Previous section: Web Page to Applet Communication

Applet to Web Page Communication

o       The MAYSCRIPT Tag

o       How Applet to Java-Script communication works in Java Plug-in

o       JSObject Support in Java Plug-in

The MAYSCRIPT Tag

The MAYSCRIPT tag for the Applet was introduced with Netscape Navigator 3. It allows a Java applet to communicate and interact with the document. If an applet tries to access JavaScript without this attribute, the browser will generate an error. There is no value assigned to this attribute, as the presence of the MAYSCRIPT attribute activates the on state

The Browser versions that support the MAYSCRIPT attribute are –

o       Internet Explorer 4.0 and above

o       Netscape Navigator 3.0 and above

How Applet to Java-Script Communication works in Java Plug-in

The Applet to JavaScript communication needs the MAYSCRIPT attribute to be activated before it can take place.

Java applets may need to perform Java to JavaScript communication to access the Document Object Model (DOM) or to call JavaScript functions on an HTML page. Internet Explorer and Navigator allow communication between Java to JavaScript through the Java wrapper class netscape.javascript.JSObject.

Because of differences in DOM implementations between browsers, Java Plug-in 1.2.2 provides different degrees of support for JSObject in Internet Explorer and Netscape Navigator.

In general, the applets can access the JavaScript objects in the following way –

import netscape.javascript.*;

import java.applet.*;

class MyApplet extends Applet {

     public void init() {

         JSObject win = JSObject.getWindow(this);

         JSObject doc = (JSObject) win.getMember("document");

         JSObject loc = (JSObject) doc.getMember("location");

         String s = (String) loc.getMember("href");  // document.location.href

         win.call("func", null);            // Call func() in HTML page

     }

}

The getWindow() function will return a JSObject that represents the Window object in the JavaScript. This function takes an applet as a parameter. Once the Window object is got, the DOM of the HTML page can be accessed with following functions –

public Object call(String methodName, Object args[])

public Object eval(String s)

public Object getMember(String name)

public Object getSlot(int index)

public void removeMember(String name)

public void setMember(String name, Object value)

public void setSlot(int index, Object value)

public String toString()

The implementations of getSlot(), setSlot(), removeMember() and toString() are browser dependant.

JSObject Support in Java Plug-in

The Java Plug-in by default does not enable the MAYSCRIPT tag. For the Java Plug-in to support the JSObject, the MAYSCRIPT tag should be enabled in the <OBJECT> and <EMBED> tags as follows –

APPLET TAG

<APPLET code="XYZApp.class" codebase="html/" align="baseline"  width="200" height="200"  MAYSCRIPT>

<PARAM NAME="car" VALUE="Mercedes">

</APPLET>

NEW OBJECT TAG

            <OBJECT classid="clsid:8AD9C840-044E-11D1-B3E9-00805F499D93"

                        width="200" height="200" align="baseline"

                        codebase="http://java.sun.com/products/plugin/1.2/jinstall-12-win.cab#Version=1,2,0,0">

                        <PARAM NAME="code" VALUE="XYZApp.class">

            <PARAM NAME="codebase" VALUE="html/">

                        <PARAM NAME= "type"VALUE="application/x-java-applet;version=1.2">

            <PARAM NAME="MAYSCRIPT" VALUE="true">

                        <PARAM NAME="car" VALUE="Mercedes">

</OBJECT>

NEW EMBED TAG

<EMBEDtype="application/x-java-applet;version=1.2" width="200"

   height="200" align="baseline" code="XYZApp.class"

   codebase="html/" car="Mercedes" MAYSCRIPT=true

   pluginspage="http://java.sun.com/products/plugin/1.2/plugin-install.html">

</EMBED>

Next section: Solution – Invoking the JNI calls from Applets

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