How do you debug Java Applets?

Question

Currently, the only information I have is a one-line error message in the browser's status-bar.

Do you know how I could get a stack-trace for example ?

Answer 1

This article is a bit old but is still relevant (including a section entitled "How to Debug Applets in Java Plug-in").

Edit: perhaps a better way to get stacktraces is to use the Java plugin console. If you hit "t" in that window, you'll see the following:

Prints out all the existing thread groups. The first group shown is Group main. ac stands for active count; it is the total number of active threads in a thread group and its child thread groups. agc stands for active group count; it is the number of active child thread groups of a thread group. pri stands for priority; it is the priority of a thread group. Following Group main, other thread groups will be shown as Group , where name is the URL associated with an applet. Individual listings of threads will show the thread name, the thread priority, alive if the thread is alive or destroyed if the thread is in the process of being destroyed, and daemon if the thread is a daemon thread.

The other command that I've used most often from that console is the trace level from 0-5:

This sets the trace-level options as described in the next section, Tracing and Logging.

From that page, you'll see that the levels look like this:

• 0 — off
• 1 — basic
• 2 — network, cache, and basic
• 3 — security, network and basic
• 4 — extension, security, network and basic
• 5 — LiveConnect, extension, security, network, temp, and basic

These tools can all be fairly useful as you're trying to unravel what in the world has gotten into the head of your applets. I know that they've worked for me.

Answer 2

Aside from the obvious use of the Java console and the applet viewer, starting from Java 6 update 7, you can use the VisualVM that comes with the JDK (JDK_HOME/bin/visualvm). It allows you to view the stack traces of each thread and even view all object instances.

AppletViewer is very handy, you can do a "Run as / Java Applet" from Eclipse to run, or "Debug As / Java Applet" to debug your applet classes.

However, sometimes to debug some security related stuff the browser plugin environment is just too different from appletviewer. Here's what you can do to effectively debug applets in the browser:

1) Obtain debugging info for the binaries

Backup the .jar files from JRE_HOME/lib

(Download and) Install a JDK for the same version as your JRE.

Copy the .jar files from JDK_HOME/jre/lib to JRE_HOME/lib

The files inside the JDK were compiled with the debugging information included (source-code line number information, variable names, etc) and the JRE files don't have this information.

Without this you won't be able to meaningfully step into core class code in your debugger.

2) Enable debugging for the Java Plug-in

Go to the Java Control Panel / Java / Java Runtime Settings / View / User / Runtime Parameters

And add the options to enable debugging. Something like this:

-Djava.compiler=NONE -Xnoagent -Xdebug -Xrunjdwp:transport=dt_socket,address=2502,server=y,suspend=n 

The interesting options are the port (using 2502 here, you can use pretty much any free port, just write it down for later) and the suspend - if you need to debug the applet startup, classloading, etc, set this to "y". That way when you access an applet page, the browser will appear to freeze as the JVM immediately gets suspended waiting for a debugger to connect.

3) Use your favorite IDE to Remotely debug the Java Plug-in

In Eclipse, for instance, choose Run / Debug Configurations ... / Remote Java Application

Click on the "New" button.

Make sure connection type is "Socket Attach", choose localhost as the host if your browser is local, and the port you chose earlier (2502 in the example).

You might have to inlude the src.zip in your JDK on the sources tab to have the Java core class sources available.

Save the configuration, and once your browser is running the plug-in (with the JVM suspended or not) run the remote debugger to connect to the plug-in JVM, with a project containing your applet sources open.