api/src/main/java/jakarta/el/MapELResolver.java

/*
 * Copyright (c) 1997, 2022 Oracle and/or its affiliates and others.
 * All rights reserved.
 * Copyright 2004 The Apache Software Foundation
 *
 * Licensed 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.Collections;
import java.util.HashMap;
import java.util.Map;

/**
 * Defines property resolution behavior on instances of {@link java.util.Map}.
 *
 * <p>
 * This resolver handles base objects of type <code>java.util.Map</code>. It accepts any object as a property and uses
 * that object as a key in the map. The resulting value is the value in the map that is associated with that key.
 * </p>
 *
 * <p>
 * This resolver can be constructed in read-only mode, which means that {@link #isReadOnly} will always return
 * <code>true</code> and {@link #setValue} will always throw <code>PropertyNotWritableException</code>.
 * </p>
 *
 * <p>
 * <code>ELResolver</code>s are combined together using {@link CompositeELResolver}s, to define rich semantics for
 * evaluating an expression. See the javadocs for {@link ELResolver} for details.
 * </p>
 *
 * @see CompositeELResolver
 * @see ELResolver
 * @see java.util.Map
 * @since Jakarta Server Pages 2.1
 */
public class MapELResolver extends ELResolver {

    static private Class<?> theUnmodifiableMapClass = Collections.unmodifiableMap(new HashMap<>()).getClass();
    private boolean isReadOnly;

    /**
     * Creates a new read/write <code>MapELResolver</code>.
     */
    public MapELResolver() {
        isReadOnly = false;
    }

    /**
     * Creates a new <code>MapELResolver</code> whose read-only status is determined by the given parameter.
     *
     * @param isReadOnly <code>true</code> if this resolver cannot modify maps; <code>false</code> otherwise.
     */
    public MapELResolver(boolean isReadOnly) {
        this.isReadOnly = isReadOnly;
    }

    /**
     * If the base object is a map, returns the most general acceptable type for a value in this map.
     *
     * <p>
     * If the base is a <code>Map</code>, the <code>propertyResolved</code> property of the <code>ELContext</code> object
     * must be set to <code>true</code> by this resolver, before returning. If this property is not <code>true</code> after
     * this method is called, the caller should ignore the return value.
     * </p>
     *
     * <p>
     * Assuming the base is a <code>Map</code>, this method will always return <code>Object.class</code> unless the
     * resolver is constructed in read-only mode in which case {@code null} will be returned. This is because
     * <code>Map</code>s accept any object as the value for a given key.
     * </p>
     *
     * @param context The context of this evaluation.
     * @param base The map to analyze. Only bases of type <code>Map</code> are handled by this resolver.
     * @param property The key to return the acceptable type for. Ignored by this resolver.
     * @return If the <code>propertyResolved</code> property of <code>ELContext</code> was set to <code>true</code>, then
     * the most general acceptable type which must be {@code null} if the either the property or the resolver is
     * read-only; otherwise undefined
     * @throws NullPointerException if context is <code>null</code>
     * @throws ELException if an exception was thrown while performing the property or variable resolution. The thrown
     * exception must be included as the cause property of this exception, if available.
     */
    @Override
    public Class<?> getType(ELContext context, Object base, Object property) {
        if (context == null) {
            throw new NullPointerException();
        }

        if (base != null && base instanceof Map) {
            context.setPropertyResolved(true);
            
            Map<?, ?> map = (Map<?, ?>) base;
            if (isReadOnly || map.getClass() == theUnmodifiableMapClass) {
                return null;
            }

            return Object.class;
        }

        return null;
    }

    /**
     * If the base object is a map, returns the value associated with the given key, as specified by the
     * <code>property</code> argument. If the key was not found, <code>null</code> is returned.
     *
     * <p>
     * If the base is a <code>Map</code>, the <code>propertyResolved</code> property of the <code>ELContext</code> object
     * must be set to <code>true</code> by this resolver, before returning. If this property is not <code>true</code> after
     * this method is called, the caller should ignore the return value.
     * </p>
     *
     * <p>
     * Just as in {@link java.util.Map#get}, just because <code>null</code> is returned doesn't mean there is no mapping for
     * the key; it's also possible that the <code>Map</code> explicitly maps the key to <code>null</code>.
     * </p>
     *
     * @param context The context of this evaluation.
     * @param base The map to be analyzed. Only bases of type <code>Map</code> are handled by this resolver.
     * @param property The key whose associated value is to be returned.
     * @return If the <code>propertyResolved</code> property of <code>ELContext</code> was set to <code>true</code>, then
     * the value associated with the given key or <code>null</code> if the key was not found. Otherwise, undefined.
     * @throws ClassCastException if the key is of an inappropriate type for this map (optionally thrown by the underlying
     * <code>Map</code>).
     * @throws NullPointerException if context is <code>null</code>, or if the key is null and this map does not permit null
     * keys (the latter is optionally thrown by the underlying <code>Map</code>).
     * @throws ELException if an exception was thrown while performing the property or variable resolution. The thrown
     * exception must be included as the cause property of this exception, if available.
     */
    @Override
    public Object getValue(ELContext context, Object base, Object property) {
        if (context == null) {
            throw new NullPointerException();
        }

        if (base != null && base instanceof Map) {
            context.setPropertyResolved(base, property);
            Map<?, ?> map = (Map<?, ?>) base;
            return map.get(property);
        }

        return null;
    }

    /**
     * If the base object is a map, attempts to set the value associated with the given key, as specified by the
     * <code>property</code> argument.
     *
     * <p>
     * If the base is a <code>Map</code>, the <code>propertyResolved</code> property of the <code>ELContext</code> object
     * must be set to <code>true</code> by this resolver, before returning. If this property is not <code>true</code> after
     * this method is called, the caller can safely assume no value was set.
     * </p>
     *
     * <p>
     * If this resolver was constructed in read-only mode, this method will always throw
     * <code>PropertyNotWritableException</code>.
     * </p>
     *
     * <p>
     * If a <code>Map</code> was created using {@link java.util.Collections#unmodifiableMap}, this method must throw
     * <code>PropertyNotWritableException</code>. Unfortunately, there is no Collections API method to detect this. However,
     * an implementation can create a prototype unmodifiable <code>Map</code> and query its runtime type to see if it
     * matches the runtime type of the base object as a workaround.
     * </p>
     *
     * @param context The context of this evaluation.
     * @param base The map to be modified. Only bases of type <code>Map</code> are handled by this resolver.
     * @param property The key with which the specified value is to be associated.
     * @param val The value to be associated with the specified key.
     * @throws ClassCastException if the class of the specified key or value prevents it from being stored in this map.
     * @throws NullPointerException if context is <code>null</code>, or if this map does not permit <code>null</code> keys
     * or values, and the specified key or value is <code>null</code>.
     * @throws IllegalArgumentException if some aspect of this key or value prevents it from being stored in this map.
     * @throws ELException if an exception was thrown while performing the property or variable resolution. The thrown
     * exception must be included as the cause property of this exception, if available.
     * @throws PropertyNotWritableException if this resolver was constructed in read-only mode, or if the put operation is
     * not supported by the underlying map.
     */
    @Override
    public void setValue(ELContext context, Object base, Object property, Object val) {
        if (context == null) {
            throw new NullPointerException();
        }

        if (base != null && base instanceof Map) {
            context.setPropertyResolved(base, property);

            // The cast is safe
            @SuppressWarnings("unchecked")
            Map<Object, Object> map = (Map<Object, Object>) base;
            if (isReadOnly || map.getClass() == theUnmodifiableMapClass) {
                throw new PropertyNotWritableException();
            }

            try {
                map.put(property, val);
            } catch (UnsupportedOperationException ex) {
                throw new PropertyNotWritableException();
            }
        }
    }

    /**
     * If the base object is a map, returns whether a call to {@link #setValue} will always fail.
     *
     * <p>
     * If the base is a <code>Map</code>, the <code>propertyResolved</code> property of the <code>ELContext</code> object
     * must be set to <code>true</code> by this resolver, before returning. If this property is not <code>true</code> after
     * this method is called, the caller should ignore the return value.
     * </p>
     *
     * <p>
     * If this resolver was constructed in read-only mode, this method will always return <code>true</code>.
     * </p>
     *
     * <p>
     * If a <code>Map</code> was created using {@link java.util.Collections#unmodifiableMap}, this method must return
     * <code>true</code>. Unfortunately, there is no Collections API method to detect this. However, an implementation can
     * create a prototype unmodifiable <code>Map</code> and query its runtime type to see if it matches the runtime type of
     * the base object as a workaround.
     * </p>
     *
     * @param context The context of this evaluation.
     * @param base The map to analyze. Only bases of type <code>Map</code> are handled by this resolver.
     * @param property The key to return the read-only status for. Ignored by this resolver.
     * @return If the <code>propertyResolved</code> property of <code>ELContext</code> was set to <code>true</code>, then
     * <code>true</code> if calling the <code>setValue</code> method will always fail or <code>false</code> if it is
     * possible that such a call may succeed; otherwise undefined.
     * @throws NullPointerException if context is <code>null</code>
     * @throws ELException if an exception was thrown while performing the property or variable resolution. The thrown
     * exception must be included as the cause property of this exception, if available.
     */
    @Override
    public boolean isReadOnly(ELContext context, Object base, Object property) {
        if (context == null) {
            throw new NullPointerException();
        }

        if (base != null && base instanceof Map) {
            context.setPropertyResolved(true);
            Map<?, ?> map = (Map<?, ?>) base;
            return isReadOnly || map.getClass() == theUnmodifiableMapClass;
        }

        return false;
    }

    /**
     * If the base object is a map, returns the most general type that this resolver accepts for the <code>property</code>
     * argument. Otherwise, returns <code>null</code>.
     *
     * <p>
     * Assuming the base is a <code>Map</code>, this method will always return <code>Object.class</code>. This is because
     * <code>Map</code>s accept any object as a key.
     * </p>
     *
     * @param context The context of this evaluation.
     * @param base The map to analyze. Only bases of type <code>Map</code> are handled by this resolver.
     * @return <code>null</code> if base is not a <code>Map</code>; otherwise <code>Object.class</code>.
     */
    @Override
    public Class<?> getCommonPropertyType(ELContext context, Object base) {
        if (base != null && base instanceof Map) {
            return Object.class;
        }

        return null;
    }

}