compositeelresolver.java

java属性邦定的（JSR-295）的一个实现

/*
 * Copyright (C) 2007 Sun Microsystems, Inc. All rights reserved. Use is
 * subject to license terms.
 */

package org.jdesktop.el;

import java.util.ArrayList;
import java.util.Iterator;
import java.beans.FeatureDescriptor;

/**
 * Maintains an ordered composite list of child <code>ELResolver</code>s.
 *
 * <p>Though only a single <code>ELResolver</code> is associated with an
 * <code>ELContext</code>, there are usually multiple resolvers considered
 * for any given variable or property resolution. <code>ELResolver</code>s
 * are combined together using a <code>CompositeELResolver</code>, to define
 * rich semantics for evaluating an expression.</p>
 *
 * <p>For the {@link #getValue}, {@link #getType}, {@link #setValue} and
 * {@link #isReadOnly} methods, an <code>ELResolver</code> is not
 * responsible for resolving all possible (base, property) pairs. In fact,
 * most resolvers will only handle a <code>base</code> of a single type.
 * To indicate that a resolver has successfully resolved a particular
 * (base, property) pair, it must set the <code>propertyResolved</code>
 * property of the <code>ELContext</code> to <code>true</code>. If it could 
 * not handle the given pair, it must leave this property alone. The caller
 * must ignore the return value of the method if <code>propertyResolved</code>
 * is <code>false</code>.</p>
 *
 * <p>The <code>CompositeELResolver</code> initializes the
 * <code>ELContext.propertyResolved</code> flag to <code>false</code>, and uses 
 * it as a stop condition for iterating through its component resolvers.</p>
 *
 * <p>The <code>ELContext.propertyResolved</code> flag is not used for the 
 * design-time methods {@link #getFeatureDescriptors} and
 * {@link #getCommonPropertyType}. Instead, results are collected and 
 * combined from all child <code>ELResolver</code>s for these methods.</p>
 *
 * @see ELContext
 * @see ELResolver
 * @since JSP 2.1
 */
public class CompositeELResolver extends ELResolver {

    /**
     * Adds the given resolver to the list of component resolvers.
     *
     * <p>Resolvers are consulted in the order in which they are added.</p>
     *
     * @param elResolver The component resolver to add.
     * @throws NullPointerException If the provided resolver is
     *     <code>null</code>.
     */
    public void add(ELResolver elResolver) {

        if (elResolver == null) {
            throw new NullPointerException();
        }
                                                                                
        elResolvers.add(elResolver);
    }

    /**
     * Attempts to resolve the given <code>property</code> object on the given
     * <code>base</code> object by querying all component resolvers.
     *
     * <p>If this resolver handles the given (base, property) pair, 
     * the <code>propertyResolved</code> property of the 
     * <code>ELContext</code> object must be set to <code>true</code>
     * by the 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>First, <code>propertyResolved</code> is set to <code>false</code> on
     * the provided <code>ELContext</code>.</p>
     *
     * <p>Next, for each component resolver in this composite:
     * <ol>
     *   <li>The <code>getValue()</code> method is called, passing in
     *       the provided <code>context</code>, <code>base</code> and 
     *       <code>property</code>.</li>
     *   <li>If the <code>ELContext</code>'s <code>propertyResolved</code>
     *       flag is <code>false</code> then iteration continues.</li>
     *   <li>Otherwise, iteration stops and no more component resolvers are
     *       considered. The value returned by <code>getValue()</code> is
     *       returned by this method.</li>
     * </ol></p>
     *
     * <p>If none of the component resolvers were able to perform this
     * operation, the value <code>null</code> is returned and the
     * <code>propertyResolved</code> flag remains set to 
     * <code>false</code></p>.
     *
     * <p>Any exception thrown by component resolvers during the iteration
     * is propagated to the caller of this method.</p>
     *
     * @param context The context of this evaluation.
     * @param base The base object whose property value is to be returned,
     *     or <code>null</code> to resolve a top-level variable.
     * @param property The property or variable to be resolved.
     * @return If the <code>propertyResolved</code> property of 
     *     <code>ELContext</code> was set to <code>true</code>, then
     *     the result of the variable or property resolution; otherwise
     *     undefined.
     * @throws NullPointerException if context is <code>null</code>
     * @throws PropertyNotFoundException if the given (base, property) pair
     *     is handled by this <code>ELResolver</code> but the specified
     *     variable or property does not exist or is not readable.
     * @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.
     */
    public Object getValue(ELContext context,
                           Object base,
                           Object property) {
        context.setPropertyResolved(false);
        int i = 0, len = this.elResolvers.size();
        ELResolver elResolver;
        Object value; 
        while (i < len) {
            elResolver = this.elResolvers.get(i);
            value = elResolver.getValue(context, base, property);
            if (context.isPropertyResolved()) {
                return value;
            }
            i++;
        } 
        return ELContext.UNRESOLVABLE_RESULT;
    }

    /**
     * For a given <code>base</code> and <code>property</code>, attempts to
     * identify the most general type that is acceptable for an object to be 
     * passed as the <code>value</code> parameter in a future call 
     * to the {@link #setValue} method. The result is obtained by 
     * querying all component resolvers.
     *
     * <p>If this resolver handles the given (base, property) pair, 
     * the <code>propertyResolved</code> property of the 
     * <code>ELContext</code> object must be set to <code>true</code>
     * by the 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>First, <code>propertyResolved</code> is set to <code>false</code> on
     * the provided <code>ELContext</code>.</p>
     *
     * <p>Next, for each component resolver in this composite:
     * <ol>
     *   <li>The <code>getType()</code> method is called, passing in
     *       the provided <code>context</code>, <code>base</code> and 
     *       <code>property</code>.</li>
     *   <li>If the <code>ELContext</code>'s <code>propertyResolved</code>
     *       flag is <code>false</code> then iteration continues.</li>
     *   <li>Otherwise, iteration stops and no more component resolvers are
     *       considered. The value returned by <code>getType()</code> is
     *       returned by this method.</li>
     * </ol></p>
     *
     * <p>If none of the component resolvers were able to perform this
     * operation, the value <code>null</code> is returned and the
     * <code>propertyResolved</code> flag remains set to 
     * <code>false</code></p>.
     *
     * <p>Any exception thrown by component resolvers during the iteration
     * is propagated to the caller of this method.</p>
     *
     * @param context The context of this evaluation.
     * @param base The base object whose property value is to be analyzed,
     *     or <code>null</code> to analyze a top-level variable.
     * @param property The property or variable to return the acceptable 
     *     type for.
     * @return If the <code>propertyResolved</code> property of 
     *     <code>ELContext</code> was set to <code>true</code>, then
     *     the most general acceptable type; otherwise undefined.
     * @throws NullPointerException if context is <code>null</code>
     * @throws PropertyNotFoundException if the given (base, property) pair
     *     is handled by this <code>ELResolver</code> but the specified
     *     variable or property does not exist or is not readable.
     * @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.
     */
    public Class<?> getType(ELContext context,
                         Object base,
                         Object property) {
        context.setPropertyResolved(false);
        int i = 0, len = this.elResolvers.size();
        ELResolver elResolver;
        Class<?> type;  
        while (i < len) {
            elResolver = this.elResolvers.get(i);
            type = elResolver.getType(context, base, property);
            if (context.isPropertyResolved()) {
                return type;
            }
            i++;
        }
        return null;
    }

    /**
     * Attempts to set the value of the given <code>property</code> 
     * object on the given <code>base</code> object. All component
     * resolvers are asked to attempt to set the value.
     *
     * <p>If this resolver handles the given (base, property) pair, 
     * the <code>propertyResolved</code> property of the 
     * <code>ELContext</code> object must be set to <code>true</code>
     * by the resolver, before returning. If this property is not 
     * <code>true</code> after this method is called, the caller can
     * safely assume no value has been set.</p>
     *
     * <p>First, <code>propertyResolved</code> is set to <code>false</code> on
     * the provided <code>ELContext</code>.</p>
     *
     * <p>Next, for each component resolver in this composite:
     * <ol>
     *   <li>The <code>setValue()</code> method is called, passing in
     *       the provided <code>context</code>, <code>base</code>, 
     *       <code>property</code> and <code>value</code>.</li>
     *   <li>If the <code>ELContext</code>'s <code>propertyResolved</code>
     *       flag is <code>false</code> then iteration continues.</li>
     *   <li>Otherwise, iteration stops and no more component resolvers are
     *       considered.</li>
     * </ol></p>
     *
     * <p>If none of the component resolvers were able to perform this
     * operation, the <code>propertyResolved</code> flag remains set to 
     * <code>false</code></p>.
     *
     * <p>Any exception thrown by component resolvers during the iteration
     * is propagated to the caller of this method.</p>
     *