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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.Collections;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;

import org.directwebremoting.extend.DownloadManager;
import org.directwebremoting.extend.FileGenerator;

/**
 * A {@link DownloadManager} that simply stores the links in-memory.
 * This implementation has the advantage that it is simple - no disk storage is
 * required, however in anything but a lightly used system it could cause
 * significant memory usage.
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
public class InMemoryDownloadManager extends PurgingDownloadManager implements DownloadManager
{
    /* (non-Javadoc)
     * @see org.directwebremoting.impl.PurgingDownloadManager#putFileGenerator(java.lang.String, org.directwebremoting.extend.DownloadManager.FileGenerator)
     */
    @Override
    protected void putFileGenerator(String id, FileGenerator generator)
    {
        synchronized (contentsLock)
        {
            contents.put(id, new TimedFileGenerator(generator));
        }
    }

    /* (non-Javadoc)
     * @see org.directwebremoting.impl.PurgingDownloadManager#getFileGenerator(java.lang.String)
     */
    @Override
    protected FileGenerator getFileGenerator(String id)
    {
        synchronized (contentsLock)
        {
            TimedFileGenerator generator = contents.remove(id);
            if (generator == null)
            {
                return null;
            }
            return generator.fileGenerator;
        }
    }

    /* (non-Javadoc)
     * @see org.directwebremoting.impl.PurgingDownloadManager#purge()
     */
    @Override
    protected void purge()
    {
        long now = System.currentTimeMillis();

        synchronized (contentsLock)
        {
            for (Iterator> it = contents.entrySet().iterator(); it.hasNext();)
            {
                Map.Entry entry = it.next();

                try
                {
                    if (now > entry.getValue().timeInserted + purgeDownloadsAfter)
                    {
                        it.remove();
                    }
                }
                catch (Exception ex)
                {
                    log.warn("Deletion queue processing error: " + ex.getMessage());
                }
            }
        }
    }

    /**
     * A union of a FileGenerator and the time it was inserted into this manager
     */
    protected static class TimedFileGenerator
    {
        protected TimedFileGenerator(FileGenerator fileGenerator)
        {
            this.fileGenerator = fileGenerator;
            this.timeInserted = System.currentTimeMillis();
        }

        FileGenerator fileGenerator;
        long timeInserted;
    }

    /**
     * The lock which you must hold to read or write from the list of
     * {@link FileGenerator}s.
     */
    protected Object contentsLock = new Object();

    /**
     * The list of files in the system
     */
    protected final Map contents = Collections.synchronizedMap(new HashMap());
}