Call Javascript from a Java applet - Real‘s Java How-to

ShangShujie 2007-06-10

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Call Javascript from a Java applet

Netscape only, using the javascript: protocol
A Javascript function is called from Java by using the showDocument method. A URL is needed with "javascript:" as the protocol.

[Java applet]

import java.applet.*;
import java.net.*;
public class InJava4 extends Applet{
public void init(){
String msg = "Hello from Java (using javascript alert)";
try {
getAppletContext().showDocument
(new URL("javascript:doAlert(\"" + msg +"\")"));
}
catch (MalformedURLException me) { }
}
}

[Javascript and HTML]

<HTML><HEAD></HEAD><BODY>
<SCRIPT>
function doAlert(s) {
alert(s);
}
</SCRIPT>
<APPLET CODE="InJava4.class"
NAME="myApplet"  MAYSCRIPT
HEIGHT=10 WIDTH=10>
</APPLET>
</BODY>
</HTML>

Try it here

Netscape and IE ok, using the netscape.javascript.JSObject package

How to compile when using the netscape.javascript.JSObject package ?

For Java 1.4.2 and later: add plugin.jar to your classpath. It can be found in the lib directory of your JRE installation, e.g. C:\Program Files\Java\jre1.5.0\lib\plugin.jar

For Java 1.4.0/1.4.1: use jaws.jar (same directory).

In the following example, you type in the TextField a Javascript function and press the button to execute the function. For example, try alert(‘Hello from JAVA‘). Or you can execute function defined on the same page as the Applet. The Applet must contains the MAYSCRIPT parameter to be able to use JSObject.

Netscape and IE Ok.

import java.applet.*;
import java.awt.*;
import java.awt.event.*;
import netscape.javascript.*;
public class InJava5 extends Applet implements ActionListener {
Button b;
TextField t;
public void init() {
t = new TextField(20);
add(t);
b = new Button("execute Javascript");
add(b);
b.addActionListener(this);
}
public void actionPerformed(ActionEvent ae) {
if (ae.getSource() == b) {
JSObject win = (JSObject) JSObject.getWindow(this);
win.eval(t.getText());
}
}
}

Try it here

Another way is to use the Reflection API. That way you don‘t need to modify your CLASSPATH for compilation or even import the netscape.jsobject package.

// posted by C Werner on the realhowto list
import java.lang.reflect.*;
...
// Somewhere in the applet class ...
...
String jscmd = "window.close()";  /* JavaScript command */
String jsresult = null;
boolean success = false;
try {
Method getw = null, eval = null;
Object jswin = null;
Class c =
Class.forName("netscape.javascript.JSObject"); /* does it in IE too */
Method ms[] = c.getMethods();
for (int i = 0; i < ms.length; i++) {
if (ms[i].getName().compareTo("getWindow") == 0)
getw = ms[i];
else if (ms[i].getName().compareTo("eval") == 0)
eval = ms[i];
}
}
Object a[] = new Object[1];
a[0] = this;               /* this is the applet */
jswin = getw.invoke(c, a); /* this yields the JSObject */
a[0] = jscmd;
Object result = eval.invoke(jswin, a);
if (result instanceof String)
jsresult = (String) result;
else
jsresult = result.toString();
success = true;
}
catch (InvocationTargetException ite) {
jsresult = "" + ite.getTargetException();
}
catch (Exception e) {
jsresult = "" + e;
}
if (success)
System.out.println("eval succeeded, result is " + jsresult);
else
System.out.println("eval failed with error " + jsresult);