ajdb

Example 2. Command line use

Suppose you want to debug the file spacewar/Ship.java found in the examples directory. At the command line start up the debugger: ajdb

The debugger will first look for initialization files in your home or current directory called either ajdb.ini or .ajdbrc and execute the commands contained in them. A useful command to have in this file is the source-path command which tells the debugger where to find source files.

For this example, we need to set the source path by: use C:\src

To view the file to debug, type list spacewar/Ship.java which generates the following output:

  	  209 void fire() {
  	  210     // firing a shot takes energy
  	  211     if (!expendEnergy(BULLET_ENERGY))
  	  212     return;
  	  213
  	  214     //create a bullet object so it doesn't hit the ship that's firing it
  	  215     double xV = getXVel() + BULLET_SPEED * (Math.cos(orientation));
  	  216     double yV = getYVel() + BULLET_SPEED * (Math.sin(orientation));
  	  217
  	  218     // create the actual bullet
  	  219     new Bullet(
  	  220         getGame(),
  	  221         (getXPos() + ((getSize()/2 + 2) * (Math.cos(orientation))) + xV),
  	  222         (getYPos() + ((getSize()/2 + 2) * (Math.sin(orientation))) + yV),
  	  223         xV,
  	  224         yV);
  	  225 }
  	

This is different from jdb because it allows one to view files before the debugger has started. The list command has the following syntax:

To set a breakpoint in the method Ship.fire, we would could type stop in spacewar.Ship.fire.

The following message appears notifying the user that the breakpoint has been noted but will not be set until the class has been loaded by the VM:

  	Deferring breakpoint spacewar.Ship.fire()
  	It will be set after the class is loaded.
        

To start Spacewar we type run spacewar.Game.

When the breakpoint is set, the following message appears:

  	  Set deferred breakpoint spacewar.Ship.fire()
  	

We are notified that we've hit the breakpoint:

  	  Breakpoint hit: thread="Thread-2", spacewar.Ship.fire(), line=174, bci=0 209 void fire() {
  	

The prompt changes to present the thread that has broken, and we can view the current stack with the where command, as follows:

  	  Thread-2[1] where
  	  [1] fire (spacewar\Ship.java:209)
  	  [2] run (spacewar\Robot.java:100)
  	  [3] run [class java.lang.Thread]
  	

Next, to stop on line 216 we type stop on spacewar/Ship.java:216

The following message tells us the breakpoint was set:

  	  Set breakpoint Ship.java:216
  	

To continue execution, we type cont and the breakpoint at line 216 is hit

        
  	  Breakpoint hit: thread="Thread-2", spacewar.Ship.fire(), line=216, bci=28
  	  216 double yV = getYVel() + BULLET_SPEED * (Math.sin(orientation));
  	

To view the visible local variables, we type locals and ajdb responds with:

        
  	  Local variables
  	  xV = 12.242462584304468
  	

To change the value of the local variable i to 15, we type set xV = 16.1

  	  Changed 'xV' from '12.242462584304468' to '16.1'
  	

To see our changes we can print the value of i by the following:

  	  print xV
  	  Value for printing 'xV' = 12.242462584304468
  	

We can now type exit or quit to leave the debugger, and we receive the following message:

  	  The application has exited.