'jdb' - The Java Debugger

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(Continued from previous part...)

Debugging Commands

Ok, I think we did enough about launching the debugger and connecting to the target application. Let's now move on to look at some debugging commands.

Here is list of commonly used debugging commands. I got this list by using "help" at debugging prompt.

run [class [args]]        -- start execution of an application

threads [threadgroup]     -- list threads
thread <thread id>        -- set default thread
suspend [thread id(s)]    -- suspend threads (default: all)
resume [thread id(s)]     -- resume threads (default: all)
where [<thread id> | all] -- dump a thread's stack
up [n frames]             -- move up a thread's stack
down [n frames]           -- move down a thread's stack
kill <thread id> <expr>   -- kill a thread with the given exception
interrupt <thread id>     -- interrupt a thread

print <expr>              -- print value of expression
dump <expr>               -- print all object information
eval <expr>               -- evaluate expression (same as print)
set <lvalue> = <expr>     -- assign new value to a variable
locals                    -- print all local variables

classes                   -- list currently known classes
class <class id>          -- show details of named class
methods <class id>        -- list a class's methods
fields <class id>         -- list a class's fields

threadgroups              -- list threadgroups
threadgroup <name>        -- set current threadgroup

stop in <class id>.<method>[(argument_type,...)]
                          -- set a breakpoint in a method
stop at <class id>:<line> -- set a breakpoint at a line
clear <class id>.<method>[(argument_type,...)]
                          -- clear a breakpoint in a method
clear <class id>:<line>   -- clear a breakpoint at a line
clear                     -- list breakpoints
catch [uncaught|caught|all] <class id>|<class pattern>
                          -- break when specified exception occurs
ignore [uncaught|caught|all] <class id>|<class pattern>
                          -- cancel 'catch'

watch [access|all] <class id>.<field name>
                          -- watch access/modifications to a field
unwatch [access|all] <class id>.<field name>
                          -- discontinue watching
trace methods [thread]    -- trace method entry and exit
untrace methods [thread]  -- stop tracing method entry and exit
step                      -- execute current line
step up                   -- execute until the current method returns
stepi                     -- execute current instruction
next                      -- step one line (step OVER calls)
cont                      -- continue execution from breakpoint

list [line number|method] -- print source code
use (or sourcepath) [source file path]
                          -- display or change the source path
classpath                 -- print classpath info from target VM

monitor <command>         -- execute command each time the program stops
monitor                   -- list monitors
unmonitor <monitor#>      -- delete a monitor
read <filename>           -- read and execute a command file

lock <expr>               -- print lock info for an object
threadlocks [thread id]   -- print lock info for a thread

disablegc <expr>          -- prevent garbage collection of an object
enablegc <expr>           -- permit garbage collection of an object

!!                        -- repeat last command
<n> <command>             -- repeat command n times
help (or ?)               -- list commands
version                   -- print version information
exit (or quit)            -- exit debugger

Multi-Thread Debugging Exercise

To help me practice debugging commands, I wrote the following simple application, PrimeNumberSeeker.java:

 1 /**
 2  * PrimeNumberSeeker.java
 3  * Copyright (c) 2003 by Dr. Herong Yang, http://www.herongyang.com/
 4  */
 5 public class PrimeNumberSeeker extends Thread {
 6    private static final int ceiling = 100;
 7    private static final int interval = 1000;
 8    private static final int delay = 100;
 9    public int count = 0;
10    public int current = 2;
11    public int[] primes = null;
12    public static void main(String[] a) {
13       System.out.println("Period, Current int, # primes");
14       PrimeNumberSeeker t = new PrimeNumberSeeker();
15       t.start();
16       int i = 0;
17       while (true) {
18          i++;
19          System.out.println( i+", "+t.current+", "+t.count);
20          try {
21             sleep(interval);
22          } catch (InterruptedException e) {
23             System.out.println("Monitor interrupted.");
24          }
25       }
26    }

(Continued on next part...)

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