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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 java.util.ArrayList;
import java.util.List;

import javax.servlet.ServletContextListener;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.directwebremoting.extend.ServerLoadMonitor;
import org.directwebremoting.extend.UninitializingBean;
import org.directwebremoting.extend.WaitController;

/**
 * A base implementation of {@link ServerLoadMonitor} that implements waiting
 * functionality, mostly to provide {@link ServletContextListener#contextDestroyed}
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
public abstract class AbstractServerLoadMonitor implements ServerLoadMonitor, UninitializingBean
{
    /* (non-Javadoc)
     * @see org.directwebremoting.extend.UninitializingBean#destroy()
     */
    public void destroy()
    {
        if (shutdownCalled)
        {
            return;
        }

        synchronized (waitControllers)
        {
            List copy = new ArrayList();
            copy.addAll(waitControllers);

            for (WaitController controller : copy)
            {
                controller.shutdown();
            }

            log.debug(" - shutdown on: " + this);
            shutdownCalled = true;
        }
    }

    /* (non-Javadoc)
     * @see org.directwebremoting.extend.ServerLoadMonitor#threadWaitStarting(org.directwebremoting.extend.WaitController)
     */
    public void threadWaitStarting(WaitController controller)
    {
        synchronized (waitControllers)
        {
            waitControllers.add(controller);
        }
    }

    /* (non-Javadoc)
     * @see org.directwebremoting.extend.ServerLoadMonitor#threadWaitEnding(org.directwebremoting.extend.WaitController)
     */
    public void threadWaitEnding(WaitController controller)
    {
        synchronized (waitControllers)
        {
            waitControllers.remove(controller);
        }
    }

    /**
     * If there are too many WaitControllers waiting then we can kill one off at
     * random.
     * @param count How many {@link WaitController}s do we shutdown?
     */
    public void shutdownRandomWaitControllers(int count)
    {
        synchronized (waitControllers)
        {
            for (int i = 0; i < count && !waitControllers.isEmpty(); i++)
            {
                waitControllers.get(0).shutdown();
            }
        }
    }

    /**
     * Have we been shutdown already?
     */
    private boolean shutdownCalled = false;

    /**
     * The known wait controllers
     */
    protected final List waitControllers = new ArrayList();

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