Applet間的通訊方法是什么

本篇內(nèi)容主要講解“Applet間的通訊方法是什么”，感興趣的朋友不妨來看看。本文介紹的方法操作簡單快捷，實用性強。下面就讓小編來帶大家學(xué)習(xí)“Applet間的通訊方法是什么”吧!

Applet間的通訊(2)

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  使用靜態(tài)變量進行通訊并不意味著applet就可以同時被初始化。不同的實例還是先后啟動的，我們并不能保證它們的啟動次序。然而，有一點我們可以確定：在第一個ColorRelay applet實例被創(chuàng)建之前，類ColorRelay已經(jīng)初始化了，因此，當(dāng)所有applet實例啟動時，它們都將獲得類的靜態(tài)變量。

  但是，當(dāng)你使用靜態(tài)變量時必須小心，因為多個實例可能同時訪問靜態(tài)變量。為了解決這個問題，我使用了兩個同步方法（synchronized methods）從鏈表中增減applet實例。因為它們是synchronized static方法，當(dāng)它們運行時，ColorRelay 類會被索住，以避免并發(fā)訪問。清單1.3是這兩個方法的代碼。值得注意的是，當(dāng)?shù)谝粋€元素被加到鏈表之后，主控線程（controller thread）將啟動。我們隨后會看到當(dāng)最后一個元素從鏈表中被移出后，這個線程將自動停止。

清單 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 not 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;
 }

初始化共享數(shù)據(jù)

  Applet被創(chuàng)建之后init方法被調(diào)用，這個方法檢查、轉(zhuǎn)換和存儲applet的參數(shù)。對image參數(shù)需要額外的注意，因為它是存儲在另一個靜態(tài)變量中的。在試圖訪問originalImage靜態(tài)變量之前，要鎖住ColorRelay類，我們沒有使用synchronized 方法，而是使用一段synchronized 監(jiān)測語句來實現(xiàn)這個目的。（事實上，應(yīng)該只有一個ColorRelay實例獲得image參數(shù)，但為了防止HTML中的編碼錯誤，我們采取了上述的預(yù)防措施）。清單1.4是init的代碼。

清單 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

  當(dāng)瀏覽器準備執(zhí)行applet時，start 方法被調(diào)用，將applet加入鏈表。在stop方法中，applet被譯出鏈表。在前面的代碼中你已經(jīng)看到，第一個鏈表元素的添加將導(dǎo)致控制線程的啟動。這個控制線程僅僅是循環(huán)讀取鏈表元素，一次將鏈表中的某個元素點亮（顯示彩色圖片）。至于顯示的持續(xù)時間，是由applet自己決定的。如果鏈表中沒有元素了，控制線程自動終止。清單1.5是控制線程的strat、stop以及run方法。

清單 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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