Using a signed Java applet as a Flex helper – Part 1

I want to talk about a proof-of-concept I developed a few months back. The problem was that we needed a Flex 2 application to be able to launch an external, native application to edit a file. Since Flex is strictly limited in how it can interact with the file system and the native OS, this seemed impossible. But then I had the idea of using a signed Java applet to do the hard work. Because it’s Java code, it can interact with the file system and do things that Flex can’t do — like launch external applications.

OK — it’s not an original idea. The Artemis project from EffectiveUI is doing something similar, but for AIR applications.

Keep in mind that I’m not a Java programmer. Nor am I a JavaScript programmer. I’m probably making all sorts of mistakes that will induce a lot of eye rolling. Just try to keep it to yourselves. (I do know Flex and Actionscript, so feel free to openly mock egregious errors in those domains.)

I’m going to break this posting into two parts. In the first part I’ll show how to create a trivial Java applet, have it load simultaneously with a Flex application, and show how the Flex application can communicate with the applet. In the second part, I’ll provide a slightly less trivial Java applet that will open an external editor, and show how to sign the applet and deal with thread synchronization issues (!!).

Step 1. Create the applet

Use your favorite text editor and create the following Java source file:

import java.awt.*;
import java.awt.event.*;
import java.applet.*;
import javax.swing.*;
public class HelloWorld extends Applet
{
	public void sayHelloWorld()
	{
           JOptionPane.showMessageDialog( null, "Hello World", "Hello World", JOptionPane.INFORMATION_MESSAGE );
	}
}

Save the file as “HelloWorld.java” and compile it:
javac HelloWorld
which should create the file HelloWorld.class.

Step 2. Create the Flex project

In Flex builder, create a project called JavaApplet, and replace the contents of JavaApplet.mxml with:

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute">
	<mx:Script>
		<![CDATA[
			private function onClick() : void
			{
                      ExternalInterface.call( "document.helloWorldApplet.sayHelloWorld()", null );
			}
		]]>
	</mx:Script>
	<mx:Button label="Hello World" click="onClick()"/>
</mx:Application>

Copy the HelloWorld.class file created in Step 1 to the bin directory of the Flex project.

Step 3. Change the html wrapper

Edit the Flex applications HTML wrapper template. Insert the following just before the closing </body> tag:

<APPLET id="helloWorldApplet" code="HelloWorld.class" codebase="." align="baseline"
    width="200" height="200">

Step 4. There’s no step 4

Just run the application. If you’ve done everything right, clicking the button should display a JOption pane with the message “hello world”.

(This might not work in Internet Explorer. I think with IE, you need to replace the <APPLET> tag with <OBJECT>. The syntax is subtly different and I haven’t tried it — I don’t have access to a Windows machine — so you’re on your own.)

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