Implement java.util.Optional support for the EL 6.0 API

See also:
jakartaee/expression-language#176

java/jakarta/el/OptionalELResolver.java

@@ -0,0 +1,182 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package jakarta.el;
+
+import java.util.Objects;
+import java.util.Optional;
+
+/**
+ * Defines property resolution behaviour on {@link Optional}s.
+ *
+ * <p>
+ * This resolver handles base objects that are instances of {@link Optional}.
+ *
+ * <p>
+ * If the {@link Optional#isEmpty()} is {@code true} for the base object and the property is {@code null} then the
+ * resulting value is {@code null}.
+ *
+ * <p>
+ * If the {@link Optional#isEmpty()} is {@code true} for the base object and the property is not {@code null} then the
+ * resulting value is the base object (an empty {@link Optional}).
+ *
+ * <p>
+ * If the {@link Optional#isPresent()} is {@code true} for the base object and the property is {@code null} then the
+ * resulting value is the result of calling {@link Optional#get()} on the base object.
+ *
+ * <p>
+ * If the {@link Optional#isPresent()} is {@code true} for the base object and the property is not {@code null} then the
+ * resulting value is the result of calling {@link ELResolver#getValue(ELContext, Object, Object)} using the
+ * {@link ELResolver} obtained from {@link ELContext#getELResolver()} with the following parameters:
+ * <ul>
+ * <li>The {@link ELContext} is the current context</li>
+ * <li>The base object is the result of calling {@link Optional#get()} on the current base object<li>
+ * <li>The property object is the current property object</li>
+ * </ul>
+ *
+ * <p>
+ * This resolver is always a read-only resolver.
+ */
+public class OptionalELResolver extends ELResolver {
+
+    @Override
+    public Object getValue(ELContext context, Object base, Object property) {
+        Objects.requireNonNull(context);
+
+        if (base instanceof Optional) {
+            context.setPropertyResolved(base, property);
+            if (((Optional<?>) base).isEmpty()) {
+                if (property == null) {
+                    return null;
+                } else {
+                    return base;
+                }
+            } else {
+                if (property == null) {
+                    return ((Optional<?>) base).get();
+                } else {
+                    Object resolvedBase = ((Optional<?>) base).get();
+                    return context.getELResolver().getValue(context, resolvedBase, property);
+                }
+            }
+        }
+
+        return null;
+    }
+
+    /**
+     * {@inheritDoc}
+     *
+     * <p>
+     * If the base object is an {@link Optional} this method always returns {@code null} since instances of this
+     * resolver are always read-only.
+     */
+    @Override
+    public Class<?> getType(ELContext context, Object base, Object property) {
+        Objects.requireNonNull(context);
+
+        if (base instanceof Optional) {
+            context.setPropertyResolved(base, property);
+        }
+
+        return null;
+    }
+
+    /**
+     * {@inheritDoc}
+     *
+     * <p>
+     * If the base object is an {@link Optional} this method always throws a {@link PropertyNotWritableException} since
+     * instances of this resolver are always read-only.
+     */
+    @Override
+    public void setValue(ELContext context, Object base, Object property, Object value) {
+        Objects.requireNonNull(context);
+
+        if (base instanceof Optional) {
+            throw new PropertyNotWritableException(
+                    Util.message(context, "resolverNotWritable", base.getClass().getName()));
+        }
+    }
+
+    /**
+     * {@inheritDoc}
+     *
+     * <p>
+     * If the base object is an {@link Optional} this method always returns {@code true} since instances of this
+     * resolver are always read-only.
+     */
+    @Override
+    public boolean isReadOnly(ELContext context, Object base, Object property) {
+        Objects.requireNonNull(context);
+
+        if (base instanceof Optional) {
+            context.setPropertyResolved(base, property);
+            return true;
+        }
+
+        return false;
+    }
+
+    /**
+     * {@inheritDoc}
+     *
+     * <p>
+     * If the base object is an {@link Optional} this method always returns {@code Object.class}.
+     */
+    @Override
+    public Class<?> getCommonPropertyType(ELContext context, Object base) {
+        if (base instanceof Optional) {
+            return Object.class;
+        }
+
+        return null;
+    }
+
+    @Override
+    public <T> T convertToType(ELContext context, Object obj, Class<T> type) {
+        Objects.requireNonNull(context);
+        if (obj instanceof Optional) {
+            if (((Optional<?>) obj).isPresent()) {
+                Object value = ((Optional<?>) obj).get();
+                // If the value is assignable to the required type, do so.
+                if (type.isAssignableFrom(value.getClass())) {
+                    context.setPropertyResolved(true);
+                    @SuppressWarnings("unchecked")
+                    T result = (T) value;
+                    return result;
+                }
+
+                try {
+                    Object convertedValue = context.convertToType(value, type);
+                    context.setPropertyResolved(true);
+                    @SuppressWarnings("unchecked")
+                    T result = (T) convertedValue;
+                    return result;
+                } catch (ELException e) {
+                    /*
+                     *  TODO: This isn't pretty but it works. Significant refactoring would be required to avoid the
+                     *  exception. See also OptionalELResolver.convertToType().
+                     */
+                }
+            } else {
+                context.setPropertyResolved(true);
+                return null;
+            }
+        }
+        return null;
+    }
+}

java/jakarta/el/Util.java

@@ -493,8 +493,10 @@ static boolean isAssignableFrom(Class<?> src, Class<?> target) {
      * This method duplicates code in org.apache.el.util.ReflectionUtil. When making changes keep the code in sync.
      */
     private static boolean isCoercibleFrom(ELContext context, Object src, Class<?> target) {
-        // TODO: This isn't pretty but it works. Significant refactoring would
-        // be required to avoid the exception.
+        /*
+         *  TODO: This isn't pretty but it works. Significant refactoring would be required to avoid the exception. See
+         *        also OptionalELResolver.convertToType().
+         */
         try {
             context.convertToType(src, target);
         } catch (ELException e) {