The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.
See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.
See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.
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
class sleeps for two seconds to simulate a long applet initialization period.DrawingApplet
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.
registerAppletStateHandler
function that registers an onLoad
event handler if the applet has not already loaded.
<script> <!-- ... --> 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"; draw(); } <!-- ... --> </script>
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()">
java_status_events
parameter set to "true"
.
<script src= "https://www.java.com/js/deployJava.js"></script> <script> // set java_status_events parameter to true var attributes = { id:'drawApplet', code:'DrawingApplet.class', archive: 'applet_StatusAndCallback.jar', width:600, height:400} ; var parameters = {java_status_events: 'true', permissions:'sandbox' } ; deployJava.runApplet(attributes, parameters, '1.7'); </script>
Open
in a browser to view the behavior of applet event handlers. In the
AppletPage.html
page, the AppletPageUpdatedDuringLoading.html
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.
Download source code for the Status and Event Handler example to experiment further.