START A JDB SESSION¶

There are many ways to start a JDB session. The most frequently used way is to have JDB launch a new JVM with the main class of the application to be debugged. Do this by substituting the jdb command for the java command in the command line. For example, if your application's main class is MyClass, then use the following command to debug it under JDB:

jdb MyClass

When started this way, the jdb command calls a second JVM with the specified parameters, loads the specified class, and stops the JVM before executing that class's first instruction.

Another way to use the jdb command is by attaching it to a JVM that is already running. Syntax for starting a JVM to which the jdb command attaches when the JVM is running is as follows. This loads in-process debugging libraries and specifies the kind of connection to be made.

java -agentlib:jdwp=transport=dt_socket,server=y,suspend=n MyClass

You can then attach the jdb command to the JVM with the following command:

jdb -attach 8000

The MyClass argument is not specified in the jdb command line in this case because the jdb command is connecting to an existing JVM instead of launching a new JVM.

There are many other ways to connect the debugger to a JVM, and all of them are supported by the jdb command. The Java Platform Debugger Architecture has additional documentation on these connection options.

BASIC JDB COMMANDS¶

The following is a list of the basic jdb commands. The JDB supports other commands that you can list with the -help option.

help or ?

The help or ? commands display the list of recognized commands with a brief description.

run

After you start JDB and set breakpoints, you can use the run command to execute the debugged application. The run command is available only when the jdb command starts the debugged application as opposed to attaching to an existing JVM.

cont

Continues execution of the debugged application after a breakpoint, exception, or step.

print

Displays Java objects and primitive values. For variables or fields of primitive types, the actual value is printed. For objects, a short description is printed. See the dump command to find out how to get more information about an object.

Note: To display local variables, the containing class must have been compiled with the javac -g option.

The print command supports many simple Java expressions including those with method invocations, for example:

print MyClass.myStaticField

print myObj.myInstanceField

print i + j + k (i, j, k are primities and either fields or local variables)

print myObj.myMethod() (if myMethod returns a non-null)

print new java.lang.String("Hello").length()

dump

For primitive values, the dump command is identical to the print command. For objects, the dump command prints the current value of each field defined in the object. Static and instance fields are included. The dump command supports the same set of expressions as the print command.

threads

List the threads that are currently running. For each thread, its name and current status are printed and an index that can be used in other commands. In this example, the thread index is 4, the thread is an instance of java.lang.Thread, the thread name is main, and it is currently running.

4. (java.lang.Thread)0x1 main      running

thread

Select a thread to be the current thread. Many jdb commands are based on the setting of the current thread. The thread is specified with the thread index described in the threads command.

where

The where command with no arguments dumps the stack of the current thread. The whereall command dumps the stack of all threads in the current thread group. The wherethreadindex command dumps the stack of the specified thread.

If the current thread is suspended either through an event such as a breakpoint or through the suspend command, then local variables and fields can be displayed with the print and dump commands. The up and down commands select which stack frame is the current stack frame.

BREAKPOINTS¶

Breakpoints can be set in JDB at line numbers or at the first instruction of a method, for example:

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The command stop at MyClass:22 sets a breakpoint at the first instruction for line 22 of the source file containing MyClass.

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The command stop in java.lang.String.length sets a breakpoint at the beginning of the method java.lang.String.length.

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The command stop in MyClass.<clinit> uses <clinit> to identify the static initialization code for MyClass.

When a method is overloaded, you must also specify its argument types so that the proper method can be selected for a breakpoint. For example, MyClass.myMethod(int,java.lang.String) or MyClass.myMethod().

The clear command removes breakpoints using the following syntax: clear MyClass:45. Using the clear or stop command with no argument displays a list of all breakpoints currently set. The cont command continues execution.

STEPPING¶

The step command advances execution to the next line whether it is in the current stack frame or a called method. The next command advances execution to the next line in the current stack frame.

EXCEPTIONS¶

When an exception occurs for which there is not a catch statement anywhere in the throwing thread's call stack, the JVM typically prints an exception trace and exits. When running under JDB, however, control returns to JDB at the offending throw. You can then use the jdb command to diagnose the cause of the exception.

Use the catch command to cause the debugged application to stop at other thrown exceptions, for example: catch java.io.FileNotFoundException or catchmypackage.BigTroubleException. Any exception that is an instance of the specified class or subclass stops the application at the point where it is thrown.

The ignore command negates the effect of an earlier catch command. The ignore command does not cause the debugged JVM to ignore specific exceptions, but only to ignore the debugger.