How do you debug Java Applets?

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.

The applet viewer supports debug options.

Stack traces from uncaught exceptions will appear to the console. This can be enabled from the Java Control Panel (Advanced > Java console > Show console) or some browsers have various options or plugins for enabling it.

You can attach a debugger to the running PlugIn process.

Perhaps the best way is not to debug at all. Write tests. Write code that doesn't couple to unnecessary assumptions - for instance that you are running as an applet. Unfortunately most GUI/applet example code is written very badly.

Uncaught exceptions are sent to the console. You can also use System.out to write your own messages to the console. To view the results you'll need to open the console by right clicking the Java icon in the systray and opening the console (note this is different for Microsoft's VM).

To debug applets properly, you can setup Eclipse to debug applets. Right click the applet source file and click Debug as Applet. (If you have parameters for the applet you'll need to set this up.) Then you can step through the applet code as you would debug any other Java code.