PC World Komputer 1996 May/PCW596.iso/wtest/clico/sunsoft/java/solaris.lzh/SOLARIS.TAZ/SOLARIS/hotjava/classsrc/browser/ImageReader.java

Java Source | 1995-05-15 | 4.7 KB | 187 lines

/*
 * @(#)ImageReader.java    1.11 94/12/26 Jonathan Payne
 *
 * Copyright (c) 1994 Sun Microsystems, Inc. All Rights Reserved.
 *
 * Permission to use, copy, modify, and distribute this software
 * and its documentation for NON-COMMERCIAL purposes and without
 * fee is hereby granted provided that this copyright notice
 * appears in all copies. Please refer to the file "copyright.html"
 * for further important copyright and licensing information.
 *
 * SUN MAKES NO REPRESENTATIONS OR WARRANTIES ABOUT THE SUITABILITY OF
 * THE SOFTWARE, EITHER EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED
 * TO THE IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
 * PARTICULAR PURPOSE, OR NON-INFRINGEMENT. SUN SHALL NOT BE LIABLE FOR
 * ANY DAMAGES SUFFERED BY LICENSEE AS A RESULT OF USING, MODIFYING OR
 * DISTRIBUTING THIS SOFTWARE OR ITS DERIVATIVES.
 */

package browser;

import awt.*;
import java.util.*;
import java.io.*;
import net.www.html.*;

class ImageFetcher implements Runnable {
    ImageReader        master;
    ImageHandle        h;

    ImageFetcher(ImageReader master, ImageHandle h) {
    this.master = master;
    this.h = h;
    }

    public void run() {
    try {
        h.fetchImage();
    } catch(ThreadDeath e) {
        h.setImage("Image fetch cancelled, try again");
    } catch(Exception e) {
        System.out.println("Error " + e + " reading " + h);
    }
    master.imageFetcherCompletes(h, this);
    }
}

/**
 * Class ImageReader is a thread which sits around waiting for
 * requests to fetch images in behalf of ImageHandles.  It performs
 * fetches in background threads, and allows up to
 * <b>ImageReader.simultaneousFetcherLimit</b> requests at the same
 * time.  ImageReader is created and managed by instances of WRWindow.
 * @see WRWindow
 * @version 1.11, 26 Dec 1994
 * @author Jonathan Payne
 */

public class ImageReader extends Thread {
    static int    simultaneousFetcherLimit = 4;

    Vector handles = new Vector();
    Hashtable allFetchers = new Hashtable(simultaneousFetcherLimit);
    URL ThisURL;
    int    fetcherCount = 0;
    boolean paused = true;

    public ImageReader () {
    setName("Background Image Reader");
    setDaemon(true);
    start();
    }

    synchronized boolean emptyQueue() {
    return (fetcherCount == 1 && handles.size() == 0);
    }

    synchronized void waitForRequest() {
    while (fetcherCount >= simultaneousFetcherLimit) {
        wait(1000);
    }
    while (handles.size() == 0) {
        wait(1000);
    }
    }

    private synchronized ImageHandle getNextRequest() {
    ImageHandle h;

    if (handles.size() == 0) {
        return null;
    }
    h = (ImageHandle) handles.elementAt(0);
    handles.removeElementAt(0);
    return h;
    }

    synchronized void flushPending() {
    handles.setSize(0);
    }

    synchronized void stopFetches() {
    for (Enumeration e = allFetchers.elements() ; e.hasMoreElements() ;) {
        Thread th = (Thread)e.nextElement();
        if (th.isAlive()) {
        th.stop();
        }
    }
    allFetchers = new Hashtable(simultaneousFetcherLimit);
    }

    /*
     * Fork a thread to fetch a single image.  If there are already
     * some maximum number of image fetches outstanding, this
     * routine blocks waiting for one of the others to complete.
     */
    private synchronized void forkImageFetcher(ImageHandle h) {
    fetcherCount += 1;
    ImageFetcher fetcher = new ImageFetcher(this, h);
    Thread th = new Thread(fetcher, "ImageFetcher " + fetcherCount);
    th.start();
    allFetchers.put(fetcher, th);
    }

    synchronized void imageFetcherCompletes(Observable o,
                        ImageFetcher fetch) {
    o.notifyObservers();

    allFetchers.remove(fetch);
    if (fetcherCount-- == simultaneousFetcherLimit) {
        notifyAll();
    }
    }

    /**
     * Add an ImageHandle to the background image reader.  This
     * causes the specified ImageHandle to have its image loaded by a
     * background thread.
     * @param h    the ImageHandle to load.
     */
    synchronized void add(ImageHandle h) {
    handles.addElement(h);
    notifyAll();
    }

    /**
     * Remove an ImageHandle from the image read queue. It won't
     * cancel the request if it has already started.
     */
    synchronized void remove(ImageHandle h) {
    handles.removeElement(h);
    }

    synchronized int imagesPending() {
    return handles.size() + fetcherCount;
    }


    public void run() {
    try {
        ImageHandle h;

        setPriority(Thread.MIN_PRIORITY+1);
        while (true) {
        waitForRequest();

        h = getNextRequest();

        if (h != null) {
            try {
            forkImageFetcher(h);
            } catch(Exception e) {
//            System.out.print("Error in image reader: ");
//            e.printStackTrace();
            }
        }
        }
    } catch (ThreadDeath d) {
        for (Enumeration e = handles.elements() ; e.hasMoreElements() ;) {
        ImageHandle h = (ImageHandle)e.nextElement();
        h.setImage("Image fetch cancelled, try again");
        }
        stopFetches();
    }
    }
}