Switching Execution Threads in a Debugging Session

<< 'jdb' - The Java Debugger

<< Java Tool Tutorials - Herong's Tutorial Notes

This section provides a tutorial example on how to list executiong threads and switch debugging control to different threads.

5. Switching threads:

main[1] cont
>
Breakpoint hit: "thread=main", PrimeNumberSeeker.main(), line=19 bci=25
19             System.out.println( i+", "+t.current+", "+t.count);

main[1] print t.current
 t.current = 14

main[1] print t.count
 t.count = 6

main[1] where all
...
main:
  [1] PrimeNumberSeeker.main (PrimeNumberSeeker.java:19)
Thread-0:
  [1] java.lang.Thread.sleep (native method)
  [2] PrimeNumberSeeker.run (PrimeNumberSeeker.java:42)

main[1] threads
...
Group main:
  (java.lang.Thread)0x1         main          running
  (PrimeNumberSeeker)0x118      Thread-0      running (at breakpoint)

main[1] thread 280

Thread-0[1] list
Current method is native

Thread-0[1] step out
11, 22, 8
>
Step completed: "thread=Thread-0", PrimeNumberSeeker.run(), line=45
45             }

Thread-0[1] list
41             try {
42                sleep(delay);
43             } catch (InterruptedException e) {
44                System.out.println("Runner interrupted.");
45 =>          }
46          }
47       }
48    }

Thread-0[1] print count
 count = 8

What I have done here:

• I let the main thread executed a full period stopping at line 19 again. Of course, the sub thread execute some number of statements and stopped inside the sleep() native method.
• Then I listed all threads and want to find a way to switch the command prompt to the sub thread to check local variable there. The manual says to use "thread n", where n is the thread index to change the current thread. But where can I to get the thread index? I tried to search the Web and did not get any clear answer.
• However I got the answer from the output of the threads output. The thread index was listed as a hex number next to the class name. For example, "(PrimeNumberSeeker)0x118" means thread 0x118 = 280.
• So I used "thread 280" to switch to the sub thread as the current thread. Notice that the command prompt changed.
• The first "list" command did not work, because the sub thread was stopped inside the "sleep()" method. The "step out" command continued the execution just enough to finish "sleep()" and back to the caller, run().

Sections in This Chapter

'jdb' - Java Debugger Command and Options

Starting a Debugging Session with 'jdb'

Debugging Applications with Separate 'jdb' Sessions

Debugging Java Applications Remotely

Listing Debugging Commands with 'help' Command

PrimeNumberSeeker.java - Multi-Thread Sample Program

Starting Debugging Session on a Multi-Thread Application

Stepping through Statements of the Child Thread

Checking Variable Values in a Debugging Session

Debugging the Main Thread of a Multi-Thread Application

Switching Execution Threads in a Debugging Session

Suspending Main Thread to Debug Child Thread