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DWR is easy Ajax for Java. It makes it simple to call Java code directly from Javascript. It gets rid of almost all the boiler plate code between the web browser and your Java code.

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/*
 * Copyright 2005 Joe Walker
 *
 * 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 org.directwebremoting.ui;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.List;

import org.directwebremoting.ScriptSession;
import org.directwebremoting.WebContext;
import org.directwebremoting.WebContextFactory;

/**
 * A way to call functions in JavaScript that return data using a reverse ajax
 * proxy.
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
public abstract class Callback
{
    /**
     * Create a Callback from a DWR thread,
     * i.e. where {@link WebContextFactory#get()} =! null
     */
    public Callback()
    {
        WebContext context = WebContextFactory.get();
        if (context == null)
        {
            throw new IllegalStateException("Attempt to use Callback without any ScriptSessions, from a non DWR thread. There is nowhere for replies to go.");
        }

        sessions.add(context.getScriptSession());
    }

    /**
     * Used when you need to specify the browser that will be providing the
     * response
     * @param session The browser to answer the question
     */
    public Callback(ScriptSession session)
    {
        sessions.add(session);
    }

    /**
     * Used when you need to specify a group of browsers that will be providing
     * the responses. The callback will be executed once per browser that
     * replies.
     * @param sessionList The browsers to answer the question
     */
    public Callback(Collection sessionList)
    {
        sessions.addAll(sessionList);
    }

    /**
     * A browser has completed some remote call as has data for you
     * @param data The data returned by the browser
     */
    public abstract void dataReturned(T data);

    /**
     * Accessor for the ScriptSessions that will reply to the question.
     * This method is generally for DWR internal use, but only because it's
     * unlikely to be useful to others.
     * @return An immutable list of browsers that may reply to the question.
     */
    public Collection getScriptSessions()
    {
        return Collections.unmodifiableCollection(sessions);
    }

    /**
     * We store the ScriptSessions that we send replies to, here.
     */
    private final List sessions = new ArrayList();
}