Question

I have a Java applet on a webp age which I am trying to code against with Javascript.

public class WebCapture extends Applet {

  public String[] getSourceDeviceNames() {
    return new String[] {'Device 1', 'Device 2'};
  }

  public void acquireImage() {
    //...
  }

}

I compile this applet and supporting libraries into a jar file and it is included in my HTML page like so:

<embed id="webCaptureApplet"
 code="com.brian.webcapture.applets.WebCapture"
 archive="jar/WebCapture.jar"
 type="application/x-java-applet">

    <noembed>
      Oops! It looks like Java is not installed. =[
    </noembed>

</embed>  

This seems to be effective, I see a blank Java applet rendered to the screen and if I call

document.getElementById('webCaptureApplet');

I can alert() the object and receive a response [object HTMLEmbedElement]

Great. Now, when I try to execute the following code:

var applet = document.getElementById('webCaptureApplet');
var devices = applet.getSourceDeviceNames();

for(var i = 0; i < devices.length; i++) {
  alert(devices[i]);
}

When I attach a breakpoint to the line containing for() and inspect the object with firebug, the toString() value of devices is [Ljava.lang.String;@1d0b3465. I think this makes sense as an array is being passed back and it is not toString() friendly for showing a simple value.

When I look at devices.length in the foor loop, the value shown is the method stub for a public netscape.javascript.JSException(java.lang.String, java.lang.String, int, java.lang.String, int)

I have read countless how to’s on Javascript calling Java and I can not see where I have gone wrong with what seems to be pretty seamless interop.

I should say that at one point it was working through Firefox on OS X, but it is holding on to a cached applet, so I am confused what the problem is there. I have tried deleting my cache on both my work station and my laptop and they still seem to cling to old jar’s. I am not sure how much this is effecting me.

It seems to be that it does not make a difference if my applet’s getSourceDeviceNames returns a string or an array of strings. It will still show the same errors on inspection. I am not sure if the cache is effecting that.

Answer 1

According to http://www.uow.edu.au/~hasan/buss930/jscript/packages.htm#1014128
“Java arrays are wrapped with a JavaScript object that understands array.length and array[index].”
which works fine with Sun Java, but returns the JSException while running on OpenJDK.

The only method I was able to find, is to count the elements of the array.

var length = 0;
while(array[length])
    length++;

It seems, that this feature is buggy in OpenJDK.