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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.io.IOException;
import java.util.concurrent.ScheduledThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

import javax.servlet.http.HttpServletRequest;

import org.directwebremoting.WebContextFactory;
import org.directwebremoting.extend.DownloadManager;
import org.directwebremoting.extend.IdGenerator;
import org.directwebremoting.io.FileTransfer;

/**
 * A {@link DownloadManager} that simply stores downloads in memory until they
 * are requested and then removes them.
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
public abstract class PurgingDownloadManager implements DownloadManager
{
    /* (non-Javadoc)
     * @see org.directwebremoting.extend.DownloadManager#addFile(org.directwebremoting.extend.DownloadManager.FileGenerator)
     */
    public String addFileTransfer(FileTransfer generator) throws IOException
    {
        String id = idGenerator.generate();
        putFileTransfer(id, generator);

        HttpServletRequest request = WebContextFactory.get().getHttpServletRequest();
        return "'" + request.getContextPath() + request.getServletPath() + downloadHandlerUrl + id + "'";
    }

    /**
     * The URL part which we attach to the downloads.
     * @param downloadHandlerUrl The URL for this Handler.
     */
    public void setDownloadHandlerUrl(String downloadHandlerUrl)
    {
        this.downloadHandlerUrl = downloadHandlerUrl;
    }

    /**
     * @param purgeDownloadsAfter the purgeDownloadsAfter to set
     */
    public void setPurgeDownloadsAfter(long purgeDownloadsAfter)
    {
        this.purgeDownloadsAfter = purgeDownloadsAfter;
    }

    /**
     * @param executor The DWR provided shutdown system
     */
    public void setScheduledThreadPoolExecutor(ScheduledThreadPoolExecutor executor)
    {
        this.executor = executor;
        this.executor.scheduleWithFixedDelay(downloadPurge, queueSleepTime, queueSleepTime, TimeUnit.MILLISECONDS);
    }

    /**
     * @param idGenerator The configured ID generator
     */
    public void setIdGenerator(IdGenerator idGenerator)
    {
        this.idGenerator = idGenerator;
    }

    /**
     * Store a {@link FileTransfer} against a given id for later retrieval.
     * @param id The id of the given FileGenerator
     * @param generator The FileGenerator to store against the id
     */
    protected abstract void putFileTransfer(String id, FileTransfer generator);

    /**
     * Remove all the stale entries from the cache
     */
    protected abstract void purge();

    /**
     * Loop over the known downloads removing the ones that are out of date
     */
    public class DownloadPurge implements Runnable
    {
        /* (non-Javadoc)
         * @see java.lang.Runnable#run()
         */
        public void run()
        {
            purge();
        }
    }

    /**
     * The cron system
     */
    protected ScheduledThreadPoolExecutor executor = null;

    /**
     * Check that the cache of files does not contain out-of-date items
     */
    protected DownloadPurge downloadPurge = new DownloadPurge();

    /**
     * How often do we run purges on the data?
     */
    protected int queueSleepTime = 15000;

    /**
     * After some time we will give up waiting for a file to be downloaded.
     * By default this is 2 minutes. After this time we expect that the user
     * is no longer waiting and has gone away.
     */
    protected long purgeDownloadsAfter = 120000;

    /**
     * The URL part which we attach to the downloads.
     */
    protected String downloadHandlerUrl;

    /**
     * Unique id so we can retrieve downloads when asked
     */
    protected IdGenerator idGenerator;
}