Post-process classes from 1.6 to 1.4 via antrun

I don't want to make a Maven Mojo out of it, because it will also need
to work with Gradle. So a standalone Java tool is the right way to go.

buildhelper/Java6to4.java

@@ -0,0 +1,53 @@
+import java.io.ByteArrayOutputStream;
+import java.io.File;
+import java.io.FileInputStream;
+import java.io.FileOutputStream;
+import java.io.FilenameFilter;
+import java.io.IOException;
+import java.io.InputStream;
+
+public class Java6to4 {
+
+    private static byte[] transform(InputStream classfile, ByteArrayOutputStream buffer) throws IOException {
+        buffer.reset();
+        int nRead;
+        byte[] data = new byte[1024];
+        while ((nRead = classfile.read(data, 0, data.length)) != -1) {
+            buffer.write(data, 0, nRead);
+        }
+        byte[] arr = buffer.toByteArray();
+        // patch byte 8
+        arr[7] = 48; // <- Java 1.4
+        return arr;
+    }
+
+    private static void walk(File root, ByteArrayOutputStream buffer) throws IOException {
+        File[] list = root.listFiles(new FilenameFilter() {
+            public boolean accept(File dir, String name) {
+                File f = new File(dir, name);
+                return f.isDirectory() || name.endsWith(".class");
+            }
+        });
+        if (list == null)
+            return;
+        for (int i = 0; i < list.length; i++) {
+            File f = list[i];
+            if (f.isDirectory()) {
+                walk(f, buffer);
+            } else {
+                FileInputStream fis = new FileInputStream(f);
+                byte[] transformed = transform(fis, buffer);
+                fis.close();
+                FileOutputStream fos = new FileOutputStream(f);
+                fos.write(transformed);
+                fos.close();
+            }
+        }
+    }
+
+    public static void main(String[] args) throws IOException {
+        ByteArrayOutputStream buffer = new ByteArrayOutputStream();
+        walk(new File(args[0]), buffer);
+    }
+
+}

pom.xml

@@ -68,6 +68,71 @@
 				<maven.compiler.source>1.6</maven.compiler.source>
 				<maven.compiler.target>1.6</maven.compiler.target>
 			</properties>
+			<build>
+				<plugins>
+					<!--
+						Make sure the jdk9 profile is only used with a
+						1.9 JDK, and not forcefully enabled via -Pjdk9, because
+						antrun will not work on earlier JDKs, because of missing
+						tools.jar.
+					-->
+					<plugin>
+						<artifactId>maven-enforcer-plugin</artifactId>
+						<version>1.4.1</version>
+						<executions>
+							<execution>
+								<id>enforce-versions</id>
+								<goals>
+									<goal>enforce</goal>
+								</goals>
+								<configuration>
+									<rules>
+										<requireJavaVersion>
+											<version>1.9</version>
+										</requireJavaVersion>
+									</rules>
+								</configuration>
+							</execution>
+						</executions>
+					</plugin>
+					<!--
+						Use the antrun plugin to build and run a post-processing
+						Java application to rewrite the Java class file
+						version from 1.6 back to 1.4 in all compiled class files.
+					-->
+					<plugin>
+						<artifactId>maven-antrun-plugin</artifactId>
+						<version>1.8</version>
+						<executions>
+							<execution>
+								<phase>process-classes</phase>
+								<configuration>
+									<target>
+										<echo message="Post-process compiled class files..."></echo>
+										<mkdir dir="${project.build.directory}/buildhelper" />
+										<javac
+											srcdir="buildhelper"
+											destdir="${project.build.directory}/buildhelper"
+											includeantruntime="false">
+											<classpath refid="maven.plugin.classpath" />
+										</javac>
+										<java classname="Java6to4" classpathref="maven.plugin.classpath">
+											<classpath>
+												<pathelement location="${project.build.directory}/buildhelper" />
+											</classpath>
+											<arg value="${project.build.outputDirectory}" />
+										</java>
+										<echo message="Post-processing done."></echo>
+									</target>
+								</configuration>
+								<goals>
+									<goal>run</goal>
+								</goals>
+							</execution>
+						</executions>
+					</plugin>
+				</plugins>
+			</build>
 		</profile>
 		<profile>
 			<!--