'jdb' - The Java Debugger

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This chapter describes:

• "jdb" Command
• Launching and Debugging Java Applications
• Attaching "jdb" to Running Applications
• Debugging Commands
• Multi-Thread Debugging Exercise

"jdb" Command

"jdb": A command line tool that allows you to debug a Java application interactively with in a command line mode. "javac" is distributed as part of the Sun JDK package. It has the following syntax:

jdb [options] main_class_name
jdb [options] -attach <address> 

where "options" is a list of options, "main_class_name" is a the name of the main class of a Java application, and "address" is the debugging connection address of a running Java application.

As you can see from the syntax, there are two ways of running "jdb":

1. Running "jdb" to launch a Java application and start a debug session on that application.

2. Running "jdb" to connect to a separately launched Java application and start a debug session on that application.

Commonly used options are:

• "-help" - Displays a short help text.
• "-verbose" - Generates verbose output to standard output.
• "-classpath classpath" - Specifies a list of path names where the launcher will search for compiled type definitions.
• "-Dproperty=value" - Defines a new system property, which can be accessed by the application.
• "-launch" - Launches the debugged application immediately upon startup of jdb. This option removes the need for using the run command. The target application is launched and then stopped just before the initial application class is loaded. At that point you can set any necessary breakpoints and use the cont to continue execution.

Note that in order to use all debugging commands, the target application must be compiled with "-g" option, which will generate all debugging information, including source file, line number, and local variables, into the class file.

Launching and Debugging Java Applications

To test the debugger, I wrote the following simple Java application, Hello.java:

class Hello {
   public static void main(String[] a) {
      System.out.println("Hello world!"); 	
   }
}

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