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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.Collection;
import java.util.Date;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.directwebremoting.ScriptSession;
import org.directwebremoting.ScriptSessionFilter;
import org.directwebremoting.extend.ScriptSessionManager;
import org.directwebremoting.extend.TaskDispatcher;

/**
 * Default single node implementation of TaskDispatcher
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
public class DefaultTaskDispatcher implements TaskDispatcher
{
    /* (non-Javadoc)
     * @see org.directwebremoting.extend.TaskDispatcher#dispatchTask(org.directwebremoting.ScriptSessionFilter, java.lang.Runnable)
     */
    public void dispatchTask(ScriptSessionFilter filter, Runnable task)
    {
        Collection all = scriptSessionManager.getAllScriptSessions();
        Collection use = new ArrayList();

        for (ScriptSession session : all)
        {
            if (filter.match(session))
            {
                use.add(session);
            }
        }

        if (use.size() > 0)
        {
            if (log.isDebugEnabled()) {
        	    log.debug("Execution time: " + new Date().toString() + " - Executing task (" + task.getClass().getSimpleName() + ") against " + use.size() + " sessions.");
            }
            target.set(use);
            task.run();
            target.remove();
        }
    }

    /* (non-Javadoc)
     * @see org.directwebremoting.extend.TaskDispatcher#getTargetSessions()
     */
    public Collection getTargetSessions()
    {
        return target.get();
    }

    /**
     * Connection to the DwrServlet that we are attached to
     */
    public void setScriptSessionManager(ScriptSessionManager scriptSessionManager)
    {
        this.scriptSessionManager = scriptSessionManager;
    }

    /**
     * @see #setScriptSessionManager(ScriptSessionManager)
     */
    private ScriptSessionManager scriptSessionManager;

    /**
     * ThreadLocal in which the list of sessions are stored.
     */
    private final ThreadLocal> target = new ThreadLocal>();

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