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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.impl;

import javax.servlet.http.HttpServletRequest;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.directwebremoting.extend.Sleeper;
import org.directwebremoting.util.Continuation;

/**
 * A Sleeper that works with Grizzly Continuations
 * @author Jeanfrancois Arcand [jeanfrancois dot arcand at sun dot com]
 */
public class GrizzlyContinuationSleeper implements Sleeper
{
    /**
     * @param request The request into which we store this as an attribute
     */
    public GrizzlyContinuationSleeper(HttpServletRequest request)
    {
        continuation = new Continuation(request);
    }

    /* (non-Javadoc)
     * @see org.directwebremoting.dwrp.Sleeper#goToSleep(java.lang.Runnable)
     */
    public void goToSleep(Runnable awakening)
    {
        this.onAwakening = awakening;

        try
        {
            continuation.suspend(-1);
        }
        catch (Exception ex)
        {
            Continuation.rethrowIfContinuation(ex);

            log.warn("Exception", ex);
            proxy = new ThreadWaitSleeper();
            proxy.goToSleep(onAwakening);
        }
    }

    
    /* (non-Javadoc)
     * @see org.directwebremoting.dwrp.PollHandler.Sleeper#wakeUp()
     */
    public void wakeUp()
    {
        if (proxy != null)
        {
            proxy.wakeUp();
        }
        else
        {
            synchronized (continuation)
            {
                if (!resumed)
                {
                    try
                    {
                        /*
                         * Flush bytes if any first as before resuming the 
                         * as Grizzly Comet isn't allowing writes once the
                         * continuation is resumed. 
                         *
                         * This can be achieved
                         * by using Grizzly CometHandler, which isn't exposed
                         * with DWR.
                         */
                        onAwakening.run();
                        continuation.resume();                      
                    }
                    catch (Exception ex)
                    {
                        log.error("Broken reflection", ex);
                    }

                    resumed = true;
                }
            }
        }
    }
    
    /**
     * If continuations fail, we proxy to a Thread Wait version
     */
    protected ThreadWaitSleeper proxy = null;

    /**
     * What we do when we are woken up
     */
    protected Runnable onAwakening;

    /**
     * The continuation object
     */
    protected final Continuation continuation;

    /**
     * Has the continuation been restarted already?
     */
    protected boolean resumed = false;

    /**
     * We remember the notify conduit so we can reuse it
     */
    public static final String ATTRIBUTE_JETTY_CONDUIT = "org.directwebremoting.dwrp.notifyConduit";
   
    private static final Log log = LogFactory.getLog(GrizzlyContinuationSleeper.class);
}