作者：未知 来源：月光软件站 加入时间：2005-2-28　月光软件站

Tricks
     of the
      Java Programming Gurus

by Glenn L. Vanderburg. et al.

1.Applet间的通讯(2)

相互查找

    使用静态变量进行通讯并不意味着applet就可以同时被初始化。不同的实例还是先后启动的，我们并不能保证它们的启动次序。然而，有一点我们可以确定：在第一个ColorRelay applet实例被创建之前，类ColorRelay已经初始化了，因此，当所有applet实例启动时，它们都将获得类的静态变量。

    但是，当你使用静态变量时必须小心，因为多个实例可能同时访问静态变量。为了解决这个问题，我使用了两个同步方法（synchronized methods）从链表中增减applet实例。因为它们是synchronized static方法，当它们运行时，ColorRelay 类会被索住，以避免并发访问。清单1.3是这两个方法的代码。值得注意的是，当第一个元素被加到链表之后，主控线程（controller thread）将启动。我们随后会看到当最后一个元素从链表中被移出后，这个线程将自动停止。

清单 1.3. ColorRelay.java (part 2).

    /**
     * Adds an instance to the list of active instances maintained in the
     * class.  No check is made to prevent adding the same instance twice.
     * @param elem the ColorRelay instance to add to the list.
     * @see #removeFromList
     */
    static synchronized void addToList(ColorRelay elem) {
        if (list == null) {
            list = listTail = elem;
            elem.next = elem.prev = null;

            // Because the list has elements now, we should start the thread.
            relayer = new Thread(new ColorRelay());
            relayer.start();
        }
        else {
            elem.prev = listTail;
            listTail.next = listTail = elem;
            elem.next = null;
        }
    }

    /**
     * Removes an instance from the list of active instances maintained in
     * the class.  Works properly but does <em>not</em> signal an error if
     * the element was not actually on the list.
     * @param elem the ColorRelay instance to be removed from the list.
     * @see #addToList
     */
    static synchronized void removeFromList(ColorRelay elem) {
        ColorRelay curr = list;
        while (curr != null && curr != elem) {
            curr = curr.next;
        }

        if (curr == elem) {     // We found it!
            if (list == curr) {
                list = curr.next;
            }
            if (listTail == curr) {
                listTail = curr.prev;
            }
            if (curr.next != null) {
                curr.next.prev = curr.prev;
            }
            if (curr.prev != null) {
                curr.prev.next = curr.next;
            }
            curr.next = curr.prev = null;
        }
        // If curr is null, then the element is not on the list
        // at all.  We could treat that as an error, but I'm
        // choosing to report success.

        return;
    }

初始化共享数据

    Applet被创建之后init方法被调用，这个方法检查、转换和存储applet的参数。对image参数需要额外的注意，因为它是存储在另一个静态变量中的。在试图访问originalImage静态变量之前，要锁住ColorRelay类，我们没有使用synchronized 方法，而是使用一段synchronized 监测语句来实现这个目的。（事实上，应该只有一个ColorRelay实例获得image参数，但为了防止HTML中的编码错误，我们采取了上述的预防措施）。清单1.4是init的代码。

清单 1.4. ColorRelay.java (part 3).

    /**
     * Initializes the applet instance.  Checks and stores
     * parameters and initializes other instance variables.
     */
    public void init() {
        String flash = getParameter("flashColor");
        if (flash != null) {
            try {
                flashColor = new Color(parseRGB(flash));
            }
            catch (NumberFormatException e) {
                // Ignore a bad parameter and just go with the default.
            }
        }

        String sleep = getParameter("sleepTime");
        if (sleep != null) {
            try {
                sleepSecs = Integer.parseInt(sleep);
            }
            catch (NumberFormatException e) {
                // Ignore a bad parameter and just go with the default.
            }
        }
        
        String imageURL = getParameter("image");
        if (imageURL != null) {
            Class cr = Class.forName("COM.MCP.Samsnet.tjg.ColorRelay");
            synchronized (cr) {
                if (originalImage == null) {
                    originalImage = getImage(getDocumentBase(), imageURL);
                }
            }
        }

        tracker = new MediaTracker(this);
    }

Working Together

    当浏览器准备执行applet时，start 方法被调用，将applet加入链表。在stop方法中，applet被译出链表。在前面的代码中你已经看到，第一个链表元素的添加将导致控制线程的启动。这个控制线程仅仅是循环读取链表元素，一次将链表中的某个元素点亮（显示彩色图片）。至于显示的持续时间，是由applet自己决定的。如果链表中没有元素了，控制线程自动终止。清单1.5是控制线程的strat、stop以及run方法。

清单 1.5. ColorRelay.java (part 4).

    /**
     * Starts the applet running.  The ColorRelay hooks up with
     * other instances on the same page and begins coordinating
     * when this method is called.
     */
    public void start() {
        // Ordinarily, we want to display the original image.
        image = originalImage;

        ColorRelay.addToList(this); // Let's get to work!
    }

    /**
     * Stops the applet.  The ColorRelay instance removes itself from the
     * group of cooperating applets when this method is called.
     */
    public void  stop() {
        ColorRelay.removeFromList(this);
    }

    /**
     * Loops through the list of active instances for as long as it is
     * non-empty, calling each instance's 'flash' method.
     * @see #flash
     */
    public void run () {
        ColorRelay curr;

        // Continue running through the list until it's empty ...
        while (list != null) {
            for (curr = list; curr != null; curr = curr.next) {
                try {
                    curr.flash();
                }
                catch (InterruptedException e) {
                }
            }
        }   
    }

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