The Java™ Tutorials
Hide TOC
Handling Initialization Status With Event Handlers
Trail: Deployment
Lesson: Java Applets
Section: Doing More With Applets

Handling Initialization Status With Event Handlers

An applet cannot handle requests from JavaScript code in the web page until the applet has been initialized. A call to an applet method or access to an applet variable from JavaScript code will be blocked until the applet's init() method is complete or the applet first invokes JavaScript code from the web page in which it is deployed. As the JavaScript implementation is single-threaded in many browsers, the web page may appear to be frozen during applet startup.

Beginning in the JDK 7 release, you can check the status variable of the applet while it is loading to determine if the applet is ready to handle requests from JavaScript code. You can also register event handlers that will automatically be invoked during various stages of applet initialization. To leverage this functionality, the applet should be deployed with the java_status_events parameter set to "true".

In the Status and Event Handler example, JavaScript code registers an onLoad handler with the applet. The onLoad handler is automatically invoked by the Java Plug-in software when the applet has been initialized. The onLoad handler invokes other methods of the applet to draw the graph on the web page. The init method of the DrawingApplet class sleeps for two seconds to simulate a long applet initialization period.

The following steps describe how to register event handlers and check an applet's status. See Applet Status and Event Handlers for a complete list of applet status values and applet events for which event handlers can be registered.

  1. Create a JavaScript function to register event handlers. The following code snippet shows the registerAppletStateHandler function that registers an onLoad event handler if the applet has not already loaded.
    <!-- ... -->
        var READY = 2;
        function registerAppletStateHandler() {
            // register onLoad handler if applet has
            // not loaded yet
            if (drawApplet.status < READY)  {                 
                drawApplet.onLoad = onLoadHandler;
            } else if (drawApplet.status >= READY) {
                // applet has already loaded or there
                // was an error
                document.getElementById("mydiv").innerHTML = 
                  "Applet event handler not registered because applet status is: "
                   + drawApplet.status;    
        function onLoadHandler() {
            // event handler for ready state
            document.getElementById("mydiv").innerHTML =
                "Applet has loaded";
    <!-- ... -->
  2. Invoke the previously created registerAppletStateHandler function in the body tag's onload method. This ensures that the HTML tag for the applet has been created in the Document Object Model (DOM) tree of the web page before the applet's event handlers are registered.
    <body onload="registerAppletStateHandler()">
  3. Deploy the applet with the java_status_events parameter set to "true".
    <script src=
        // set java_status_events parameter to true 
        var attributes = { id:'drawApplet',
            archive: 'applet_StatusAndCallback.jar',
            width:600, height:400} ;
        var parameters = {java_status_events: 'true', permissions:'sandbox' } ;
        deployJava.runApplet(attributes, parameters, '1.7');

Open AppletPage.html in a browser to view the behavior of applet event handlers. In the AppletPageUpdatedDuringLoading.html page, the status variable of the applet is checked to determine if the applet has been loaded. Based on the status, the web page is continuously updated while the applet is being loaded.

Note:  If you don't see the applet running, you need to install at least the Java SE Development Kit (JDK) 7 release.

Note:  If you don't see the example running, you might need to enable the JavaScript interpreter in your browser so that the Deployment Toolkit script can function properly.

Download source code for the Status and Event Handler example to experiment further.

Previous page: Invoking Applet Methods From JavaScript Code
Next page: Manipulating DOM of Applet's Web Page