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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.extend;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.directwebremoting.Container;
import org.directwebremoting.ServerContext;

/**
 * Factory is not a user facing object, it is designed for system implementors.
 * Factory objects are generally use as helper classes by Factory classes
 * whose methods reflect the methods of Factory, but which are static and
 * proxy the call to the contained Factory instance.
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
public class Factory
{
    /**
     * Make it easy for Factories to create a Factory instance.
     */
    public static  Factory create(Class> created)
    {
        return new Factory(created);
    }

    /**
     * We need to know what type of builder to extract from the Container
     */
    public Factory(Class> created)
    {
        this.created = created;
    }

    /**
     * Accessor for the current object managed by this factory instance.
     */
    public T get()
    {
        Builder b = this.builder;
        if (b == null)
        {
            log.warn("DWR has not been initialized properly");
            return null;
        }

        return b.get();
    }

    /**
     * Accessor for the current object in more complex setups.
     * For some setups DWR may not be able to discover the correct environment
     * (i.e. ServletContext), so we need to tell it. This generally happens if
     * you have DWR configured twice in a single context. Unless you are writing
     * code that someone else will configure, it is probably safe to use the
     * simpler {@link #get()} method.
     * @param ctx The servlet context to allow us to bootstrap
     * @return The current object accessed by this Factory.
     */
    public T get(ServerContext ctx)
    {
        Builder b = this.builder;
        if (b == null)
        {
            log.warn("DWR has not been initialized properly");
            return null;
        }

        return b.get(ctx);
    }

    /**
     * Internal method to allow us to get the Builder from which we will get
     * created objects. Do NOT call this method from outside of DWR.
     * This function should only be called during startup.
     * our Container Builder from.
     */
    public T attach(Container container)
    {
        this.builder = container.getBean(created);
        return builder.attach(container);
    }

    /**
     * The type of builder that we get out of the container
     */
    private final Class> created;

    /**
     * The Builder from which we will get created objects
     */
    private volatile Builder builder = null;

    /**
     * The log stream
     */
    private static final Log log = LogFactory.getLog(Factory.class);
}