listelresolver.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.List;
import java.util.Iterator;
import java.util.Collections;
import java.util.ArrayList;
import java.beans.FeatureDescriptor;


/**
 * Defines property resolution behavior on instances of {@link java.util.List}.
 *
 * <p>This resolver handles base objects of type <code>java.util.List</code>.
 * It accepts any object as a property and coerces that object into an
 * integer index into the list. The resulting value is the value in the list
 * at that index.</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.List
 * @since JSP 2.1
 */
public class ListELResolver extends ELResolver {

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

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

    /**
     * If the base object is a list, returns the most general acceptable type 
     * for a value in this list.
     *
     * <p>If the base is a <code>List</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>List</code>, this method will always
     * return <code>Object.class</code>. This is because <code>List</code>s
     * accept any object as an element.</p>
     *
     * @param context The context of this evaluation.
     * @param base The list to analyze. Only bases of type <code>List</code>
     *     are handled by this resolver.
     * @param property The index of the element in the list to return the 
     *     acceptable type for. Will be coerced into an integer, but 
     *     otherwise 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; otherwise undefined.
     * @throws PropertyNotFoundException if the given index is out of 
     *     bounds for this list.
     * @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.
     */
    public Class<?> getType(ELContext context,
                         Object base,
                         Object property) {

        if (context == null) {
            throw new NullPointerException();
        }

        if (base != null && base instanceof List) {
            context.setPropertyResolved(true);
            List list = (List) base;
            int index = toInteger(property);
            if (index < 0 || index >= list.size()) {
                throw new PropertyNotFoundException();
            } 
            return Object.class;
        }
        return null;
    }

    /**
     * If the base object is a list, returns the value at the given index.
     * The index is specified by the <code>property</code> argument, and
     * coerced into an integer. If the coercion could not be performed,
     * an <code>IllegalArgumentException</code> is thrown. If the index is
     * out of bounds, <code>null</code> is returned.
     *
     * <p>If the base is a <code>List</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>
     *
     * @param context The context of this evaluation.
     * @param base The list to be analyzed. Only bases of type 
     *     <code>List</code> are handled by this resolver.
     * @param property The index of the value to be returned. Will be coerced
     *     into an integer.
     * @return If the <code>propertyResolved</code> property of 
     *     <code>ELContext</code> was set to <code>true</code>, then
     *     the value at the given index or <code>null</code>
     *     if the index was out of bounds. Otherwise, undefined.
     * @throws IllegalArgumentException if the property could not be coerced
     *     into an integer.
     * @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.
     */
    public Object getValue(ELContext context,
                           Object base,
                           Object property) {

        if (context == null) {
            throw new NullPointerException();
        }

        if (base != null && base instanceof List) {
            context.setPropertyResolved(true);
            List list = (List) base;
            int index = toInteger(property);
            if (index < 0 || index >= list.size()) {
                return null;
            } 
            return list.get(index);
        }
        return null;
    }

    /**
     * If the base object is a list, attempts to set the value at the
     * given index with the given value. The index is specified by the
     * <code>property</code> argument, and coerced into an integer. If the 
     * coercion could not be performed, an 
     * <code>IllegalArgumentException</code> is thrown. If the index is
     * out of bounds, a <code>PropertyNotFoundException</code> is thrown.
     *
     * <p>If the base is a <code>List</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>List</code> was created using 
     * {@link java.util.Collections#unmodifiableList}, 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>List</code>
     * and query its runtime type to see if it matches the runtime type of 
     * the base object as a workaround.</p>
     *