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Applets can find other applets and send messages to them, with the following security restrictions:
- Many browsers require that the applets originate from the same server.
- Many browsers further require that the applets originate from the same directory on the server (the same code base).
- The Java API requires that the applets be running on the same page, in the same browser window.
Note: Some browsers let applets invoke methods on other applets -- even applets on different pages in the same browser -- as long as all of the applets come from the same code base. This method of interapplet communication isn't supported by the Java API, so it's possible that it will not be supported by all browsers.An applet can find another applet either by looking it up by name (using the
AppletContext getAppletmethod) or by finding all the applets on the page (using theAppletContext getAppletsmethod). Both methods, if successful, give the caller one or moreAppletobjects. Once the caller finds anAppletobject, the caller can invoke methods on the object.
The
getAppletmethod looks through all of the applets on the current page to see if one of them has the specified name. If so,getAppletreturns the applet'sAppletobject.By default, an applet has no name. For an applet to have a name, one must be specified in the HTML code that adds the applet to a page. You can specify an applet's name in two ways:
- By specifying a
NAMEattribute within the applet's<APPLET>tag. For example:<APPLET CODEBASE=example/ CODE=Sender.class WIDTH=450 HEIGHT=200 NAME="buddy"> . . . </applet>- By specifying a
NAMEparameter with a<PARAM>tag. For example:<APPLET CODEBASE=example/ CODE=Receiver.class WIDTH=450 HEIGHT=35> <PARAM NAME="name" value="old pal"> . . . </applet>
Browser Note: Although at least one Java-enabled browser conducts a case-sensitive search, the expected behavior is for thegetAppletmethod to perform a case-insensitive search. For example,getApplet("old pal")andgetApplet("OLD PAL")should both find an applet named "Old Pal".Below are two applets that illustrate lookup by name. The first, the Sender, looks up the second, the Receiver. When the Sender finds the Receiver, the Sender sends a message to the Receiver by invoking one of the Receiver's methods (passing the Sender's name as an argument). The Receiver reacts to this method call by changing its leftmost text string to "Received message from sender-name!".
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Note: If you don't see the applet running above, you need to install Java Plug-in, which happens automatically when you install the J2SE JRE or SDK. We strongly recommend that you install the latest version; at least 1.3.1 is required for all our applets. You can find more information in the Java Plug-in home page.
Try this: Click the Send message button of the top applet (the Sender). Some status information will appear in the Sender's window, and the Receiver will confirm (with its own status string) that it received a message, After you've read the Receiver status string, press the Receiver's Clear button to reset the Receiver. In the Sender's text field labeled "Receiver name:," type inbuddyand press Return. Since "buddy" is the Sender's own name, the Sender will find an applet named buddy but won't send it a message, since it isn't a Receiver instance.Here's the
whole Sender program. The code it uses to look up and communicate with the Receiver is listed below. Code that you can use without change in your own applet is in bold font.
The Sender goes on to make sure that the Receiver was found and that it's an instance of the correct class (Applet receiver = null; String receiverName = nameField.getText(); //Get name to search for. receiver = getAppletContext().getApplet(receiverName);Receiver). If all goes well, the Sender sends a message to the Receiver. (Here's theReceiver program.)
if (receiver != null) { //Use the instanceof operator to make sure the applet //we found is a Receiver object. if (!(receiver instanceof Receiver)) { status.appendText("Found applet named " + receiverName + ", " + "but it's not a Receiver object.\n"); } else { status.appendText("Found applet named " + receiverName + ".\n" + " Sending message to it.\n"); //Cast the receiver to be a Receiver object //(instead of just an Applet object) so that the //compiler will let us call a Receiver method. ((Receiver)receiver).processRequestFrom(myName); } } . . .From an applet's point of view, its name is stored in a parameter named
NAME. It can get the value of the parameter using theApplet getParametermethod. For example, Sender gets its own name with the following code:myName = getParameter("NAME");For more information on using
getParameter, see Writing the Code to Support Parameters.The example applets in this page perform one-way communication -- from the Sender to the Receiver. If you want your receiver to be able to send messages to the sender, then you just need to have the sender give a reference to itself (
this) to the receiver. For example:((Receiver)receiver).startCommunicating(this);
ThegetAppletsmethod returns a list (anEnumeration, to be precise) of all the applets on the page. For security reasons, many browsers and applet viewers implement
getAppletsso that it returns only those applets that originated from the same host as the applet callinggetApplets. Here's an applet that simply lists all the applets it can find on this page:
Note: If you don't see the applet running above, you need to install Java Plug-in, which happens automatically when you install the J2SE JRE or SDK. We strongly recommend that you install the latest version; at least 1.3.1 is required for all our applets. You can find more information in the Java Plug-in home page.Below are the relevant parts of the method that calls
getApplets. (Here's thewhole program.)
public void printApplets() { //Enumeration will contain all applets on this page (including //this one) that we can send messages to. Enumeration e = getAppletContext().getApplets(); . . . while (e.hasMoreElements()) { Applet applet = (Applet)e.nextElement(); String info = ((Applet)applet).getAppletInfo(); if (info != null) { textArea.appendText("- " + info + "\n"); } else { textArea.appendText("- " + applet.getClass().getName() + "\n"); } } . . . }
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