jdb - Unix, Linux Command

NAME

jdb - Java debugger

SYNOPSIS

jdb [ options ] [ class ] [ arguments ]

PARAMETERS

Tag	Description
options	Command-line options.
class	Name of the class to begin debugging.
arguments	Arguments passed to the main() method of class.

DESCRIPTION

The Java debugger, jdb, is a simple command-line debugger for Java classes. It is a demonstration of the Java Platform Debugger Architecture that provides inspection and debugging of a local or remote Java Virtual Machine.

Starting a jdb Session

There are many ways to start a jdb session. The most frequently used way is to have jdb launch a new Java Virtual Machine (VM) with the main class of the application to be debugged. This is done by substituting the command jdb for java(1) in the command line. For example, if your application’s main class is MyClass, you use the following command to debug it under jdb:

example% jdb MyClass

When started this way, jdb invokes a second Java VM with any specified parameters, loads the specified class, and stops the VM before executing that class’s first instruction.

Another way to use jdb is by attaching it to a Java VM that is already running. A VM that is to be debugged with jdb must be started with the following options:

option	purpose
-Xdebug	Enables debugging support in the VM.
-Xrunjdwp:transport=dt_socket, server=y,suspend=n	Loads in-process debugging libraries and specifies the kind of connection to be made.

For example, the following command will run the MyClass application and allow jdb to connect to it at a later time:

example% java -Xdebug \
-Xrunjdwp:transport=dt_socket,address=8000,server=y,suspend=n \
\MyClass

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

example% jdb -attach 8000

Note that MyClass is not specified in the jdb command line in this case because jdb is connecting to an existing VM instead of launching a new one.

There are many other ways to connect the debugger to a VM, and all of them are supported by jdb. 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 Java debugger supports other commands listed with the help command.

Notice that to display local (stack) variables, the class must have been compiled with the javac -g option.

Tag

Description

cont

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

dump

For primitive values, this command is identical to print. For objects, it 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.

help, or ?

As the most important jdb command, help displays the list of recognized commands with a brief description.

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 for getting more information about an object.

print supports many simple Java expressions including those with method invocations. For example:

Tag	Description
o	print MyClass.myStaticField
o	print myObj.myInstanceField
o	print i + j + k ... where i, j, and k are primitives and either fields or local variables.
o	print myObj.myMethod() ... if myMethod returns a non-null.
o	print new java.lang.String( Hello ).length()

thread

Selects 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.

threads

Lists the threads that are currently running. For each thread, its name and current status are printed, as well as an index that can be used for other commands. For example:

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

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

run

After starting jdb, and setting any necessary breakpoints, use this command to start the execution of the debugged application. This command is available only when jdb launches the debugged application (as opposed to attaching to an existing VM).

where

The where subcommand with no arguments dumps the stack of the current thread (which is set with the thread command). Using where all dumps the stack of all threads in the current thread group. Using where threadindex 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), local variables and fields can be displayed with the print and dump commands. The up and down commands select which stack frame is current.

Breakpoint Commands

Breakpoints are set in jdb at line numbers or at the first instruction of a method. For example:

Tag	Description
stop at MyClass:22	Sets a breakpoint at the first instruction for line 22 of the source file containing MyClass.
stop in java.lang.String.length
	Sets a breakpoint at the beginning of the method java.lang.String.length.
stop in MyClass.init	init identifies the MyClass constructor.
stop in MyClass.clinit	clinit identifies the static initialization code for MyClass.

If 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)

MyClass.myMethod()

The clear command removes breakpoints using a syntax as in clearMyClass:45. Using the clear command with no argument displays a list of all breakpoints currently set. The cont command continues execution.

Stepping Commands

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.

Exception Commands

When an exception occurs for which there is no catch statement anywhere in the throwing thread’s call stack, the VM normally prints an exception trace and exits. When running under jdb, however, control returns to jdb at the offending throw. Use jdb to determine the cause of the exception.

Tag

Description

catch

Causes the debugged application to stop at other thrown exceptions. For example:

Tag	Description
	catch java.io.FileNotFoundException

Tag	Description
	catch mypackage.BigTroubleException

Any exception which is an instance of the specified class (or of a subclass) will stop the application at the point where it is thrown.

ignore

Negates the effect of a previous catch command. Notice that the ignore command does not cause the debugged VM to ignore specific exceptions, only the debugger.

OPTIONS

When using jdb in place of the Java application launcher on the command line, jdb accepts many of the same options as the java(1) command, including -D, -classpath, and -Xoption.

The following additional options are accepted by jdb:

Tag	Description
-sourcepath dir1:dir2:...
	Uses the given path in searching for source files in the specified path. If this option is not specified, the default path of "." is used.
-attach address	Attaches the debugger to previously running VM using the default connection mechanism.
-launch	Launches the debugged application immediately upon startup of jdb. This option removes the need for using the run command. The debuged 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 command to continue execution.
-J option	Pass option to the Java virtual machine, where option is one of the options described on the man page for the java application launcher, java(1). For example, -J-Xms48m sets the startup memory to 48 megabytes. It is a common convention for -J to pass options to the underlying virtual machine.

Other options are supported for alternate mechanisms for connecting the debugger and the VM it is to debug. The Java Platform Debugger Architecture has additional documentation on these connection alternatives.