Starting Debugging Session on a Multi-Thread Application

Java Tools Tutorials - Herong's Tutorial Notes - Version 5.32, by Dr. Herong Yang

Java Tools Tutorials - Herong's Tutorial Notes

∟Starting Debugging Session on a Multi-Thread Application

This section provides a tutorial example on how to start a debugging session on a multi-thread Java application, PrimeNumberSeeker.java.

1. Setting up breakpoints and getting the debugging session going:

C:\herong>jdb PrimeNumberSeeker

Initializing jdb ...
> stop in PrimeNumberSeeker.main
Deferring breakpoint PrimeNumberSeeker.main.
It will be set after the class is loaded.

> run
run PrimeNumberSeeker
Set uncaught java.lang.Throwable
Set deferred uncaught java.lang.Throwable
>
VM Started: Set deferred breakpoint PrimeNumberSeeker.main

Breakpoint hit: "thread=main", PrimeNumberSeeker.main(), line=13 
13          System.out.println("Period, Current int, # primes");

main[1] stop in PrimeNumberSeeker.run
Set breakpoint PrimeNumberSeeker.run

main[1] cont
Period, Current int, # primes
1, 2, 0
>
Breakpoint hit: "thread=Thread-0", PrimeNumberSeeker.run(), line=28
28          primes = new int[ceiling];

Thread-0[1] threads
Group system:
  (java.lang.ref.Reference$ReferenceHandler)0x157 Reference Handler...
  (java.lang.ref.Finalizer$FinalizerThread)0x156  Finalizer        ...
  (java.lang.Thread)0x155                         Signal Dispatcher...
  (java.lang.Thread)0x154                         Attach Listener  ...
Group main:
  (java.lang.Thread)0x1         main              sleeping
  (PrimeNumberSeeker)0x189      Thread-0          running (breakpoint)

Ok. What I have done so far:

Started the debugging session with the first breakpoint at the beginning of the main() method, which cause the execution to stop at line 13.
Then I created the second breakpoint at the beginning of the run() method in order to catch the sub thread.
Then I issued the "cont" command. My application continued with the sub thread created stopped at the breakpoint at line 28. The main thread also stopped.
Noticed that the debugger prompt is changed from "main[1]" to "Thread-0[1]". This is to inform you that you are currently in the sub thread, no longer in the main thread.
Then I used the "threads" command to list all threads. I saw two thread groups: "system" and "main". Of course, I am not interested in the "system" group at this point. The "main" shows two threads: "main" (my monitoring thread) and "Thread-0" (my sub thread working on the calculation).
Thread "Thread-0" status shows "running (at breakpoint)". But the word "running" is referring to the thread execution mode, not the current execution status.
Notice that tread "main" is also stopped at this moment. As a general rule, if one thread is stopped, all other threads are stopped also.

Last update: 2015.

Table of Contents

About This Book

Java Tools Terminology

Installing Java 8 on Windows

'javac' - The Java Program Compiler

'java' - The Java Program Launcher

►'jdb' - The Java Debugger

'jdb' - Java Debugger Command and Options

Starting a Debugging Session with 'jdb'

Debugging Applications with Separate 'jdb' Sessions

Debugging Java Applications Remotely