path: root/netx/net/sourceforge/jnlp/cache/ResourceTracker.java

// Copyright (C) 2001-2003 Jon A. Maxwell (JAM)
//
// This library is free software; you can redistribute it and/or
// modify it under the terms of the GNU Lesser General Public
// License as published by the Free Software Foundation; either
// version 2.1 of the License, or (at your option) any later version.
//
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
// Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public
// License along with this library; if not, write to the Free Software
// Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.

package net.sourceforge.jnlp.cache;

import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLConnection;
import java.util.ArrayList;
import java.util.List;
import java.util.jar.JarOutputStream;
import java.util.jar.Pack200;
import java.util.jar.Pack200.Unpacker;
import java.util.zip.GZIPInputStream;

import net.sourceforge.jnlp.Version;
import net.sourceforge.jnlp.event.DownloadEvent;
import net.sourceforge.jnlp.event.DownloadListener;
import net.sourceforge.jnlp.runtime.JNLPRuntime;
import net.sourceforge.jnlp.util.WeakList;

/**
 * This class tracks the downloading of various resources of a
 * JNLP file to local files in the cache.  It can be used to
 * download icons, jnlp and extension files, jars, and jardiff
 * files using the version based protocol or any file using the
 * basic download protocol (jardiff and version not implemented
 * yet).<p>
 *
 * The resource tracker can be configured to prefetch resources,
 * which are downloaded in the order added to the media
 * tracker.<p>
 *
 * Multiple threads are used to download and cache resources that
 * are actively being waited for (blocking a caller) or those that
 * have been started downloading by calling the startDownload
 * method.  Resources that are prefetched are downloaded one at a
 * time and only if no other trackers have requested downloads.
 * This allows the tracker to start downloading many items without
 * using many system resources, but still quickly download items
 * as needed.<p>
 *
 * @author <a href="mailto:jmaxwell@users.sourceforge.net">Jon A. Maxwell (JAM)</a> - initial author
 * @version $Revision: 1.22 $
 */
public class ResourceTracker {

    // todo: use event listener arrays instead of lists

    // todo: see if there is a way to set the socket options just
    // for use by the tracker so checks for updates don't hang for
    // a long time.

    // todo: ability to restart/retry a hung download

    // todo: move resource downloading/processing code into Resource
    // class, threading stays in ResourceTracker

    // todo: get status method? and some way to convey error status
    // to the caller.

    // todo: might make a tracker be able to download more than one
    // version of a resource, but probably not very useful.

    // defines
    //    ResourceTracker.Downloader (download threads)

    // separately locks on (in order of aquire order, ie, sync on prefetch never syncs on lock):
    //   lock, prefetch, this.resources, each resource, listeners

    /** notified on initialization or download of a resource */
    private static Object lock = new Integer(0); // used to lock static structures

    // shortcuts
    private static final int UNINITIALIZED = Resource.UNINITIALIZED;
    private static final int CONNECT = Resource.CONNECT;
    private static final int CONNECTING = Resource.CONNECTING;
    private static final int CONNECTED = Resource.CONNECTED;
    private static final int DOWNLOAD = Resource.DOWNLOAD;
    private static final int DOWNLOADING = Resource.DOWNLOADING;
    private static final int DOWNLOADED = Resource.DOWNLOADED;
    private static final int ERROR = Resource.ERROR;
    private static final int STARTED = Resource.STARTED;

    /** max threads */
    private static final int maxThreads = 5;

    /** running threads */
    private static int threads = 0;

    /** weak list of resource trackers with resources to prefetch */
    private static WeakList<ResourceTracker> prefetchTrackers =
            new WeakList<ResourceTracker>();

    /** resources requested to be downloaded */
    private static ArrayList<Resource> queue = new ArrayList<Resource>();

    /** resource trackers threads are working for (used for load balancing across multi-tracker downloads) */
    private static ArrayList<ResourceTracker> active =
            new ArrayList<ResourceTracker>(); //

    /** the resources known about by this resource tracker */
    private List<Resource> resources = new ArrayList<Resource>();

    /** download listeners for this tracker */
    private List<DownloadListener> listeners = new ArrayList<DownloadListener>();

    /** whether to download parts before requested */
    private boolean prefetch;

    /**
     * Creates a resource tracker that does not prefetch resources.
     */
    public ResourceTracker() {
        this(false);
    }

    /**
     * Creates a resource tracker.
     *
     * @param prefetch whether to download resources before requested.
     */
    public ResourceTracker(boolean prefetch) {
        this.prefetch = prefetch;

        if (prefetch) {
            synchronized (prefetchTrackers) {
                prefetchTrackers.add(this);
                prefetchTrackers.trimToSize();
            }
        }
    }

    /**
     * Add a resource identified by the specified location and
     * version.  The tracker only downloads one version of a given
     * resource per instance (ie cannot download both versions 1 and
     * 2 of a resource in the same tracker).
     *
     * @param location the location of the resource
     * @param version the resource version
     * @param updatePolicy whether to check for updates if already in cache
     */
    public void addResource(URL location, Version version, UpdatePolicy updatePolicy) {
        if (location == null)
            throw new IllegalArgumentException("location==null");

        Resource resource = Resource.getResource(location, version, updatePolicy);
        boolean downloaded = false;

        synchronized (resources) {
            if (resources.contains(resource))
                return;
            resource.addTracker(this);
            resources.add(resource);
        }

        // checkCache may take a while (loads properties file).  this
        // should really be synchronized on resources, but the worst
        // case should be that the resource will be updated once even
        // if unnecessary.
        downloaded = checkCache(resource, updatePolicy);

        synchronized (lock) {
            if (!downloaded)
                if (prefetch && threads == 0) // existing threads do pre-fetch when queue empty
                    startThread();
        }
    }

    /**
     * Removes a resource from the tracker.  This method is useful
     * to allow memory to be reclaimed, but calling this method is
     * not required as resources are reclaimed when the tracker is
     * collected.
     *
     * @throws IllegalArgumentException if the resource is not being tracked
     */
    public void removeResource(URL location) {
        synchronized (resources) {
            Resource resource = getResource(location);

            if (resource != null) {
                resources.remove(resource);
                resource.removeTracker(this);
            }

            // should remove from queue? probably doesn't matter
        }
    }

    /**
     * Check the cache for a resource, and initialize the resource
     * as already downloaded if found. <p>
     *
     * @param updatePolicy whether to check for updates if already in cache
     * @return whether the resource are already downloaded
     */
    private boolean checkCache(Resource resource, UpdatePolicy updatePolicy) {
        if (!CacheUtil.isCacheable(resource.location, resource.downloadVersion)) {
            // pretend that they are already downloaded; essentially
            // they will just 'pass through' the tracker as if they were
            // never added (for example, not affecting the total download size).
            synchronized (resource) {
                resource.changeStatus(0, DOWNLOADED | CONNECTED | STARTED);
            }
            fireDownloadEvent(resource);
            return true;
        }

        if (updatePolicy != UpdatePolicy.ALWAYS && updatePolicy != UpdatePolicy.FORCE) { // save loading entry props file
            CacheEntry entry = new CacheEntry(resource.location, resource.downloadVersion);

            if (entry.isCached() && !updatePolicy.shouldUpdate(entry)) {
                if (JNLPRuntime.isDebug())
                    System.out.println("not updating: " + resource.location);

                synchronized (resource) {
                    resource.localFile = CacheUtil.getCacheFile(resource.location, resource.downloadVersion);
                    resource.size = resource.localFile.length();
                    resource.transferred = resource.localFile.length();
                    resource.changeStatus(0, DOWNLOADED | CONNECTED | STARTED);
                }
                fireDownloadEvent(resource);
                return true;
            }
        }

        if (updatePolicy == UpdatePolicy.FORCE) { // ALWAYS update
            // When we are "always" updating, we update for each instance. Reset resource status.
            resource.changeStatus(Integer.MAX_VALUE, 0);
        }

        // may or may not be cached, but check update when connection
        // is open to possibly save network communication time if it
        // has to be downloaded, and allow this call to return quickly
        return false;
    }

    /**
     * Adds the listener to the list of objects interested in
     * receivind DownloadEvents.<p>
     *
     * @param listener the listener to add.
     */
    public void addDownloadListener(DownloadListener listener) {
        synchronized (listeners) {
            if (!listeners.contains(listener))
                listeners.add(listener);
        }
    }

    /**
     * Removes a download listener.
     *
     * @param listener the listener to remove.
     */
    public void removeDownloadListener(DownloadListener listener) {
        synchronized (listeners) {
            listeners.remove(listener);
        }
    }

    /**
     * Fires the download event corresponding to the resource's
     * state.  This method is typicall called by the Resource itself
     * on each tracker that is monitoring the resource.  Do not call
     * this method with any locks because the listeners may call
     * back to this ResourceTracker.
     */
    protected void fireDownloadEvent(Resource resource) {
        DownloadListener l[] = null;
        synchronized (listeners) {
            l = listeners.toArray(new DownloadListener[0]);
        }

        int status;
        synchronized (resource) {
            status = resource.status;
        }

        DownloadEvent event = new DownloadEvent(this, resource);
        for (int i = 0; i < l.length; i++) {
            if (0 != ((ERROR | DOWNLOADED) & status))
                l[i].downloadCompleted(event);
            else if (0 != (DOWNLOADING & status))
                l[i].downloadStarted(event);
            else if (0 != (CONNECTING & status))
                l[i].updateStarted(event);
        }
    }

    /**
     * Returns a URL pointing to the cached location of the
     * resource, or the resource itself if it is a non-cacheable
     * resource.<p>
     *
     * If the resource has not downloaded yet, the method will block
     * until it has been transferred to the cache.<p>
     *
     * @param location the resource location
     * @return the resource, or null if it could not be downloaded
     * @throws IllegalArgumentException if the resource is not being tracked
     * @see CacheUtil#isCacheable
     */
    public URL getCacheURL(URL location) {
        try {
            File f = getCacheFile(location);
            if (f != null)
                // TODO: Should be toURI().toURL()
                return f.toURL();
        } catch (MalformedURLException ex) {
            if (JNLPRuntime.isDebug())
                ex.printStackTrace();
        }

        return location;
    }

    /**
     * Returns a file containing the downloaded resource.  If the
     * resource is non-cacheable then null is returned unless the
     * resource is a local file (the original file is returned).<p>
     *
     * If the resource has not downloaded yet, the method will block
     * until it has been transferred to the cache.<p>
     *
     * @param location the resource location
     * @return a local file containing the resource, or null
     * @throws IllegalArgumentException if the resource is not being tracked
     * @see CacheUtil#isCacheable
     */
    public File getCacheFile(URL location) {
        try {
            Resource resource = getResource(location);
            if (!resource.isSet(DOWNLOADED | ERROR))
                waitForResource(location, 0);

            if (resource.isSet(ERROR))
                return null;

            if (resource.localFile != null)
                return resource.localFile;

            if (location.getProtocol().equalsIgnoreCase("file")) {
                File file = new File(location.getFile());
                if (file.exists())
                    return file;
            }

            return null;
        } catch (InterruptedException ex) {
            if (JNLPRuntime.isDebug())
                ex.printStackTrace();

            return null; // need an error exception to throw
        }
    }

    /**
     * Returns an input stream that reads the contents of the
     * resource.  For non-cacheable resources, an InputStream that
     * reads from the source location is returned.  Otherwise the
     * InputStream reads the cached resource.<p>
     *
     * This method will block while the resource is downloaded to
     * the cache.
     *
     * @throws IOException if there was an error opening the stream
     * @throws IllegalArgumentException if the resource is not being tracked
     */
    public InputStream getInputStream(URL location) throws IOException {
        try {
            Resource resource = getResource(location);
            if (!resource.isSet(DOWNLOADED | ERROR))
                waitForResource(location, 0);

            if (resource.localFile != null)
                return new FileInputStream(resource.localFile);

            return resource.location.openStream();
        } catch (InterruptedException ex) {
            throw new IOException("wait was interrupted");
        }
    }

    /**
     * Wait for a group of resources to be downloaded and made
     * available locally.
     *
     * @param urls the resources to wait for
     * @param timeout the time in ms to wait before returning, 0 for no timeout
     * @return whether the resources downloaded before the timeout
     * @throws IllegalArgumentException if the resource is not being tracked
     */
    public boolean waitForResources(URL urls[], long timeout) throws InterruptedException {
        Resource resources[] = new Resource[urls.length];

        synchronized (resources) {
            // keep the lock so getResource doesn't have to aquire it each time
            for (int i = 0; i < urls.length; i++)
                resources[i] = getResource(urls[i]);
        }

        if (resources.length > 0)
            return wait(resources, timeout);

        return true;
    }

    /**
     * Wait for a particular resource to be downloaded and made
     * available.
     *
     * @param location the resource to wait for
     * @param timeout the timeout, or 0 to wait until completed
     * @return whether the resource downloaded before the timeout
     * @throws InterruptedException if another thread interrupted the wait
     * @throws IllegalArgumentException if the resource is not being tracked
     */
    public boolean waitForResource(URL location, long timeout) throws InterruptedException {
        return wait(new Resource[] { getResource(location) }, timeout);
    }

    /**
     * Returns the number of bytes downloaded for a resource.
     *
     * @param location the resource location
     * @return the number of bytes transferred
     * @throws IllegalArgumentException if the resource is not being tracked
     */
    public long getAmountRead(URL location) {
        // not atomic b/c transferred is a long, but so what (each
        // byte atomic? so probably won't affect anything...)
        return getResource(location).transferred;
    }

    /**
     * Returns whether a resource is available for use (ie, can be
     * accessed with the getCacheFile method).
     *
     * @throws IllegalArgumentException if the resource is not being tracked
     */
    public boolean checkResource(URL location) {
        return getResource(location).isSet(DOWNLOADED | ERROR); // isSet atomic
    }

    /**
     * Starts loading the resource if it is not already being
     * downloaded or already cached.  Resources started downloading
     * using this method may download faster than those prefetched
     * by the tracker because the tracker will only prefetch one
     * resource at a time to conserve system resources.
     *
     * @return true if the resource is already downloaded (or an error occurred)
     * @throws IllegalArgumentException if the resource is not being tracked
     */
    public boolean startResource(URL location) {
        Resource resource = getResource(location);

        return startResource(resource);
    }

    /**
     * Sets the resource status to connect and download, and
     * enqueues the resource if not already started.
     *
     * @return true if the resource is already downloaded (or an error occurred)
     * @throws IllegalArgumentException if the resource is not being tracked
     */
    private boolean startResource(Resource resource) {
        boolean enqueue = false;

        synchronized (resource) {
            if (resource.isSet(ERROR))
                return true;

            enqueue = !resource.isSet(STARTED);

            if (!resource.isSet(CONNECTED | CONNECTING))
                resource.changeStatus(0, CONNECT | STARTED);
            if (!resource.isSet(DOWNLOADED | DOWNLOADING))
                resource.changeStatus(0, DOWNLOAD | STARTED);

            if (!resource.isSet(DOWNLOAD | CONNECT))
                enqueue = false;
        }

        if (enqueue)
            queueResource(resource);

        return !enqueue;
    }

    /**
     * Returns the number of total size in bytes of a resource, or
     * -1 it the size is not known.
     *
     * @param location the resource location
     * @return the number of bytes, or -1
     * @throws IllegalArgumentException if the resource is not being tracked
     */
    public long getTotalSize(URL location) {
        return getResource(location).size; // atomic
    }

    /**
     * Start a new download thread if there are not too many threads
     * already running.<p>
     *
     * Calls to this method should be synchronized on lock.
     */
    protected void startThread() {
        if (threads < maxThreads) {
            threads++;

            Thread thread = new Thread(new Downloader());
            thread.start();
        }
    }

    /**
     * A thread is ending, called by the thread itself.<p>
     *
     * Calls to this method should be synchronized.
     */
    private void endThread() {
        threads--;

        if (threads < 0) {
            // this should never happen but try to recover
            threads = 0;

            if (queue.size() > 0) // if any on queue make sure a thread is running
                startThread(); // look into whether this could create a loop

            throw new RuntimeException("tracker threads < 0");
        }

        if (threads == 0) {
            synchronized (prefetchTrackers) {
                queue.trimToSize(); // these only accessed by threads so no sync needed
                active.clear(); // no threads so no trackers actively downloading
                active.trimToSize();
                prefetchTrackers.trimToSize();
            }
        }
    }

    /**
     * Add a resource to the queue and start a thread to download or
     * initialize it.
     */
    private void queueResource(Resource resource) {
        synchronized (lock) {
            if (!resource.isSet(CONNECT | DOWNLOAD))
                throw new IllegalArgumentException("Invalid resource state (resource: " + resource + ")");

            queue.add(resource);
            startThread();
        }
    }

    /**
     * Process the resource by either downloading it or initializing
     * it.
     */
    private void processResource(Resource resource) {
        boolean doConnect = false;
        boolean doDownload = false;

        synchronized (resource) {
            if (resource.isSet(CONNECTING))
                doConnect = true;
        }
        if (doConnect)
            initializeResource(resource);

        synchronized (resource) {
            // return to queue if we just initalized but it still needs
            // to download (not cached locally / out of date)
            if (resource.isSet(DOWNLOAD)) // would be DOWNLOADING if connected before this method
                queueResource(resource);

            if (resource.isSet(DOWNLOADING))
                doDownload = true;
        }
        if (doDownload)
            downloadResource(resource);
    }

    /**
     * Downloads a resource to a file, uncompressing it if required
     *
     * @param resource the resource to download
     */
    private void downloadResource(Resource resource) {
        resource.fireDownloadEvent(); // fire DOWNLOADING

        try {
            // create out second in case in does not exist
            URLConnection con = getVersionedResourceURL(resource).openConnection();
            con.addRequestProperty("Accept-Encoding", "pack200-gzip, gzip");

            con.connect();

            /*
             * We dont really know what we are downloading. If we ask for
             * foo.jar, the server might send us foo.jar.pack.gz or foo.jar.gz
             * instead. So we save the file with the appropriate extension
             */
            URL downloadLocation = resource.location;

            String contentEncoding = con.getContentEncoding();

            if (JNLPRuntime.isDebug()) {
                System.err.println("Content encoding for " + resource.location + ": "
                        + contentEncoding);
            }

            if (contentEncoding != null) {
                if (contentEncoding.equals("gzip")) {
                    downloadLocation = new URL(downloadLocation.toString() + ".gz");
                } else if (contentEncoding.equals("pack200-gzip")) {
                    downloadLocation = new URL(downloadLocation.toString() + ".pack.gz");
                }
            }

            InputStream in = new BufferedInputStream(con.getInputStream());
            OutputStream out = CacheUtil.getOutputStream(downloadLocation, resource.downloadVersion);
            byte buf[] = new byte[1024];
            int rlen;

            while (-1 != (rlen = in.read(buf))) {
                resource.transferred += rlen;
                out.write(buf, 0, rlen);
            }

            in.close();
            out.close();

            // explicitly close the URLConnection.
            if (con instanceof HttpURLConnection)
                ((HttpURLConnection) con).disconnect();

            /*
             * If the file was compressed, uncompress it.
             */
            if (contentEncoding != null) {
                if (contentEncoding.equals("gzip")) {
                    GZIPInputStream gzInputStream = new GZIPInputStream(new FileInputStream(CacheUtil
                            .getCacheFile(downloadLocation, resource.downloadVersion)));
                    InputStream inputStream = new BufferedInputStream(gzInputStream);

                    BufferedOutputStream outputStream = new BufferedOutputStream(
                            new FileOutputStream(CacheUtil.getCacheFile(resource.location,
                                    resource.downloadVersion)));

                    while (-1 != (rlen = inputStream.read(buf))) {
                        outputStream.write(buf, 0, rlen);
                    }

                    outputStream.close();
                    inputStream.close();
                    gzInputStream.close();

                } else if (contentEncoding.equals("pack200-gzip")) {
                    GZIPInputStream gzInputStream = new GZIPInputStream(new FileInputStream(
                            CacheUtil.getCacheFile(downloadLocation, resource.downloadVersion)));
                    InputStream inputStream = new BufferedInputStream(gzInputStream);

                    JarOutputStream outputStream = new JarOutputStream(new FileOutputStream(
                            CacheUtil.getCacheFile(resource.location, resource.downloadVersion)));

                    Unpacker unpacker = Pack200.newUnpacker();
                    unpacker.unpack(inputStream, outputStream);

                    outputStream.close();
                    inputStream.close();
                    gzInputStream.close();
                }
            }

            resource.changeStatus(DOWNLOADING, DOWNLOADED);
            synchronized (lock) {
                lock.notifyAll(); // wake up wait's to check for completion
            }
            resource.fireDownloadEvent(); // fire DOWNLOADED
        } catch (Exception ex) {
            if (JNLPRuntime.isDebug())
                ex.printStackTrace();

            resource.changeStatus(0, ERROR);
            synchronized (lock) {
                lock.notifyAll(); // wake up wait's to check for completion
            }
            resource.fireDownloadEvent(); // fire ERROR
        }
    }

    /**
     * Open a URL connection and get the content length and other
     * fields.
     */
    private void initializeResource(Resource resource) {
        resource.fireDownloadEvent(); // fire CONNECTING

        try {
            File localFile = CacheUtil.getCacheFile(resource.location, resource.downloadVersion);

            // connect
            URLConnection connection = getVersionedResourceURL(resource).openConnection(); // this won't change so should be okay unsynchronized
            connection.addRequestProperty("Accept-Encoding", "pack200-gzip, gzip");

            int size = connection.getContentLength();
            boolean current = CacheUtil.isCurrent(resource.location, resource.requestVersion, connection) && resource.getUpdatePolicy() != UpdatePolicy.FORCE;

            synchronized (resource) {
                resource.localFile = localFile;
                // resource.connection = connection;
                resource.size = size;
                resource.changeStatus(CONNECT | CONNECTING, CONNECTED);

                // check if up-to-date; if so set as downloaded
                if (current)
                    resource.changeStatus(DOWNLOAD | DOWNLOADING, DOWNLOADED);
            }

            // update cache entry
            CacheEntry entry = new CacheEntry(resource.location, resource.requestVersion);
            if (!current)
                entry.initialize(connection);

            entry.setLastUpdated(System.currentTimeMillis());
            entry.store();

            synchronized (lock) {
                lock.notifyAll(); // wake up wait's to check for completion
            }
            resource.fireDownloadEvent(); // fire CONNECTED

            // explicitly close the URLConnection.
            if (connection instanceof HttpURLConnection)
                ((HttpURLConnection) connection).disconnect();
        } catch (Exception ex) {
            if (JNLPRuntime.isDebug())
                ex.printStackTrace();

            resource.changeStatus(0, ERROR);
            synchronized (lock) {
                lock.notifyAll(); // wake up wait's to check for completion
            }
            resource.fireDownloadEvent(); // fire ERROR
        }
    }

    /**
     * Returns the versioned url for a resource
     * @param resource the resource to get the url for
     */
    private URL getVersionedResourceURL(Resource resource) {
        String actualLocation = resource.location.getProtocol() + "://"
                + resource.location.getHost();
        if (resource.location.getPort() != -1) {
            actualLocation += ":" + resource.location.getPort();
        }
        actualLocation += resource.location.getPath();
        if (resource.requestVersion != null
                && resource.requestVersion.isVersionId()) {
            actualLocation += "?version-id=" + resource.requestVersion;
        }
        URL versionedURL;
        try {
            versionedURL = new URL(actualLocation);
        } catch (MalformedURLException e) {
            return resource.location;
        }
        return versionedURL;
    }

    /**
     * Pick the next resource to download or initialize.  If there
     * are no more resources requested then one is taken from a
     * resource tracker with prefetch enabled.<p>
     *
     * The resource state is advanced before it is returned
     * (CONNECT-&gt;CONNECTING).<p>
     *
     * Calls to this method should be synchronized on lock.<p>
     *
     * @return the resource to initialize or download, or null
     */
    private static Resource selectNextResource() {
        Resource result;

        // pick from queue
        result = selectByFlag(queue, CONNECT, ERROR); // connect but not error
        if (result == null)
            result = selectByFlag(queue, DOWNLOAD, ERROR | CONNECT | CONNECTING);

        // remove from queue if found
        if (result != null)
            queue.remove(result);

        // prefetch if nothing found so far and this is the last thread
        if (result == null && threads == 1)
            result = getPrefetch();

        if (result == null)
            return null;

        synchronized (result) {
            if (result.isSet(CONNECT)) {
                result.changeStatus(CONNECT, CONNECTING);
            } else if (result.isSet(DOWNLOAD)) {
                // only download if *not* connecting, when done connecting
                // select next will pick up the download part.  This makes
                // all requested connects happen before any downloads, so
                // the size is known as early as possible.
                result.changeStatus(DOWNLOAD, DOWNLOADING);
            }
        }

        return result;
    }

    /**
     * Returns the next resource to be prefetched before
     * requested.<p>
     *
     * Calls to this method should be synchronized on lock.<p>
     */
    private static Resource getPrefetch() {
        Resource result = null;
        Resource alternate = null;

        // first find one to initialize
        synchronized (prefetchTrackers) {
            for (int i = 0; i < prefetchTrackers.size() && result == null; i++) {
                ResourceTracker tracker = prefetchTrackers.get(i);
                if (tracker == null)
                    continue;

                synchronized (tracker.resources) {
                    result = selectByFlag(tracker.resources, UNINITIALIZED, ERROR);

                    if (result == null && alternate == null)
                        alternate = selectByFlag(tracker.resources, CONNECTED, ERROR | DOWNLOADED | DOWNLOADING | DOWNLOAD);
                }
            }
        }

        // if none to initialize, switch to download
        if (result == null)
            result = alternate;

        if (result == null)
            return null;

        synchronized (result) {
            ResourceTracker tracker = result.getTracker();
            if (tracker == null)
                return null; // GC of tracker happened between above code and here

            // prevents startResource from putting it on queue since
            // we're going to return it.
            result.changeStatus(0, STARTED);

            tracker.startResource(result);
        }

        return result;
    }

    /**
     * Selects a resource from the source list that has the
     * specified flag set.<p>
     *
     * Calls to this method should be synchronized on lock and
     * source list.<p>
     */
    private static Resource selectByFlag(List<Resource> source, int flag,
                                         int notflag) {
        Resource result = null;
        int score = Integer.MAX_VALUE;

        for (int i = 0; i < source.size(); i++) {
            Resource resource = source.get(i);
            boolean selectable = false;

            synchronized (resource) {
                if (resource.isSet(flag) && !resource.isSet(notflag))
                    selectable = true;
            }

            if (selectable) {
                int activeCount = 0;

                for (int j = 0; j < active.size(); j++)
                    if (active.get(j) == resource.getTracker())
                        activeCount++;

                // try to spread out the downloads so that a slow host
                // won't monopolize the downloads
                if (activeCount < score) {
                    result = resource;
                    score = activeCount;
                }
            }
        }

        return result;
    }

    /**
     * Return the resource matching the specified URL.
     *
     * @throws IllegalArgumentException if the resource is not being tracked
     */
    private Resource getResource(URL location) {
        synchronized (resources) {
            for (int i = 0; i < resources.size(); i++) {
                Resource resource = resources.get(i);

                if (CacheUtil.urlEquals(resource.location, location))
                    return resource;
            }
        }

        throw new IllegalArgumentException("Location does not specify a resource being tracked.");
    }

    /**
     * Wait for some resources.
     *
     * @param resources the resources to wait for
     * @param timeout the timeout, or 0 to wait until completed
     * @returns true if the resources were downloaded or had errors,
     * false if the timeout was reached
     * @throws InterruptedException if another thread interrupted the wait
     */
    private boolean wait(Resource resources[], long timeout) throws InterruptedException {
        long startTime = System.currentTimeMillis();

        // start them downloading / connecting in background
        for (int i = 0; i < resources.length; i++)
            startResource(resources[i]);

        // wait for completion
        while (true) {
            boolean finished = true;

            synchronized (lock) {
                // check for completion
                for (int i = 0; i < resources.length; i++) {
                    //NetX Deadlocking may be solved by removing this
                    //synch block.
                    synchronized (resources[i]) {
                        if (!resources[i].isSet(DOWNLOADED | ERROR)) {
                            finished = false;
                            break;
                        }
                    }
                }
                if (finished)
                    return true;

                // wait
                long waitTime = 0;

                if (timeout > 0) {
                    waitTime = timeout - (System.currentTimeMillis() - startTime);
                    if (waitTime <= 0)
                        return false;
                }

                lock.wait(waitTime);
            }
        }
    }

    // inner classes

    /**
     * This class downloads and initializes the queued resources.
     */
    class Downloader implements Runnable {
        Resource resource = null;

        public void run() {
            while (true) {
                synchronized (lock) {
                    // remove from active list, used for load balancing
                    if (resource != null)
                        active.remove(resource.getTracker());

                    resource = selectNextResource();

                    if (resource == null) {
                        endThread();
                        break;
                    }

                    // add to active list, used for load balancing
                    active.add(resource.getTracker());
                }

                try {
                    processResource(resource);
                } catch (Exception ex) {
                    if (JNLPRuntime.isDebug())
                        ex.printStackTrace();
                }
            }
            // should have a finally in case some exception is thrown by
            // selectNextResource();
        }
    };

}