Sending Messages to Other Applets

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Trail: Writing Applets
Lesson: Taking Advantage of the Applet API

Sending Messages to Other Applets

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 getApplet method) or by finding all the applets on the page (using the AppletContext getApplets method). Both methods, if successful, give the caller one or more Applet objects. Once the caller finds an Applet object, the caller can invoke methods on the object.

Finding an Applet by Name: The getApplet Method

The getApplet method looks through all of the applets on the current page to see if one of them has the specified name. If so, getApplet returns the applet's Applet object.
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 NAME attribute within the applet's <APPLET> tag. For example:
<APPLET CODEBASE=example/ CODE=Sender.class
    WIDTH=450
    HEIGHT=200
    NAME="buddy">
. . .
</applet>
By specifying a NAME parameter 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 the getApplet method to perform a case-insensitive search. For example, getApplet("old pal") and getApplet("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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This figure has been reduced to fit on the page.
Click the image to view it at its natural size.

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 in buddy and 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.
Applet receiver = null;
String receiverName = nameField.getText(); //Get name to search for.
receiver = getAppletContext().getApplet(receiverName);
The Sender goes on to make sure that the Receiver was found and that it's an instance of the correct class (Receiver). If all goes well, the Sender sends a message to the Receiver. (Here's the Receiver 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 the Applet getParameter method. 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);

Finding All the Applets on a Page: The getApplets Method

The getApplets method returns a list (an Enumeration, to be precise) of all the applets on the page. For security reasons, many browsers and applet viewers implement getApplets so that it returns only those applets that originated from the same host as the applet calling getApplets. 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 the whole 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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