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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 boilerplate code between the web browser and your Java code. This version 4.0.2 works with Jakarta Servlet 4.0.2.

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package org.directwebremoting.impl;

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

import jakarta.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);
}