package) use ($realInstallationPath) : array {
+                            return $this->prefixWithPackagePath(
+                                $this->packageToPsr4AutoloadNamespaces($package),
+                                $realInstallationPath,
+                                $package
+                            );
+                        }, $installed)
+                    )),
+                    $astLocator
+                ),
+                new PsrAutoloaderLocator(
+                    Psr0Mapping::fromArrayMappings(array_merge_recursive(
+                        [],
+                        ...array_map(function (array $package) use ($realInstallationPath) : array {
+                            return $this->prefixWithPackagePath(
+                                $this->packageToPsr0AutoloadNamespaces($package),
+                                $realInstallationPath,
+                                $package
+                            );
+                        }, $installed)
+                    )),
+                    $astLocator
+                ),
+                new DirectoriesSourceLocator($classMapDirectories, $astLocator),
+            ],
+            ...array_map(static function (string $file) use ($astLocator) : array {
+                return [new SingleFileSourceLocator($file, $astLocator)];
+            }, array_merge($classMapFiles, $filePaths))
+        ));
+    }
+
+    /**
+     * @param mixed[] $package
+     *
+     * @return array<string, array<int, string>>
+     */
+    private function packageToPsr4AutoloadNamespaces(array $package) : array
+    {
+        return array_map(static function ($namespacePaths) : array {
+            return (array) $namespacePaths;
+        }, $package['autoload']['psr-4'] ?? []);
+    }
+
+    /**
+     * @param mixed[] $package
+     *
+     * @return array<string, array<int, string>>
+     */
+    private function packageToPsr0AutoloadNamespaces(array $package) : array
+    {
+        return array_map(static function ($namespacePaths) : array {
+            return (array) $namespacePaths;
+        }, $package['autoload']['psr-0'] ?? []);
+    }
+
+    /**
+     * @param mixed[] $package
+     *
+     * @return array<int, string>
+     */
+    private function packageToClassMapPaths(array $package) : array
+    {
+        return $package['autoload']['classmap'] ?? [];
+    }
+
+    /**
+     * @param mixed[] $package
+     *
+     * @return array<int, string>
+     */
+    private function packageToFilePaths(array $package) : array
+    {
+        return $package['autoload']['files'] ?? [];
+    }
+
+    /**
+     * @param mixed[] $package
+     *
+     * @psalm-param array{name: string} $package
+     */
+    private function packagePrefixPath(string $trimmedInstallationPath, array $package) : string
+    {
+        return $trimmedInstallationPath . '/vendor/' . $package['name'] . '/';
+    }
+
+    /**
+     * @param array<int|string, array<string>> $paths
+     * @param array<string, array<string>>     $package
+     *
+     * @return array<int|string, string|array<string>>
+     *
+     * @psalm-param array{name: string} $package
+     */
+    private function prefixWithPackagePath(array $paths, string $trimmedInstallationPath, array $package) : array
+    {
+        $prefix = $this->packagePrefixPath($trimmedInstallationPath, $package);
+
+        return array_map(function (array $paths) use ($prefix) : array {
+            return $this->prefixPaths($paths, $prefix);
+        }, $paths);
+    }
+
+    /**
+     * @param array<int|string, string> $paths
+     *
+     * @return array<int|string, string>
+     */
+    private function prefixPaths(array $paths, string $prefix) : array
+    {
+        return array_map(static function (string $path) use ($prefix) {
+            return $prefix . $path;
+        }, $paths);
+    }
+}
diff --git a/src/SourceLocator/Type/Composer/Psr/Exception/Exception.php b/src/SourceLocator/Type/Composer/Psr/Exception/Exception.php
new file mode 100644
index 000000000..581ab648d
--- /dev/null
+++ b/src/SourceLocator/Type/Composer/Psr/Exception/Exception.php
@@ -0,0 +1,11 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Roave\BetterReflection\SourceLocator\Type\Composer\Psr\Exception;
+
+use Throwable;
+
+interface Exception extends Throwable
+{
+}
diff --git a/src/SourceLocator/Type/Composer/Psr/Exception/InvalidPrefixMapping.php b/src/SourceLocator/Type/Composer/Psr/Exception/InvalidPrefixMapping.php
new file mode 100644
index 000000000..d5a817275
--- /dev/null
+++ b/src/SourceLocator/Type/Composer/Psr/Exception/InvalidPrefixMapping.php
@@ -0,0 +1,33 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Roave\BetterReflection\SourceLocator\Type\Composer\Psr\Exception;
+
+use InvalidArgumentException;
+use function sprintf;
+
+class InvalidPrefixMapping extends InvalidArgumentException implements Exception
+{
+    public static function emptyPrefixGiven() : self
+    {
+        return new self('An invalid empty string provided as a PSR mapping prefix');
+    }
+
+    public static function emptyPrefixMappingGiven(string $prefix) : self
+    {
+        return new self(sprintf(
+            'An invalid empty list of paths was provided for PSR mapping prefix "%s"',
+            $prefix
+        ));
+    }
+
+    public static function prefixMappingIsNotADirectory(string $prefix, string $path) : self
+    {
+        return new self(sprintf(
+            'Provided path "%s" for prefix "%s" is not a directory',
+            $prefix,
+            $path
+        ));
+    }
+}
diff --git a/src/SourceLocator/Type/Composer/Psr/Psr0Mapping.php b/src/SourceLocator/Type/Composer/Psr/Psr0Mapping.php
new file mode 100644
index 000000000..d1f288bbd
--- /dev/null
+++ b/src/SourceLocator/Type/Composer/Psr/Psr0Mapping.php
@@ -0,0 +1,98 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Roave\BetterReflection\SourceLocator\Type\Composer\Psr;
+
+use Roave\BetterReflection\Identifier\Identifier;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Psr\Exception\InvalidPrefixMapping;
+use function array_map;
+use function array_merge;
+use function array_unique;
+use function array_values;
+use function is_dir;
+use function rtrim;
+use function str_replace;
+use function strpos;
+
+final class Psr0Mapping implements PsrAutoloaderMapping
+{
+    /** @var array<string, array<int, string>> */
+    private $mappings = [];
+
+    private function __construct()
+    {
+    }
+
+    /** @param array<string, array<int, string>> $mappings */
+    public static function fromArrayMappings(array $mappings) : self
+    {
+        self::assertValidMapping($mappings);
+
+        $instance = new self();
+
+        $instance->mappings = array_map(
+            static function (array $directories) : array {
+                return array_map(static function (string $directory) : string {
+                    return rtrim($directory, '/');
+                }, $directories);
+            },
+            $mappings
+        );
+
+        return $instance;
+    }
+
+    /** {@inheritDoc} */
+    public function resolvePossibleFilePaths(Identifier $identifier) : array
+    {
+        if (! $identifier->isClass()) {
+            return [];
+        }
+
+        $className = $identifier->getName();
+
+        foreach ($this->mappings as $prefix => $paths) {
+            if (strpos($className, $prefix) === 0) {
+                return array_map(
+                    static function (string $path) use ($className) : string {
+                        return rtrim($path, '/') . '/' . str_replace(['\\', '_'], '/', $className) . '.php';
+                    },
+                    $paths
+                );
+            }
+        }
+
+        return [];
+    }
+
+    /** {@inheritDoc} */
+    public function directories() : array
+    {
+        return array_values(array_unique(array_merge([], ...array_values($this->mappings))));
+    }
+
+    /**
+     * @param array<string, array<int, string>> $mappings
+     *
+     * @throws InvalidPrefixMapping
+     */
+    private static function assertValidMapping(array $mappings) : void
+    {
+        foreach ($mappings as $prefix => $paths) {
+            if ($prefix === '') {
+                throw InvalidPrefixMapping::emptyPrefixGiven();
+            }
+
+            if ($paths === []) {
+                throw InvalidPrefixMapping::emptyPrefixMappingGiven($prefix);
+            }
+
+            foreach ($paths as $path) {
+                if (! is_dir($path)) {
+                    throw InvalidPrefixMapping::prefixMappingIsNotADirectory($prefix, $path);
+                }
+            }
+        }
+    }
+}
diff --git a/src/SourceLocator/Type/Composer/Psr/Psr4Mapping.php b/src/SourceLocator/Type/Composer/Psr/Psr4Mapping.php
new file mode 100644
index 000000000..ce9a2edb8
--- /dev/null
+++ b/src/SourceLocator/Type/Composer/Psr/Psr4Mapping.php
@@ -0,0 +1,119 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Roave\BetterReflection\SourceLocator\Type\Composer\Psr;
+
+use Roave\BetterReflection\Identifier\Identifier;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Psr\Exception\InvalidPrefixMapping;
+use const ARRAY_FILTER_USE_KEY;
+use function array_filter;
+use function array_keys;
+use function array_map;
+use function array_merge;
+use function array_unique;
+use function array_values;
+use function is_dir;
+use function ltrim;
+use function rtrim;
+use function str_replace;
+use function strlen;
+use function strpos;
+use function substr;
+
+final class Psr4Mapping implements PsrAutoloaderMapping
+{
+    /** @var array<string, array<int, string>> */
+    private $mappings = [];
+
+    private function __construct()
+    {
+    }
+
+    /** @param array<string, array<int, string>> $mappings */
+    public static function fromArrayMappings(array $mappings) : self
+    {
+        self::assertValidMapping($mappings);
+
+        $instance = new self();
+
+        $instance->mappings = array_map(
+            static function (array $directories) : array {
+                return array_map(static function (string $directory) : string {
+                    return rtrim($directory, '/');
+                }, $directories);
+            },
+            $mappings
+        );
+
+        return $instance;
+    }
+
+    /** {@inheritDoc} */
+    public function resolvePossibleFilePaths(Identifier $identifier) : array
+    {
+        if (! $identifier->isClass()) {
+            return [];
+        }
+
+        $className        = $identifier->getName();
+        $matchingPrefixes = $this->matchingPrefixes($className);
+
+        return array_values(array_filter(array_merge(
+            [],
+            ...array_map(static function (array $paths, string $prefix) use ($className) : array {
+                $subPath = ltrim(str_replace('\\', '/', substr($className, strlen($prefix))), '/');
+
+                if ($subPath === '') {
+                    return [];
+                }
+
+                return array_map(static function (string $path) use ($subPath) : string {
+                    return rtrim($path, '/') . '/' . $subPath . '.php';
+                }, $paths);
+            }, $matchingPrefixes, array_keys($matchingPrefixes))
+        )));
+    }
+
+    /** @return array<string, array<int, string>> */
+    private function matchingPrefixes(string $className) : array
+    {
+        return array_filter(
+            $this->mappings,
+            static function (string $prefix) use ($className) : bool {
+                return strpos($className, $prefix) === 0;
+            },
+            ARRAY_FILTER_USE_KEY
+        );
+    }
+
+    /** {@inheritDoc} */
+    public function directories() : array
+    {
+        return array_values(array_unique(array_merge([], ...array_values($this->mappings))));
+    }
+
+    /**
+     * @param array<string, array<int, string>> $mappings
+     *
+     * @throws InvalidPrefixMapping
+     */
+    private static function assertValidMapping(array $mappings) : void
+    {
+        foreach ($mappings as $prefix => $paths) {
+            if ($prefix === '') {
+                throw InvalidPrefixMapping::emptyPrefixGiven();
+            }
+
+            if ($paths === []) {
+                throw InvalidPrefixMapping::emptyPrefixMappingGiven($prefix);
+            }
+
+            foreach ($paths as $path) {
+                if (! is_dir($path)) {
+                    throw InvalidPrefixMapping::prefixMappingIsNotADirectory($prefix, $path);
+                }
+            }
+        }
+    }
+}
diff --git a/src/SourceLocator/Type/Composer/Psr/PsrAutoloaderMapping.php b/src/SourceLocator/Type/Composer/Psr/PsrAutoloaderMapping.php
new file mode 100644
index 000000000..6ddf0e8e1
--- /dev/null
+++ b/src/SourceLocator/Type/Composer/Psr/PsrAutoloaderMapping.php
@@ -0,0 +1,16 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Roave\BetterReflection\SourceLocator\Type\Composer\Psr;
+
+use Roave\BetterReflection\Identifier\Identifier;
+
+interface PsrAutoloaderMapping
+{
+    /** @return array<int, string> */
+    public function resolvePossibleFilePaths(Identifier $identifier) : array;
+
+    /** @return array<int, string> */
+    public function directories() : array;
+}
diff --git a/src/SourceLocator/Type/Composer/PsrAutoloaderLocator.php b/src/SourceLocator/Type/Composer/PsrAutoloaderLocator.php
new file mode 100644
index 000000000..9864da72d
--- /dev/null
+++ b/src/SourceLocator/Type/Composer/PsrAutoloaderLocator.php
@@ -0,0 +1,73 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Roave\BetterReflection\SourceLocator\Type\Composer;
+
+use Roave\BetterReflection\Identifier\Identifier;
+use Roave\BetterReflection\Identifier\IdentifierType;
+use Roave\BetterReflection\Reflection\Reflection;
+use Roave\BetterReflection\Reflector\Exception\IdentifierNotFound;
+use Roave\BetterReflection\Reflector\Reflector;
+use Roave\BetterReflection\SourceLocator\Ast\Locator;
+use Roave\BetterReflection\SourceLocator\Located\LocatedSource;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Psr\PsrAutoloaderMapping;
+use Roave\BetterReflection\SourceLocator\Type\DirectoriesSourceLocator;
+use Roave\BetterReflection\SourceLocator\Type\SourceLocator;
+use function file_exists;
+use function file_get_contents;
+
+final class PsrAutoloaderLocator implements SourceLocator
+{
+    /** @var PsrAutoloaderMapping */
+    private $mapping;
+
+    /** @var Locator */
+    private $astLocator;
+
+    public function __construct(PsrAutoloaderMapping $mapping, Locator $astLocator)
+    {
+        $this->mapping    = $mapping;
+        $this->astLocator = $astLocator;
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    public function locateIdentifier(Reflector $reflector, Identifier $identifier) : ?Reflection
+    {
+        foreach ($this->mapping->resolvePossibleFilePaths($identifier) as $file) {
+            if (! file_exists($file)) {
+                continue;
+            }
+
+            try {
+                return $this->astLocator->findReflection(
+                    $reflector,
+                    new LocatedSource(
+                        file_get_contents($file),
+                        $file
+                    ),
+                    $identifier
+                );
+            } catch (IdentifierNotFound $exception) {
+                // on purpose - autoloading is allowed to fail, and silently-failing autoloaders are normal/endorsed
+            }
+        }
+
+        return null;
+    }
+
+    /**
+     * Find all identifiers of a type
+     *
+     * @return Reflection[]
+     */
+    public function locateIdentifiersByType(Reflector $reflector, IdentifierType $identifierType) : array
+    {
+        return (new DirectoriesSourceLocator(
+            $this->mapping->directories(),
+            $this->astLocator
+        ))->locateIdentifiersByType($reflector, $identifierType);
+    }
+}
diff --git a/test/unit/Assets/ComposerLocators/empty-project/.gitignore b/test/unit/Assets/ComposerLocators/empty-project/.gitignore
new file mode 100644
index 000000000..d6b7ef32c
--- /dev/null
+++ b/test/unit/Assets/ComposerLocators/empty-project/.gitignore
@@ -0,0 +1,2 @@
+*
+!.gitignore
diff --git a/test/unit/Assets/ComposerLocators/project-a/composer.json b/test/unit/Assets/ComposerLocators/project-a/composer.json
new file mode 100644
index 000000000..cc90907d5
--- /dev/null
+++ b/test/unit/Assets/ComposerLocators/project-a/composer.json
@@ -0,0 +1,20 @@
+{
+  "autoload": {
+    "psr-4": {
+      "ProjectA\\": "src/root_PSR-4_Sources",
+      "ProjectA\\B\\": "src/root_PSR-4_Sources"
+    },
+    "psr-0": {
+      "ProjectA_A_": "src/root_PSR-0_Sources",
+      "ProjectA_B_": "src/root_PSR-0_Sources"
+    },
+    "classmap": [
+      "src/root_ClassmapDir",
+      "src/root_ClassmapFile"
+    ],
+    "files": [
+      "src/root_File1.php",
+      "src/root_File2.php"
+    ]
+  }
+}
diff --git a/test/unit/Assets/ComposerLocators/project-a/src/root_ClassmapDir/.gitignore b/test/unit/Assets/ComposerLocators/project-a/src/root_ClassmapDir/.gitignore
new file mode 100644
index 000000000..d6b7ef32c
--- /dev/null
+++ b/test/unit/Assets/ComposerLocators/project-a/src/root_ClassmapDir/.gitignore
@@ -0,0 +1,2 @@
+*
+!.gitignore
diff --git a/test/unit/Assets/ComposerLocators/project-a/src/root_ClassmapFile b/test/unit/Assets/ComposerLocators/project-a/src/root_ClassmapFile
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/unit/Assets/ComposerLocators/project-a/src/root_File1.php b/test/unit/Assets/ComposerLocators/project-a/src/root_File1.php
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/unit/Assets/ComposerLocators/project-a/src/root_File2.php b/test/unit/Assets/ComposerLocators/project-a/src/root_File2.php
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/unit/Assets/ComposerLocators/project-a/src/root_PSR-0_Sources/.gitignore b/test/unit/Assets/ComposerLocators/project-a/src/root_PSR-0_Sources/.gitignore
new file mode 100644
index 000000000..d6b7ef32c
--- /dev/null
+++ b/test/unit/Assets/ComposerLocators/project-a/src/root_PSR-0_Sources/.gitignore
@@ -0,0 +1,2 @@
+*
+!.gitignore
diff --git a/test/unit/Assets/ComposerLocators/project-a/src/root_PSR-4_Sources/.gitignore b/test/unit/Assets/ComposerLocators/project-a/src/root_PSR-4_Sources/.gitignore
new file mode 100644
index 000000000..d6b7ef32c
--- /dev/null
+++ b/test/unit/Assets/ComposerLocators/project-a/src/root_PSR-4_Sources/.gitignore
@@ -0,0 +1,2 @@
+*
+!.gitignore
diff --git a/test/unit/Assets/ComposerLocators/project-a/vendor/a/b/src/ab_ClassmapDir/.gitignore b/test/unit/Assets/ComposerLocators/project-a/vendor/a/b/src/ab_ClassmapDir/.gitignore
new file mode 100644
index 000000000..d6b7ef32c
--- /dev/null
+++ b/test/unit/Assets/ComposerLocators/project-a/vendor/a/b/src/ab_ClassmapDir/.gitignore
@@ -0,0 +1,2 @@
+*
+!.gitignore
diff --git a/test/unit/Assets/ComposerLocators/project-a/vendor/a/b/src/ab_ClassmapFile b/test/unit/Assets/ComposerLocators/project-a/vendor/a/b/src/ab_ClassmapFile
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/unit/Assets/ComposerLocators/project-a/vendor/a/b/src/ab_File1.php b/test/unit/Assets/ComposerLocators/project-a/vendor/a/b/src/ab_File1.php
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/unit/Assets/ComposerLocators/project-a/vendor/a/b/src/ab_File2.php b/test/unit/Assets/ComposerLocators/project-a/vendor/a/b/src/ab_File2.php
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/unit/Assets/ComposerLocators/project-a/vendor/a/b/src/ab_PSR-0_Sources/.gitignore b/test/unit/Assets/ComposerLocators/project-a/vendor/a/b/src/ab_PSR-0_Sources/.gitignore
new file mode 100644
index 000000000..d6b7ef32c
--- /dev/null
+++ b/test/unit/Assets/ComposerLocators/project-a/vendor/a/b/src/ab_PSR-0_Sources/.gitignore
@@ -0,0 +1,2 @@
+*
+!.gitignore
diff --git a/test/unit/Assets/ComposerLocators/project-a/vendor/a/b/src/ab_PSR-4_Sources/.gitignore b/test/unit/Assets/ComposerLocators/project-a/vendor/a/b/src/ab_PSR-4_Sources/.gitignore
new file mode 100644
index 000000000..d6b7ef32c
--- /dev/null
+++ b/test/unit/Assets/ComposerLocators/project-a/vendor/a/b/src/ab_PSR-4_Sources/.gitignore
@@ -0,0 +1,2 @@
+*
+!.gitignore
diff --git a/test/unit/Assets/ComposerLocators/project-a/vendor/composer/installed.json b/test/unit/Assets/ComposerLocators/project-a/vendor/composer/installed.json
new file mode 100644
index 000000000..e9cd7722e
--- /dev/null
+++ b/test/unit/Assets/ComposerLocators/project-a/vendor/composer/installed.json
@@ -0,0 +1,66 @@
+[
+  {
+    "name": "a/b",
+    "autoload": {
+      "psr-4": {
+        "A\\B\\": "src/ab_PSR-4_Sources",
+        "C\\D\\": [
+          "src/ab_PSR-4_Sources"
+        ]
+      },
+      "psr-0": {
+        "A_B_": "src/ab_PSR-0_Sources",
+        "C_D_": [
+          "src/ab_PSR-0_Sources"
+        ]
+      },
+      "classmap": [
+        "src/ab_ClassmapDir",
+        "src/ab_ClassmapFile"
+      ],
+      "files": [
+        "src/ab_File1.php",
+        "src/ab_File2.php"
+      ]
+    }
+  },
+  {
+    "name": "e/f",
+    "autoload": {
+      "psr-4": {
+        "E\\F\\": [
+          "src/ef_PSR-4_Sources"
+        ]
+      },
+      "psr-0": {
+        "E_F_": [
+          "src/ef_PSR-0_Sources"
+        ]
+      },
+      "classmap": [
+        "src/ef_ClassmapDir",
+        "src/ef_ClassmapFile"
+      ],
+      "files": [
+        "src/ef_File1.php",
+        "src/ef_File2.php"
+      ]
+    }
+  },
+  {
+    "name": "this-has/no-autoload-definitions"
+  },
+  {
+    "name": "this-has/empty-autoload-definitions",
+    "autoload": {}
+  },
+  {
+    "name": "this-has/empty-autoload-definition-blocks",
+    "autoload": {
+      "psr-0": {},
+      "psr-4": {},
+      "classmap": [],
+      "files": []
+    }
+  }
+]
diff --git a/test/unit/Assets/ComposerLocators/project-a/vendor/e/f/src/ef_ClassmapDir/.gitignore b/test/unit/Assets/ComposerLocators/project-a/vendor/e/f/src/ef_ClassmapDir/.gitignore
new file mode 100644
index 000000000..d6b7ef32c
--- /dev/null
+++ b/test/unit/Assets/ComposerLocators/project-a/vendor/e/f/src/ef_ClassmapDir/.gitignore
@@ -0,0 +1,2 @@
+*
+!.gitignore
diff --git a/test/unit/Assets/ComposerLocators/project-a/vendor/e/f/src/ef_ClassmapFile b/test/unit/Assets/ComposerLocators/project-a/vendor/e/f/src/ef_ClassmapFile
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/unit/Assets/ComposerLocators/project-a/vendor/e/f/src/ef_File1.php b/test/unit/Assets/ComposerLocators/project-a/vendor/e/f/src/ef_File1.php
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/unit/Assets/ComposerLocators/project-a/vendor/e/f/src/ef_File2.php b/test/unit/Assets/ComposerLocators/project-a/vendor/e/f/src/ef_File2.php
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/unit/Assets/ComposerLocators/project-a/vendor/e/f/src/ef_PSR-0_Sources/.gitignore b/test/unit/Assets/ComposerLocators/project-a/vendor/e/f/src/ef_PSR-0_Sources/.gitignore
new file mode 100644
index 000000000..d6b7ef32c
--- /dev/null
+++ b/test/unit/Assets/ComposerLocators/project-a/vendor/e/f/src/ef_PSR-0_Sources/.gitignore
@@ -0,0 +1,2 @@
+*
+!.gitignore
diff --git a/test/unit/Assets/ComposerLocators/project-a/vendor/e/f/src/ef_PSR-4_Sources/.gitignore b/test/unit/Assets/ComposerLocators/project-a/vendor/e/f/src/ef_PSR-4_Sources/.gitignore
new file mode 100644
index 000000000..d6b7ef32c
--- /dev/null
+++ b/test/unit/Assets/ComposerLocators/project-a/vendor/e/f/src/ef_PSR-4_Sources/.gitignore
@@ -0,0 +1,2 @@
+*
+!.gitignore
diff --git a/test/unit/Assets/ComposerLocators/project-with-invalid-composer-json/composer.json b/test/unit/Assets/ComposerLocators/project-with-invalid-composer-json/composer.json
new file mode 100644
index 000000000..72943a16f
--- /dev/null
+++ b/test/unit/Assets/ComposerLocators/project-with-invalid-composer-json/composer.json
@@ -0,0 +1 @@
+aaa
diff --git a/test/unit/Assets/ComposerLocators/project-with-invalid-composer-json/vendor/composer/installed.json b/test/unit/Assets/ComposerLocators/project-with-invalid-composer-json/vendor/composer/installed.json
new file mode 100644
index 000000000..2c63c0851
--- /dev/null
+++ b/test/unit/Assets/ComposerLocators/project-with-invalid-composer-json/vendor/composer/installed.json
@@ -0,0 +1,2 @@
+{
+}
diff --git a/test/unit/Assets/ComposerLocators/project-with-invalid-installed-json/composer.json b/test/unit/Assets/ComposerLocators/project-with-invalid-installed-json/composer.json
new file mode 100644
index 000000000..2c63c0851
--- /dev/null
+++ b/test/unit/Assets/ComposerLocators/project-with-invalid-installed-json/composer.json
@@ -0,0 +1,2 @@
+{
+}
diff --git a/test/unit/Assets/ComposerLocators/project-with-invalid-installed-json/vendor/composer/installed.json b/test/unit/Assets/ComposerLocators/project-with-invalid-installed-json/vendor/composer/installed.json
new file mode 100644
index 000000000..72943a16f
--- /dev/null
+++ b/test/unit/Assets/ComposerLocators/project-with-invalid-installed-json/vendor/composer/installed.json
@@ -0,0 +1 @@
+aaa
diff --git a/test/unit/Assets/ComposerLocators/project-with-psr-collisions/composer.json b/test/unit/Assets/ComposerLocators/project-with-psr-collisions/composer.json
new file mode 100644
index 000000000..e45ac6f6b
--- /dev/null
+++ b/test/unit/Assets/ComposerLocators/project-with-psr-collisions/composer.json
@@ -0,0 +1,10 @@
+{
+  "autoload": {
+    "psr-4": {
+      "A\\": "src/root_PSR-4_Sources"
+    },
+    "psr-0": {
+      "A_": "src/root_PSR-0_Sources"
+    }
+  }
+}
diff --git a/test/unit/Assets/ComposerLocators/project-with-psr-collisions/src/root_PSR-0_Sources/.gitignore b/test/unit/Assets/ComposerLocators/project-with-psr-collisions/src/root_PSR-0_Sources/.gitignore
new file mode 100644
index 000000000..d6b7ef32c
--- /dev/null
+++ b/test/unit/Assets/ComposerLocators/project-with-psr-collisions/src/root_PSR-0_Sources/.gitignore
@@ -0,0 +1,2 @@
+*
+!.gitignore
diff --git a/test/unit/Assets/ComposerLocators/project-with-psr-collisions/src/root_PSR-4_Sources/.gitignore b/test/unit/Assets/ComposerLocators/project-with-psr-collisions/src/root_PSR-4_Sources/.gitignore
new file mode 100644
index 000000000..d6b7ef32c
--- /dev/null
+++ b/test/unit/Assets/ComposerLocators/project-with-psr-collisions/src/root_PSR-4_Sources/.gitignore
@@ -0,0 +1,2 @@
+*
+!.gitignore
diff --git a/test/unit/Assets/ComposerLocators/project-with-psr-collisions/vendor/a/b/src/ab_PSR-0_Sources/.gitignore b/test/unit/Assets/ComposerLocators/project-with-psr-collisions/vendor/a/b/src/ab_PSR-0_Sources/.gitignore
new file mode 100644
index 000000000..d6b7ef32c
--- /dev/null
+++ b/test/unit/Assets/ComposerLocators/project-with-psr-collisions/vendor/a/b/src/ab_PSR-0_Sources/.gitignore
@@ -0,0 +1,2 @@
+*
+!.gitignore
diff --git a/test/unit/Assets/ComposerLocators/project-with-psr-collisions/vendor/a/b/src/ab_PSR-4_Sources/.gitignore b/test/unit/Assets/ComposerLocators/project-with-psr-collisions/vendor/a/b/src/ab_PSR-4_Sources/.gitignore
new file mode 100644
index 000000000..d6b7ef32c
--- /dev/null
+++ b/test/unit/Assets/ComposerLocators/project-with-psr-collisions/vendor/a/b/src/ab_PSR-4_Sources/.gitignore
@@ -0,0 +1,2 @@
+*
+!.gitignore
diff --git a/test/unit/Assets/ComposerLocators/project-with-psr-collisions/vendor/composer/installed.json b/test/unit/Assets/ComposerLocators/project-with-psr-collisions/vendor/composer/installed.json
new file mode 100644
index 000000000..d9410688e
--- /dev/null
+++ b/test/unit/Assets/ComposerLocators/project-with-psr-collisions/vendor/composer/installed.json
@@ -0,0 +1,34 @@
+[
+  {
+    "name": "a/b",
+    "autoload": {
+      "psr-4": {
+        "A\\": "src/ab_PSR-4_Sources",
+        "B\\": [
+          "src/ab_PSR-4_Sources"
+        ]
+      },
+      "psr-0": {
+        "A_": "src/ab_PSR-0_Sources",
+        "B_": [
+          "src/ab_PSR-0_Sources"
+        ]
+      }
+    }
+  },
+  {
+    "name": "e/f",
+    "autoload": {
+      "psr-4": {
+        "A\\": [
+          "src/ef_PSR-4_Sources"
+        ]
+      },
+      "psr-0": {
+        "A_": [
+          "src/ef_PSR-0_Sources"
+        ]
+      }
+    }
+  }
+]
diff --git a/test/unit/Assets/ComposerLocators/project-with-psr-collisions/vendor/e/f/src/ef_PSR-0_Sources/.gitignore b/test/unit/Assets/ComposerLocators/project-with-psr-collisions/vendor/e/f/src/ef_PSR-0_Sources/.gitignore
new file mode 100644
index 000000000..d6b7ef32c
--- /dev/null
+++ b/test/unit/Assets/ComposerLocators/project-with-psr-collisions/vendor/e/f/src/ef_PSR-0_Sources/.gitignore
@@ -0,0 +1,2 @@
+*
+!.gitignore
diff --git a/test/unit/Assets/ComposerLocators/project-with-psr-collisions/vendor/e/f/src/ef_PSR-4_Sources/.gitignore b/test/unit/Assets/ComposerLocators/project-with-psr-collisions/vendor/e/f/src/ef_PSR-4_Sources/.gitignore
new file mode 100644
index 000000000..d6b7ef32c
--- /dev/null
+++ b/test/unit/Assets/ComposerLocators/project-with-psr-collisions/vendor/e/f/src/ef_PSR-4_Sources/.gitignore
@@ -0,0 +1,2 @@
+*
+!.gitignore
diff --git a/test/unit/Assets/ComposerLocators/project-without-autoload-definitions/composer.json b/test/unit/Assets/ComposerLocators/project-without-autoload-definitions/composer.json
new file mode 100644
index 000000000..2c63c0851
--- /dev/null
+++ b/test/unit/Assets/ComposerLocators/project-without-autoload-definitions/composer.json
@@ -0,0 +1,2 @@
+{
+}
diff --git a/test/unit/Assets/ComposerLocators/project-without-autoload-definitions/vendor/composer/installed.json b/test/unit/Assets/ComposerLocators/project-without-autoload-definitions/vendor/composer/installed.json
new file mode 100644
index 000000000..2c63c0851
--- /dev/null
+++ b/test/unit/Assets/ComposerLocators/project-without-autoload-definitions/vendor/composer/installed.json
@@ -0,0 +1,2 @@
+{
+}
diff --git a/test/unit/Assets/ComposerLocators/project-without-installed.json/.gitignore b/test/unit/Assets/ComposerLocators/project-without-installed.json/.gitignore
new file mode 100644
index 000000000..d6b7ef32c
--- /dev/null
+++ b/test/unit/Assets/ComposerLocators/project-without-installed.json/.gitignore
@@ -0,0 +1,2 @@
+*
+!.gitignore
diff --git a/test/unit/Assets/ComposerLocators/project-without-installed.json/composer.json b/test/unit/Assets/ComposerLocators/project-without-installed.json/composer.json
new file mode 100644
index 000000000..2c63c0851
--- /dev/null
+++ b/test/unit/Assets/ComposerLocators/project-without-installed.json/composer.json
@@ -0,0 +1,2 @@
+{
+}
diff --git a/test/unit/SourceLocator/Type/Composer/Factory/Exception/FailedToParseJsonTest.php b/test/unit/SourceLocator/Type/Composer/Factory/Exception/FailedToParseJsonTest.php
new file mode 100644
index 000000000..979a20f35
--- /dev/null
+++ b/test/unit/SourceLocator/Type/Composer/Factory/Exception/FailedToParseJsonTest.php
@@ -0,0 +1,23 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Roave\BetterReflectionTest\SourceLocator\Type\Composer\Factory\Exception;
+
+use PHPUnit\Framework\TestCase;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Factory\Exception\FailedToParseJson;
+
+/**
+ * @covers \Roave\BetterReflection\SourceLocator\Type\Composer\Factory\Exception\FailedToParseJson
+ */
+class FailedToParseJsonTest extends TestCase
+{
+    public function testInFile() : void
+    {
+        self::assertSame(
+            'Could not parse JSON file "foo/bar"',
+            FailedToParseJson::inFile('foo/bar')
+                ->getMessage()
+        );
+    }
+}
diff --git a/test/unit/SourceLocator/Type/Composer/Factory/Exception/InvalidProjectDirectoryTest.php b/test/unit/SourceLocator/Type/Composer/Factory/Exception/InvalidProjectDirectoryTest.php
new file mode 100644
index 000000000..a2f8717b3
--- /dev/null
+++ b/test/unit/SourceLocator/Type/Composer/Factory/Exception/InvalidProjectDirectoryTest.php
@@ -0,0 +1,23 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Roave\BetterReflectionTest\SourceLocator\Type\Composer\Factory\Exception;
+
+use PHPUnit\Framework\TestCase;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Factory\Exception\InvalidProjectDirectory;
+
+/**
+ * @covers \Roave\BetterReflection\SourceLocator\Type\Composer\Factory\Exception\InvalidProjectDirectory
+ */
+class InvalidProjectDirectoryTest extends TestCase
+{
+    public function testAtPath() : void
+    {
+        self::assertSame(
+            'Could not locate project directory "foo/bar"',
+            InvalidProjectDirectory::atPath('foo/bar')
+                ->getMessage()
+        );
+    }
+}
diff --git a/test/unit/SourceLocator/Type/Composer/Factory/Exception/MissingComposerJsonTest.php b/test/unit/SourceLocator/Type/Composer/Factory/Exception/MissingComposerJsonTest.php
new file mode 100644
index 000000000..601ee7d70
--- /dev/null
+++ b/test/unit/SourceLocator/Type/Composer/Factory/Exception/MissingComposerJsonTest.php
@@ -0,0 +1,23 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Roave\BetterReflectionTest\SourceLocator\Type\Composer\Factory\Exception;
+
+use PHPUnit\Framework\TestCase;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Factory\Exception\MissingComposerJson;
+
+/**
+ * @covers \Roave\BetterReflection\SourceLocator\Type\Composer\Factory\Exception\MissingComposerJson
+ */
+class MissingComposerJsonTest extends TestCase
+{
+    public function testInProjectPath() : void
+    {
+        self::assertSame(
+            'Could not locate a "composer.json" file in "foo/bar"',
+            MissingComposerJson::inProjectPath('foo/bar')
+                ->getMessage()
+        );
+    }
+}
diff --git a/test/unit/SourceLocator/Type/Composer/Factory/Exception/MissingInstalledJsonTest.php b/test/unit/SourceLocator/Type/Composer/Factory/Exception/MissingInstalledJsonTest.php
new file mode 100644
index 000000000..73ee35a64
--- /dev/null
+++ b/test/unit/SourceLocator/Type/Composer/Factory/Exception/MissingInstalledJsonTest.php
@@ -0,0 +1,23 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Roave\BetterReflectionTest\SourceLocator\Type\Composer\Factory\Exception;
+
+use PHPUnit\Framework\TestCase;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Factory\Exception\MissingInstalledJson;
+
+/**
+ * @covers \Roave\BetterReflection\SourceLocator\Type\Composer\Factory\Exception\MissingInstalledJson
+ */
+class MissingInstalledJsonTest extends TestCase
+{
+    public function testInProjectPath() : void
+    {
+        self::assertSame(
+            'Could not locate a "vendor/composer/installed.json" file in "foo/bar"',
+            MissingInstalledJson::inProjectPath('foo/bar')
+                ->getMessage()
+        );
+    }
+}
diff --git a/test/unit/SourceLocator/Type/Composer/Factory/MakeLocatorForComposerInstalledJsonTest.php b/test/unit/SourceLocator/Type/Composer/Factory/MakeLocatorForComposerInstalledJsonTest.php
new file mode 100644
index 000000000..c55f6bab5
--- /dev/null
+++ b/test/unit/SourceLocator/Type/Composer/Factory/MakeLocatorForComposerInstalledJsonTest.php
@@ -0,0 +1,189 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Roave\BetterReflectionTest\SourceLocator\Type\Composer\Factory;
+
+use PHPUnit\Framework\TestCase;
+use Roave\BetterReflection\SourceLocator\Type\AggregateSourceLocator;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Factory\Exception\FailedToParseJson;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Factory\Exception\InvalidProjectDirectory;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Factory\Exception\MissingInstalledJson;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Factory\MakeLocatorForInstalledJson;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Psr\Psr0Mapping;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Psr\Psr4Mapping;
+use Roave\BetterReflection\SourceLocator\Type\Composer\PsrAutoloaderLocator;
+use Roave\BetterReflection\SourceLocator\Type\DirectoriesSourceLocator;
+use Roave\BetterReflection\SourceLocator\Type\SingleFileSourceLocator;
+use Roave\BetterReflection\SourceLocator\Type\SourceLocator;
+use Roave\BetterReflectionTest\BetterReflectionSingleton;
+use function array_column;
+use function array_combine;
+use function realpath;
+
+/**
+ * @covers \Roave\BetterReflection\SourceLocator\Type\Composer\Factory\MakeLocatorForInstalledJson
+ */
+class MakeLocatorForComposerInstalledJsonTest extends TestCase
+{
+    /** @dataProvider expectedLocators */
+    public function testLocatorEquality(string $projectDirectory, SourceLocator $expectedLocatorStructure) : void
+    {
+        self::assertEquals(
+            $expectedLocatorStructure,
+            (new MakeLocatorForInstalledJson())
+                ->__invoke($projectDirectory, BetterReflectionSingleton::instance()->astLocator())
+        );
+    }
+
+    /**
+     * @return array<string, array<int, string|SourceLocator>>
+     *
+     * @psalm-return array<string, array{0: string, 1: SourceLocator}>
+     */
+    public function expectedLocators() : array
+    {
+        $astLocator = BetterReflectionSingleton::instance()->astLocator();
+
+        $projectA                 = realpath(__DIR__ . '/../../../../Assets/ComposerLocators/project-a');
+        $projectWithPsrCollisions = realpath(__DIR__ . '/../../../../Assets/ComposerLocators/project-with-psr-collisions');
+        $projectALocator          = new AggregateSourceLocator([
+            new PsrAutoloaderLocator(
+                Psr4Mapping::fromArrayMappings([
+                    'A\\B\\' => [
+                        $projectA . '/vendor/a/b/src/ab_PSR-4_Sources',
+                    ],
+                    'C\\D\\' => [
+                        $projectA . '/vendor/a/b/src/ab_PSR-4_Sources',
+                    ],
+                    'E\\F\\' => [
+                        $projectA . '/vendor/e/f/src/ef_PSR-4_Sources',
+                    ],
+                ]),
+                $astLocator
+            ),
+            new PsrAutoloaderLocator(
+                Psr0Mapping::fromArrayMappings([
+                    'A_B_' => [
+                        $projectA . '/vendor/a/b/src/ab_PSR-0_Sources',
+                    ],
+                    'C_D_' => [
+                        $projectA . '/vendor/a/b/src/ab_PSR-0_Sources',
+                    ],
+                    'E_F_' => [
+                        $projectA . '/vendor/e/f/src/ef_PSR-0_Sources',
+                    ],
+                ]),
+                $astLocator
+            ),
+            new DirectoriesSourceLocator(
+                [
+                    $projectA . '/vendor/a/b/src/ab_ClassmapDir',
+                    $projectA . '/vendor/e/f/src/ef_ClassmapDir',
+                ],
+                $astLocator
+            ),
+            new SingleFileSourceLocator(
+                $projectA . '/vendor/a/b/src/ab_ClassmapFile',
+                $astLocator
+            ),
+            new SingleFileSourceLocator(
+                $projectA . '/vendor/e/f/src/ef_ClassmapFile',
+                $astLocator
+            ),
+            new SingleFileSourceLocator(
+                $projectA . '/vendor/a/b/src/ab_File1.php',
+                $astLocator
+            ),
+            new SingleFileSourceLocator(
+                $projectA . '/vendor/a/b/src/ab_File2.php',
+                $astLocator
+            ),
+            new SingleFileSourceLocator(
+                $projectA . '/vendor/e/f/src/ef_File1.php',
+                $astLocator
+            ),
+            new SingleFileSourceLocator(
+                $projectA . '/vendor/e/f/src/ef_File2.php',
+                $astLocator
+            ),
+        ]);
+
+        $expectedLocators = [
+            [
+                $projectA,
+                $projectALocator,
+            ],
+            [
+                $projectWithPsrCollisions,
+                new AggregateSourceLocator([
+                    new PsrAutoloaderLocator(
+                        Psr4Mapping::fromArrayMappings([
+                            'A\\' => [
+                                $projectWithPsrCollisions . '/vendor/a/b/src/ab_PSR-4_Sources',
+                                $projectWithPsrCollisions . '/vendor/e/f/src/ef_PSR-4_Sources',
+                            ],
+                            'B\\' => [
+                                $projectWithPsrCollisions . '/vendor/a/b/src/ab_PSR-4_Sources',
+                            ],
+                        ]),
+                        $astLocator
+                    ),
+                    new PsrAutoloaderLocator(
+                        Psr0Mapping::fromArrayMappings([
+                            'A_' => [
+                                $projectWithPsrCollisions . '/vendor/a/b/src/ab_PSR-0_Sources',
+                                $projectWithPsrCollisions . '/vendor/e/f/src/ef_PSR-0_Sources',
+                            ],
+                            'B_' => [
+                                $projectWithPsrCollisions . '/vendor/a/b/src/ab_PSR-0_Sources',
+                            ],
+                        ]),
+                        $astLocator
+                    ),
+                    new DirectoriesSourceLocator([], $astLocator),
+                ]),
+            ],
+            [
+                // Relative paths are turned into absolute paths too
+                __DIR__ . '/../../../../Assets/ComposerLocators/project-a',
+                $projectALocator,
+            ],
+        ];
+
+        return array_combine(array_column($expectedLocators, 0), $expectedLocators);
+    }
+
+    public function testWillFailToProduceLocatorForProjectWithoutInstalledJson() : void
+    {
+        $this->expectException(MissingInstalledJson::class);
+
+        (new MakeLocatorForInstalledJson())
+            ->__invoke(
+                __DIR__ . '/../../../../Assets/ComposerLocators/project-without-installed.json',
+                BetterReflectionSingleton::instance()->astLocator()
+            );
+    }
+
+    public function testWillFailToProduceLocatorForProjectWithInvalidInstalledJson() : void
+    {
+        $this->expectException(FailedToParseJson::class);
+
+        (new MakeLocatorForInstalledJson())
+            ->__invoke(
+                __DIR__ . '/../../../../Assets/ComposerLocators/project-with-invalid-installed-json',
+                BetterReflectionSingleton::instance()->astLocator()
+            );
+    }
+
+    public function testWillFailToProduceLocatorForInvalidProjectDirectory() : void
+    {
+        $this->expectException(InvalidProjectDirectory::class);
+
+        (new MakeLocatorForInstalledJson())
+            ->__invoke(
+                __DIR__ . '/../../../../Assets/ComposerLocators/non-existing',
+                BetterReflectionSingleton::instance()->astLocator()
+            );
+    }
+}
diff --git a/test/unit/SourceLocator/Type/Composer/Factory/MakeLocatorForComposerJsonAndInstalledJsonTest.php b/test/unit/SourceLocator/Type/Composer/Factory/MakeLocatorForComposerJsonAndInstalledJsonTest.php
new file mode 100644
index 000000000..443ad1fd4
--- /dev/null
+++ b/test/unit/SourceLocator/Type/Composer/Factory/MakeLocatorForComposerJsonAndInstalledJsonTest.php
@@ -0,0 +1,239 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Roave\BetterReflectionTest\SourceLocator\Type\Composer\Factory;
+
+use PHPUnit\Framework\TestCase;
+use Roave\BetterReflection\SourceLocator\Type\AggregateSourceLocator;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Factory\Exception\FailedToParseJson;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Factory\Exception\InvalidProjectDirectory;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Factory\Exception\MissingComposerJson;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Factory\Exception\MissingInstalledJson;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Factory\MakeLocatorForComposerJsonAndInstalledJson;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Psr\Psr0Mapping;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Psr\Psr4Mapping;
+use Roave\BetterReflection\SourceLocator\Type\Composer\PsrAutoloaderLocator;
+use Roave\BetterReflection\SourceLocator\Type\DirectoriesSourceLocator;
+use Roave\BetterReflection\SourceLocator\Type\SingleFileSourceLocator;
+use Roave\BetterReflection\SourceLocator\Type\SourceLocator;
+use Roave\BetterReflectionTest\BetterReflectionSingleton;
+use function array_column;
+use function array_combine;
+use function realpath;
+
+/**
+ * @covers \Roave\BetterReflection\SourceLocator\Type\Composer\Factory\MakeLocatorForComposerJsonAndInstalledJson
+ */
+class MakeLocatorForComposerJsonAndInstalledJsonTest extends TestCase
+{
+    /** @dataProvider expectedLocators */
+    public function testLocatorEquality(string $projectDirectory, SourceLocator $expectedLocatorStructure) : void
+    {
+        self::assertEquals(
+            $expectedLocatorStructure,
+            (new MakeLocatorForComposerJsonAndInstalledJson())
+                ->__invoke($projectDirectory, BetterReflectionSingleton::instance()->astLocator())
+        );
+    }
+
+    /**
+     * @return array<string, array<int, string|SourceLocator>>
+     *
+     * @psalm-return array<string, array{0: string, 1: SourceLocator}>
+     */
+    public function expectedLocators() : array
+    {
+        $astLocator = BetterReflectionSingleton::instance()->astLocator();
+
+        $projectA                 = realpath(__DIR__ . '/../../../../Assets/ComposerLocators/project-a');
+        $projectWithPsrCollisions = realpath(__DIR__ . '/../../../../Assets/ComposerLocators/project-with-psr-collisions');
+        $projectALocator          = new AggregateSourceLocator([
+            new PsrAutoloaderLocator(
+                Psr4Mapping::fromArrayMappings([
+                    'ProjectA\\'    => [
+                        $projectA . '/src/root_PSR-4_Sources',
+                    ],
+                    'ProjectA\\B\\' => [
+                        $projectA . '/src/root_PSR-4_Sources',
+                    ],
+                    'A\\B\\'        => [
+                        $projectA . '/vendor/a/b/src/ab_PSR-4_Sources',
+                    ],
+                    'C\\D\\'        => [
+                        $projectA . '/vendor/a/b/src/ab_PSR-4_Sources',
+                    ],
+                    'E\\F\\'        => [
+                        $projectA . '/vendor/e/f/src/ef_PSR-4_Sources',
+                    ],
+                ]),
+                $astLocator
+            ),
+            new PsrAutoloaderLocator(
+                Psr0Mapping::fromArrayMappings([
+                    'ProjectA_A_' => [
+                        $projectA . '/src/root_PSR-0_Sources',
+                    ],
+                    'ProjectA_B_' => [
+                        $projectA . '/src/root_PSR-0_Sources',
+                    ],
+                    'A_B_'        => [
+                        $projectA . '/vendor/a/b/src/ab_PSR-0_Sources',
+                    ],
+                    'C_D_'        => [
+                        $projectA . '/vendor/a/b/src/ab_PSR-0_Sources',
+                    ],
+                    'E_F_'        => [
+                        $projectA . '/vendor/e/f/src/ef_PSR-0_Sources',
+                    ],
+                ]),
+                $astLocator
+            ),
+            new DirectoriesSourceLocator(
+                [
+                    $projectA . '/src/root_ClassmapDir',
+                    $projectA . '/vendor/a/b/src/ab_ClassmapDir',
+                    $projectA . '/vendor/e/f/src/ef_ClassmapDir',
+                ],
+                $astLocator
+            ),
+            new SingleFileSourceLocator(
+                $projectA . '/src/root_ClassmapFile',
+                $astLocator
+            ),
+            new SingleFileSourceLocator(
+                $projectA . '/vendor/a/b/src/ab_ClassmapFile',
+                $astLocator
+            ),
+            new SingleFileSourceLocator(
+                $projectA . '/vendor/e/f/src/ef_ClassmapFile',
+                $astLocator
+            ),
+            new SingleFileSourceLocator(
+                $projectA . '/src/root_File1.php',
+                $astLocator
+            ),
+            new SingleFileSourceLocator(
+                $projectA . '/src/root_File2.php',
+                $astLocator
+            ),
+            new SingleFileSourceLocator(
+                $projectA . '/vendor/a/b/src/ab_File1.php',
+                $astLocator
+            ),
+            new SingleFileSourceLocator(
+                $projectA . '/vendor/a/b/src/ab_File2.php',
+                $astLocator
+            ),
+            new SingleFileSourceLocator(
+                $projectA . '/vendor/e/f/src/ef_File1.php',
+                $astLocator
+            ),
+            new SingleFileSourceLocator(
+                $projectA . '/vendor/e/f/src/ef_File2.php',
+                $astLocator
+            ),
+        ]);
+
+        $expectedLocators = [
+            [
+                $projectA,
+                $projectALocator,
+            ],
+            [
+                $projectWithPsrCollisions,
+                new AggregateSourceLocator([
+                    new PsrAutoloaderLocator(
+                        Psr4Mapping::fromArrayMappings([
+                            'A\\' => [
+                                $projectWithPsrCollisions . '/src/root_PSR-4_Sources',
+                                $projectWithPsrCollisions . '/vendor/a/b/src/ab_PSR-4_Sources',
+                                $projectWithPsrCollisions . '/vendor/e/f/src/ef_PSR-4_Sources',
+                            ],
+                            'B\\' => [
+                                $projectWithPsrCollisions . '/vendor/a/b/src/ab_PSR-4_Sources',
+                            ],
+                        ]),
+                        $astLocator
+                    ),
+                    new PsrAutoloaderLocator(
+                        Psr0Mapping::fromArrayMappings([
+                            'A_' => [
+                                $projectWithPsrCollisions . '/src/root_PSR-0_Sources',
+                                $projectWithPsrCollisions . '/vendor/a/b/src/ab_PSR-0_Sources',
+                                $projectWithPsrCollisions . '/vendor/e/f/src/ef_PSR-0_Sources',
+                            ],
+                            'B_' => [
+                                $projectWithPsrCollisions . '/vendor/a/b/src/ab_PSR-0_Sources',
+                            ],
+                        ]),
+                        $astLocator
+                    ),
+                    new DirectoriesSourceLocator([], $astLocator),
+                ]),
+            ],
+            [
+                // Relative paths are turned into absolute paths too
+                __DIR__ . '/../../../../Assets/ComposerLocators/project-a',
+                $projectALocator,
+            ],
+        ];
+
+        return array_combine(array_column($expectedLocators, 0), $expectedLocators);
+    }
+
+    public function testWillFailToProduceLocatorForProjectWithoutComposerJson() : void
+    {
+        $this->expectException(MissingComposerJson::class);
+
+        (new MakeLocatorForComposerJsonAndInstalledJson())
+            ->__invoke(
+                __DIR__ . '/../../../../Assets/ComposerLocators/empty-project',
+                BetterReflectionSingleton::instance()->astLocator()
+            );
+    }
+
+    public function testWillFailToProduceLocatorForProjectWithoutInstalledJson() : void
+    {
+        $this->expectException(MissingInstalledJson::class);
+
+        (new MakeLocatorForComposerJsonAndInstalledJson())
+            ->__invoke(
+                __DIR__ . '/../../../../Assets/ComposerLocators/project-without-installed.json',
+                BetterReflectionSingleton::instance()->astLocator()
+            );
+    }
+
+    public function testWillFailToProduceLocatorForProjectWithInvalidComposerJson() : void
+    {
+        $this->expectException(FailedToParseJson::class);
+
+        (new MakeLocatorForComposerJsonAndInstalledJson())
+            ->__invoke(
+                __DIR__ . '/../../../../Assets/ComposerLocators/project-with-invalid-composer-json',
+                BetterReflectionSingleton::instance()->astLocator()
+            );
+    }
+
+    public function testWillFailToProduceLocatorForProjectWithInvalidInstalledJson() : void
+    {
+        $this->expectException(FailedToParseJson::class);
+
+        (new MakeLocatorForComposerJsonAndInstalledJson())
+            ->__invoke(
+                __DIR__ . '/../../../../Assets/ComposerLocators/project-with-invalid-installed-json',
+                BetterReflectionSingleton::instance()->astLocator()
+            );
+    }
+
+    public function testWillFailToProduceLocatorForInvalidProjectDirectory() : void
+    {
+        $this->expectException(InvalidProjectDirectory::class);
+
+        (new MakeLocatorForComposerJsonAndInstalledJson())
+            ->__invoke(
+                __DIR__ . '/../../../../Assets/ComposerLocators/non-existing',
+                BetterReflectionSingleton::instance()->astLocator()
+            );
+    }
+}
diff --git a/test/unit/SourceLocator/Type/Composer/Factory/MakeLocatorForComposerJsonTest.php b/test/unit/SourceLocator/Type/Composer/Factory/MakeLocatorForComposerJsonTest.php
new file mode 100644
index 000000000..3f50c32c2
--- /dev/null
+++ b/test/unit/SourceLocator/Type/Composer/Factory/MakeLocatorForComposerJsonTest.php
@@ -0,0 +1,162 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Roave\BetterReflectionTest\SourceLocator\Type\Composer\Factory;
+
+use PHPUnit\Framework\TestCase;
+use Roave\BetterReflection\SourceLocator\Type\AggregateSourceLocator;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Factory\Exception\FailedToParseJson;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Factory\Exception\InvalidProjectDirectory;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Factory\Exception\MissingComposerJson;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Factory\MakeLocatorForComposerJson;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Psr\Psr0Mapping;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Psr\Psr4Mapping;
+use Roave\BetterReflection\SourceLocator\Type\Composer\PsrAutoloaderLocator;
+use Roave\BetterReflection\SourceLocator\Type\DirectoriesSourceLocator;
+use Roave\BetterReflection\SourceLocator\Type\SingleFileSourceLocator;
+use Roave\BetterReflection\SourceLocator\Type\SourceLocator;
+use Roave\BetterReflectionTest\BetterReflectionSingleton;
+use function array_column;
+use function array_combine;
+use function realpath;
+
+/**
+ * @covers \Roave\BetterReflection\SourceLocator\Type\Composer\Factory\MakeLocatorForComposerJson
+ */
+class MakeLocatorForComposerJsonTest extends TestCase
+{
+    /** @dataProvider expectedLocators */
+    public function testLocatorEquality(string $projectDirectory, SourceLocator $expectedLocatorStructure) : void
+    {
+        self::assertEquals(
+            $expectedLocatorStructure,
+            (new MakeLocatorForComposerJson())
+                ->__invoke($projectDirectory, BetterReflectionSingleton::instance()->astLocator())
+        );
+    }
+
+    /**
+     * @return array<string, array<int, string|SourceLocator>>
+     *
+     * @psalm-return array<string, array{0: string, 1: SourceLocator}>
+     */
+    public function expectedLocators() : array
+    {
+        $astLocator = BetterReflectionSingleton::instance()->astLocator();
+
+        $projectA                 = realpath(__DIR__ . '/../../../../Assets/ComposerLocators/project-a');
+        $projectWithPsrCollisions = realpath(__DIR__ . '/../../../../Assets/ComposerLocators/project-with-psr-collisions');
+        $projectALocator          = new AggregateSourceLocator([
+            new PsrAutoloaderLocator(
+                Psr4Mapping::fromArrayMappings([
+                    'ProjectA\\'    => [
+                        $projectA . '/src/root_PSR-4_Sources',
+                    ],
+                    'ProjectA\\B\\' => [
+                        $projectA . '/src/root_PSR-4_Sources',
+                    ],
+                ]),
+                $astLocator
+            ),
+            new PsrAutoloaderLocator(
+                Psr0Mapping::fromArrayMappings([
+                    'ProjectA_A_' => [
+                        $projectA . '/src/root_PSR-0_Sources',
+                    ],
+                    'ProjectA_B_' => [
+                        $projectA . '/src/root_PSR-0_Sources',
+                    ],
+                ]),
+                $astLocator
+            ),
+            new DirectoriesSourceLocator(
+                [
+                    $projectA . '/src/root_ClassmapDir',
+                ],
+                $astLocator
+            ),
+            new SingleFileSourceLocator(
+                $projectA . '/src/root_ClassmapFile',
+                $astLocator
+            ),
+            new SingleFileSourceLocator(
+                $projectA . '/src/root_File1.php',
+                $astLocator
+            ),
+            new SingleFileSourceLocator(
+                $projectA . '/src/root_File2.php',
+                $astLocator
+            ),
+        ]);
+
+        $expectedLocators = [
+            [
+                $projectA,
+                $projectALocator,
+            ],
+            [
+                $projectWithPsrCollisions,
+                new AggregateSourceLocator([
+                    new PsrAutoloaderLocator(
+                        Psr4Mapping::fromArrayMappings([
+                            'A\\' => [
+                                $projectWithPsrCollisions . '/src/root_PSR-4_Sources',
+                            ],
+                        ]),
+                        $astLocator
+                    ),
+                    new PsrAutoloaderLocator(
+                        Psr0Mapping::fromArrayMappings([
+                            'A_' => [
+                                $projectWithPsrCollisions . '/src/root_PSR-0_Sources',
+                            ],
+                        ]),
+                        $astLocator
+                    ),
+                    new DirectoriesSourceLocator([], $astLocator),
+                ]),
+            ],
+            [
+                // Relative paths are turned into absolute paths too
+                __DIR__ . '/../../../../Assets/ComposerLocators/project-a',
+                $projectALocator,
+            ],
+        ];
+
+        return array_combine(array_column($expectedLocators, 0), $expectedLocators);
+    }
+
+    public function testWillFailToProduceLocatorForProjectWithoutComposerJson() : void
+    {
+        $this->expectException(MissingComposerJson::class);
+
+        (new MakeLocatorForComposerJson())
+            ->__invoke(
+                __DIR__ . '/../../../../Assets/ComposerLocators/empty-project',
+                BetterReflectionSingleton::instance()->astLocator()
+            );
+    }
+
+    public function testWillFailToProduceLocatorForProjectWithInvalidComposerJson() : void
+    {
+        $this->expectException(FailedToParseJson::class);
+
+        (new MakeLocatorForComposerJson())
+            ->__invoke(
+                __DIR__ . '/../../../../Assets/ComposerLocators/project-with-invalid-composer-json',
+                BetterReflectionSingleton::instance()->astLocator()
+            );
+    }
+
+    public function testWillFailToProduceLocatorForInvalidProjectDirectory() : void
+    {
+        $this->expectException(InvalidProjectDirectory::class);
+
+        (new MakeLocatorForComposerJson())
+            ->__invoke(
+                __DIR__ . '/../../../../Assets/ComposerLocators/non-existing',
+                BetterReflectionSingleton::instance()->astLocator()
+            );
+    }
+}
diff --git a/test/unit/SourceLocator/Type/Composer/Psr/Excetion/InvalidPrefixMappingTest.php b/test/unit/SourceLocator/Type/Composer/Psr/Excetion/InvalidPrefixMappingTest.php
new file mode 100644
index 000000000..66441c3fe
--- /dev/null
+++ b/test/unit/SourceLocator/Type/Composer/Psr/Excetion/InvalidPrefixMappingTest.php
@@ -0,0 +1,41 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Roave\BetterReflectionTest\SourceLocator\Type\Composer\Psr\Exception;
+
+use PHPUnit\Framework\TestCase;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Psr\Exception\InvalidPrefixMapping;
+
+/**
+ * @covers \Roave\BetterReflection\SourceLocator\Type\Composer\Psr\Exception\InvalidPrefixMapping
+ */
+final class InvalidPrefixMappingTest extends TestCase
+{
+    public function testEmptyPrefixGiven() : void
+    {
+        self::assertSame(
+            'An invalid empty string provided as a PSR mapping prefix',
+            InvalidPrefixMapping::emptyPrefixGiven()
+                ->getMessage()
+        );
+    }
+
+    public function testPrefixMappingIsNotADirectory() : void
+    {
+        self::assertSame(
+            'Provided path "A\" for prefix "a/b" is not a directory',
+            InvalidPrefixMapping::prefixMappingIsNotADirectory('A\\', 'a/b')
+                ->getMessage()
+        );
+    }
+
+    public function testEmptyPrefixMappingGiven() : void
+    {
+        self::assertSame(
+            'An invalid empty list of paths was provided for PSR mapping prefix "A\"',
+            InvalidPrefixMapping::emptyPrefixMappingGiven('A\\')
+                ->getMessage()
+        );
+    }
+}
diff --git a/test/unit/SourceLocator/Type/Composer/Psr/Psr0MappingTest.php b/test/unit/SourceLocator/Type/Composer/Psr/Psr0MappingTest.php
new file mode 100644
index 000000000..35ae93eae
--- /dev/null
+++ b/test/unit/SourceLocator/Type/Composer/Psr/Psr0MappingTest.php
@@ -0,0 +1,159 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Roave\BetterReflectionTest\SourceLocator\Type\Composer\Psr;
+
+use PHPUnit\Framework\TestCase;
+use Roave\BetterReflection\Identifier\Identifier;
+use Roave\BetterReflection\Identifier\IdentifierType;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Psr\Exception\InvalidPrefixMapping;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Psr\Psr0Mapping;
+use function sys_get_temp_dir;
+use function tempnam;
+use function uniqid;
+
+/**
+ * @covers \Roave\BetterReflection\SourceLocator\Type\Composer\Psr\Psr0Mapping
+ */
+class Psr0MappingTest extends TestCase
+{
+    /**
+     * @param array<int, array<string, array<int, string>>> $mappings
+     * @param string[]                                      $expectedDirectories
+     *
+     * @dataProvider mappings
+     */
+    public function testExpectedDirectories(array $mappings, array $expectedDirectories) : void
+    {
+        self::assertEquals($expectedDirectories, Psr0Mapping::fromArrayMappings($mappings)->directories());
+    }
+
+    /**
+     * @param array<int, array<string, array<int, string>>> $mappings
+     *
+     * @dataProvider mappings
+     */
+    public function testIdempotentConstructor(array $mappings) : void
+    {
+        self::assertEquals(Psr0Mapping::fromArrayMappings($mappings), Psr0Mapping::fromArrayMappings($mappings));
+    }
+
+    /** @return array<string, array<int, array<string, array<int, string>>>> */
+    public function mappings() : array
+    {
+        return [
+            'one directory, one prefix'                  => [
+                ['foo' => [__DIR__]],
+                [__DIR__],
+            ],
+            'two directories, one prefix'                => [
+                ['foo' => [__DIR__, __DIR__ . '/../..']],
+                [__DIR__, __DIR__ . '/../..'],
+            ],
+            'two directories, one duplicate, one prefix' => [
+                ['foo' => [__DIR__, __DIR__, __DIR__ . '/../..']],
+                [__DIR__, __DIR__ . '/../..'],
+            ],
+            'two directories, two prefixes'              => [
+                [
+                    'foo' => [__DIR__],
+                    'bar' => [__DIR__ . '/../..'],
+                ],
+                [__DIR__, __DIR__ . '/../..'],
+            ],
+            'trailing slash in directory is trimmed'     => [
+                ['foo' => [__DIR__ . '/']],
+                [__DIR__],
+            ],
+        ];
+    }
+
+    /**
+     * @param array<int, array<string, array<int, string>>> $mappings
+     * @param string[]                                      $expectedFiles
+     *
+     * @dataProvider classLookupMappings
+     */
+    public function testClassLookups(array $mappings, Identifier $identifier, array $expectedFiles) : void
+    {
+        self::assertEquals(
+            $expectedFiles,
+            Psr0Mapping::fromArrayMappings($mappings)->resolvePossibleFilePaths($identifier)
+        );
+    }
+
+    /** @return array<string, array<int, array<string, array<int, string>>>> */
+    public function classLookupMappings() : array
+    {
+        return [
+            'empty mappings, no match'                                          => [
+                [],
+                new Identifier('Foo', new IdentifierType(IdentifierType::IDENTIFIER_CLASS)),
+                [],
+            ],
+            'one mapping, no match for function identifier'                     => [
+                ['Foo\\' => [__DIR__]],
+                new Identifier('Foo\\Bar', new IdentifierType(IdentifierType::IDENTIFIER_FUNCTION)),
+                [],
+            ],
+            'one mapping, match'                                                => [
+                ['Foo\\' => [__DIR__]],
+                new Identifier('Foo\\Bar', new IdentifierType(IdentifierType::IDENTIFIER_CLASS)),
+                [__DIR__ . '/Foo/Bar.php'],
+            ],
+            'one mapping, no match with underscore prefix and namespaced class' => [
+                ['Foo_' => [__DIR__]],
+                new Identifier('Foo\\Bar', new IdentifierType(IdentifierType::IDENTIFIER_CLASS)),
+                [],
+            ],
+            'one mapping, match with underscore replacement'                    => [
+                ['Foo_' => [__DIR__]],
+                new Identifier('Foo_Bar', new IdentifierType(IdentifierType::IDENTIFIER_CLASS)),
+                [__DIR__ . '/Foo/Bar.php'],
+            ],
+            'trailing and leading slash in mapping is trimmed'                  => [
+                ['Foo' => [__DIR__ . '/']],
+                new Identifier('Foo\\Bar', new IdentifierType(IdentifierType::IDENTIFIER_CLASS)),
+                [__DIR__ . '/Foo/Bar.php'],
+            ],
+            'one mapping, match if class === prefix'                            => [
+                ['Foo' => [__DIR__]],
+                new Identifier('Foo', new IdentifierType(IdentifierType::IDENTIFIER_CLASS)),
+                [__DIR__ . '/Foo.php'],
+            ],
+            'multiple mappings, match when class !== prefix'                    => [
+                [
+                    'Foo_Baz' => [__DIR__ . '/../..'],
+                    'Foo'     => [__DIR__ . '/..'],
+                ],
+                new Identifier('Foo_Bar', new IdentifierType(IdentifierType::IDENTIFIER_CLASS)),
+                [__DIR__ . '/../Foo/Bar.php'],
+            ],
+        ];
+    }
+
+    /**
+     * @param array<int, array<string, array<int, string>>> $invalidMappings
+     *
+     * @dataProvider invalidMappings
+     */
+    public function testRejectsInvalidMappings(array $invalidMappings) : void
+    {
+        $this->expectException(InvalidPrefixMapping::class);
+
+        Psr0Mapping::fromArrayMappings($invalidMappings);
+    }
+
+    /** @return array<string, array<int, array<string, array<int, string>>>> */
+    public function invalidMappings() : array
+    {
+        return [
+            'array contains empty prefixes'                            => [['' => 'bar']],
+            'array contains empty paths'                               => [['foo' => ['']]],
+            'array contains empty path list'                           => [['foo' => []]],
+            'array contains path pointing to a file'                   => [['foo' => [tempnam(sys_get_temp_dir(), 'non_existing')]]],
+            'array contains path pointing to a non-existing directory' => [['foo' => [sys_get_temp_dir() . '/' . uniqid('not_existing', true)]]],
+        ];
+    }
+}
diff --git a/test/unit/SourceLocator/Type/Composer/Psr/Psr4MappingTest.php b/test/unit/SourceLocator/Type/Composer/Psr/Psr4MappingTest.php
new file mode 100644
index 000000000..4975a41bb
--- /dev/null
+++ b/test/unit/SourceLocator/Type/Composer/Psr/Psr4MappingTest.php
@@ -0,0 +1,149 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Roave\BetterReflectionTest\SourceLocator\Type\Composer\Psr;
+
+use PHPUnit\Framework\TestCase;
+use Roave\BetterReflection\Identifier\Identifier;
+use Roave\BetterReflection\Identifier\IdentifierType;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Psr\Exception\InvalidPrefixMapping;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Psr\Psr4Mapping;
+use function sys_get_temp_dir;
+use function tempnam;
+use function uniqid;
+
+/**
+ * @covers \Roave\BetterReflection\SourceLocator\Type\Composer\Psr\Psr4Mapping
+ */
+class Psr4MappingTest extends TestCase
+{
+    /**
+     * @param array<int, array<string, array<int, string>>> $mappings
+     * @param string[]                                      $expectedDirectories
+     *
+     * @dataProvider mappings
+     */
+    public function testExpectedDirectories(array $mappings, array $expectedDirectories) : void
+    {
+        self::assertEquals($expectedDirectories, Psr4Mapping::fromArrayMappings($mappings)->directories());
+    }
+
+    /**
+     * @param array<int, array<string, array<int, string>>> $mappings
+     *
+     * @dataProvider mappings
+     */
+    public function testIdempotentConstructor(array $mappings) : void
+    {
+        self::assertEquals(Psr4Mapping::fromArrayMappings($mappings), Psr4Mapping::fromArrayMappings($mappings));
+    }
+
+    /** @return array<string, array<int, array<string, array<int, string>>|array<int, string>>> */
+    public function mappings() : array
+    {
+        return [
+            'one directory, one prefix'                  => [
+                ['foo' => [__DIR__]],
+                [__DIR__],
+            ],
+            'two directories, one prefix'                => [
+                ['foo' => [__DIR__, __DIR__ . '/../..']],
+                [__DIR__, __DIR__ . '/../..'],
+            ],
+            'two directories, one duplicate, one prefix' => [
+                ['foo' => [__DIR__, __DIR__, __DIR__ . '/../..']],
+                [__DIR__, __DIR__ . '/../..'],
+            ],
+            'two directories, two prefixes'              => [
+                [
+                    'foo' => [__DIR__],
+                    'bar' => [__DIR__ . '/../..'],
+                ],
+                [__DIR__, __DIR__ . '/../..'],
+            ],
+            'trailing slash in directory is trimmed'     => [
+                ['foo' => [__DIR__ . '/']],
+                [__DIR__],
+            ],
+        ];
+    }
+
+    /**
+     * @param array<int, array<string, array<int, string>>> $mappings
+     * @param string[]                                      $expectedFiles
+     *
+     * @dataProvider classLookupMappings
+     */
+    public function testClassLookups(array $mappings, Identifier $identifier, array $expectedFiles) : void
+    {
+        self::assertEquals(
+            $expectedFiles,
+            Psr4Mapping::fromArrayMappings($mappings)->resolvePossibleFilePaths($identifier)
+        );
+    }
+
+    /** @return array<string, array<int, array<string, array<int, string>>|array<int, string>>> */
+    public function classLookupMappings() : array
+    {
+        return [
+            'empty mappings, no match'                         => [
+                [],
+                new Identifier('Foo', new IdentifierType(IdentifierType::IDENTIFIER_CLASS)),
+                [],
+            ],
+            'one mapping, no match for function identifier'    => [
+                ['Foo\\' => [__DIR__]],
+                new Identifier('Foo\\Bar', new IdentifierType(IdentifierType::IDENTIFIER_FUNCTION)),
+                [],
+            ],
+            'one mapping, match'                               => [
+                ['Foo\\' => [__DIR__]],
+                new Identifier('Foo\\Bar', new IdentifierType(IdentifierType::IDENTIFIER_CLASS)),
+                [__DIR__ . '/Bar.php'],
+            ],
+            'trailing and leading slash in mapping is trimmed' => [
+                ['Foo' => [__DIR__ . '/']],
+                new Identifier('Foo\\Bar', new IdentifierType(IdentifierType::IDENTIFIER_CLASS)),
+                [__DIR__ . '/Bar.php'],
+            ],
+            'one mapping, no match if class === prefix'        => [
+                ['Foo' => [__DIR__]],
+                new Identifier('Foo', new IdentifierType(IdentifierType::IDENTIFIER_CLASS)),
+                [],
+            ],
+            'multiple mappings, match when class !== prefix'   => [
+                [
+                    'Foo\\Bar' => [__DIR__ . '/../..'],
+                    'Foo'      => [__DIR__ . '/..'],
+                ],
+                new Identifier('Foo\\Bar', new IdentifierType(IdentifierType::IDENTIFIER_CLASS)),
+                [__DIR__ . '/../Bar.php'],
+            ],
+        ];
+    }
+
+    /**
+     * @param array<int, array<string, array<int, string>>> $invalidMappings
+     *
+     * @dataProvider invalidMappings
+     */
+    public function testRejectsInvalidMappings(array $invalidMappings) : void
+    {
+        $this->expectException(InvalidPrefixMapping::class);
+
+        Psr4Mapping::fromArrayMappings($invalidMappings);
+    }
+
+    /** @return array<string, array<int, array<string, array<int, string>>>> */
+    public function invalidMappings() : array
+    {
+        return [
+            'array contains empty prefixes'                            => [['' => 'bar']],
+            'array contains empty paths'                               => [['foo' => ['']]],
+            'array contains empty path list'                           => [['foo' => []]],
+            'array contains path pointing to a file'                   => [['foo' => [tempnam(sys_get_temp_dir(), 'non_existing')]]],
+            'array contains path pointing to a non-existing directory' => [['foo' => [sys_get_temp_dir() . '/' . uniqid('not_existing', true)]]],
+        ];
+    }
+}
diff --git a/test/unit/SourceLocator/Type/Composer/PsrAutoloaderLocatorTest.php b/test/unit/SourceLocator/Type/Composer/PsrAutoloaderLocatorTest.php
new file mode 100644
index 000000000..0f17492d9
--- /dev/null
+++ b/test/unit/SourceLocator/Type/Composer/PsrAutoloaderLocatorTest.php
@@ -0,0 +1,167 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Roave\BetterReflectionTest\SourceLocator\Type\Composer;
+
+use PHPUnit\Framework\MockObject\MockObject;
+use PHPUnit\Framework\TestCase;
+use Roave\BetterReflection\Identifier\Identifier;
+use Roave\BetterReflection\Identifier\IdentifierType;
+use Roave\BetterReflection\Reflection\Reflection;
+use Roave\BetterReflection\Reflector\ClassReflector;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Psr\Psr4Mapping;
+use Roave\BetterReflection\SourceLocator\Type\Composer\Psr\PsrAutoloaderMapping;
+use Roave\BetterReflection\SourceLocator\Type\Composer\PsrAutoloaderLocator;
+use Roave\BetterReflectionTest\Assets\DirectoryScannerAssets\Bar\FooBar;
+use Roave\BetterReflectionTest\Assets\DirectoryScannerAssets\Foo;
+use Roave\BetterReflectionTest\Assets\DirectoryScannerAssetsFoo\Bar\FooBar as FooBar1;
+use Roave\BetterReflectionTest\Assets\DirectoryScannerAssetsFoo\Foo as Foo1;
+use Roave\BetterReflectionTest\BetterReflectionSingleton;
+use function array_map;
+use function sort;
+
+/**
+ * @covers \Roave\BetterReflection\SourceLocator\Type\Composer\PsrAutoloaderLocator
+ */
+class PsrAutoloaderLocatorTest extends TestCase
+{
+    /** @var PsrAutoloaderMapping&MockObject */
+    private $psrMapping;
+
+    /** @var ClassReflector */
+    private $reflector;
+
+    /** @var PsrAutoloaderLocator */
+    private $psrLocator;
+
+    protected function setUp() : void
+    {
+        parent::setUp();
+
+        $astLocator = BetterReflectionSingleton::instance()
+            ->astLocator();
+
+        $this->psrMapping = $this->createMock(PsrAutoloaderMapping::class);
+        $this->psrLocator = new PsrAutoloaderLocator(
+            $this->psrMapping,
+            BetterReflectionSingleton::instance()
+                ->astLocator()
+        );
+        $this->reflector  = new ClassReflector($this->psrLocator);
+        $this
+            ->psrMapping
+            ->method('directories')
+            ->willReturn([
+                __DIR__ . '/../../../Assets/DirectoryScannerAssets',
+                __DIR__ . '/../../../Assets/DirectoryScannerAssetsFoo',
+            ]);
+
+        $this
+            ->psrMapping
+            ->method('resolvePossibleFilePaths')
+            ->willReturnCallback(static function (Identifier $identifier) : array {
+                if ($identifier->getName() === Foo::class) {
+                    return [__DIR__ . '/../../../Assets/DirectoryScannerAssets/Foo.php'];
+                }
+
+                if ($identifier->getName() === (Foo::class . 'potato')) {
+                    return [__DIR__ . '/../../../Assets/DirectoryScannerAssets/Foopotato.php'];
+                }
+
+                if ($identifier->getName() === 'Roave\\BetterReflectionTest\\Assets\\DirectoryScannerAssets\\Bar\\Empty') {
+                    return [__DIR__ . '/../../../Assets/DirectoryScannerAssets/Bar/Empty.php'];
+                }
+
+                return [];
+            });
+    }
+
+    public function testWillLocateExistingFileWithMatchingClass() : void
+    {
+        $located = $this->psrLocator->locateIdentifier(
+            $this->reflector,
+            new Identifier(
+                Foo::class,
+                new IdentifierType(IdentifierType::IDENTIFIER_CLASS)
+            )
+        );
+
+        self::assertNotNull($located);
+        self::assertSame(Foo::class, $located->getName());
+    }
+
+    public function testWillNotLocateNonExistingFileWithMatchingPsr4Class() : void
+    {
+        self::assertNull($this->psrLocator->locateIdentifier(
+            $this->reflector,
+            new Identifier(
+                Foo::class . 'potato',
+                new IdentifierType(IdentifierType::IDENTIFIER_CLASS)
+            )
+        ));
+    }
+
+    public function testWillNotLocateExistingFileWithMatchingPsr4ClassAndNoContents() : void
+    {
+        self::assertNull($this->psrLocator->locateIdentifier(
+            $this->reflector,
+            new Identifier(
+                'Roave\\BetterReflectionTest\\Assets\\DirectoryScannerAssets\\Bar\\Empty',
+                new IdentifierType(IdentifierType::IDENTIFIER_CLASS)
+            )
+        ));
+    }
+
+    public function testWillNotLocateClassNotMatchingPsr4Mappings() : void
+    {
+        self::assertNull($this->psrLocator->locateIdentifier(
+            $this->reflector,
+            new Identifier(
+                'Blah',
+                new IdentifierType(IdentifierType::IDENTIFIER_CLASS)
+            )
+        ));
+    }
+
+    public function testWillLocateAllClassesInMappedPsr4Paths() : void
+    {
+        $astLocator = BetterReflectionSingleton::instance()
+            ->astLocator();
+
+        $locator = new PsrAutoloaderLocator(
+            Psr4Mapping::fromArrayMappings([
+                'Roave\\BetterReflectionTest\\Assets\\DirectoryScannerAssets\\' => [
+                    __DIR__ . '/../../../Assets/DirectoryScannerAssets',
+                ],
+                'Roave\\BetterReflectionTest\\Assets\\DirectoryScannerAssetsFoo\\' => [
+                    __DIR__ . '/../../../Assets/DirectoryScannerAssetsFoo',
+                ],
+            ]),
+            $astLocator
+        );
+
+        $expected = [
+            FooBar::class,
+            Foo::class,
+            FooBar1::class,
+            Foo1::class,
+        ];
+
+        $actual = array_map(
+            static function (Reflection $reflection) : string {
+                return $reflection->getName();
+            },
+            $locator->locateIdentifiersByType(
+                new ClassReflector($locator),
+                new IdentifierType(IdentifierType::IDENTIFIER_CLASS)
+            )
+        );
+
+        // Sorting may depend on filesystem here
+        sort($expected);
+        sort($actual);
+
+        self::assertSame($expected, $actual);
+    }
+}