debug example

Use jdb for java applications like HelloWorldApp, and pass a -debug flag to appletviewer to debug applets. Before you use the debugger, be sure to compile your application/applet with the -g flag

% javac -g HelloWorld.java

In our example jdb session, we'll use the appletviewer to debug the simple HelloWorld applet:

// Sample HelloWorld applet
//
     import java.awt.Graphics;
     import java.applet.Applet;
 
     public class HelloWorld extends Applet {
         int i;
 
         public void paint (Graphics g){
            i = 10;
            g.drawString("Hello world!", 25, 25);
         }
     }

% appletviewer -debug hw.html

Initializing jdb... 0xed6e1950:class(sun.applet.AppletViewer) >

The help command returns a list of available commands in the jdb
Type threadgroups to show the current list of threadgroups running. The "system" and "main" groups should be running
Type threads to show the complete list of running threads:
Type run to start the appletviewer
The appletviewer runs and displays your HelloWorld message. Try restarting the appletviewer and setting a breakpoint:
% appletviewer -debug hw.html
The where command will show the applet's execution stack:
The use command reports where
is looking for your source files. By default, the CLASSPATH is used (mine is huge so this is edited)
The list command will show the current source code at the current breakpoint
Now we can look at the value of the g object passed from the appletviewer
Finally, the step command provides a way to execute the current line: (see how the => moved from line 10 to line 11)
and exit

Good luck