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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.dwrp;

/**
 * The simplest type of Sleeper that just uses {@link #wait()} and
 * {@link #notify()} to halt a Thread and restart it.
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
public class ThreadWaitSleeper implements Sleeper
{
    /* (non-Javadoc)
     * @see org.directwebremoting.dwrp.PollHandler.Sleeper#goToSleep()
     */
    public void goToSleep(Runnable onAwakening)
    {
        synchronized (sleepLock)
        {
            try
            {
                while (!wakeUpCalled)
                {
                    sleepLock.wait();
                }
            }
            catch (InterruptedException ex)
            {
                Thread.interrupted();
            }
            finally
            {
                onAwakening.run();
            }
        }
    }

    /* (non-Javadoc)
     * @see org.directwebremoting.dwrp.PollHandler.Sleeper#wakeUp()
     */
    public void wakeUp()
    {
        synchronized (wakeUpCalledLock)
        {
            if (wakeUpCalled)
            {
                return;
            }

            wakeUpCalled = true;
        }

        synchronized (sleepLock)
        {
            sleepLock.notifyAll();
        }
    }

    /**
     * All operations that involve going to sleep of waking up must hold this
     * lock before they take action.
     */
    private final Object wakeUpCalledLock = new Object();

    /**
     * Has wakeUp been called?
     */
    private transient boolean wakeUpCalled = false;

    /**
     * The object to lock on
     */
    private final Object sleepLock = new Object();
}