The Java Debugger jdb helps you find and fix bugs in Java language programs.
options ] [ class ] [ arguments ]
Command-line options, as
Name of the class to begin
Arguments passed to the
main() method of class.
add an example, a script, a trick and tips
jdb -classpath $classpath
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.
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 in the command line. For example,
if your application’s main class is MyClass, you use
the following command to debug it under JDB:
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
Another way to
use jdb is by attaching it to a Java VM that is
already running. Syntax for Starting a VM to which jdb will
attach when the VM is running is as follows. This loads
in-process debugging libraries and specifies the kind
of connection to be made.
the following command will run the MyClass application, and
allow jdb to connect to it at a later time.
You can then
attach jdb to the VM with the following commmand:
"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. For information on starting a
J2SE 1.4.2 or early VM for use with jdb see 1.4.2
The following is a list of the basic jdb commands.
The Java debugger supports other commands which you can list
using jdb’s help command.
help, or ?
The most important jdb
command, help displays the list of recognized
commands with a brief description.
After starting jdb, and
setting any necessary breakpoints, you can use this command
to start the execution the debugged application. This
command is available only when jdb launches the
debugged application (as opposed to attaching to an existing
Continues execution of the
debugged application after a breakpoint, exception, or
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 below
for getting more information about an object.
display local variables, the containing class must have been
compiled with the javac -g option.
supports many simple Java expressions including those with
method invocations, for example:
print i + j + k (i, j, k are primities and either
fields or local variables)
print myObj.myMethod() (if myMethod returns a
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.
command supports the same set of expressions as the
List 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:
(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
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 with no arguments
dumps the stack of the current thread. where all
dumps the stack of all threads in the current thread group.
where threadindex dumps the stack of the specified
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.
Breakpoints can be set in jdb at line numbers or at
the first instruction of a method, for example:
stop at MyClass:22 (sets a breakpoint at the first
instruction for line 22 of the source file containing
stop in java.lang.String.length (sets a breakpoint at
the beginnig 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
If a method is
overloaded, you must also specify its argument types so that
the proper method can be selected for a breakpoint. For
clear command removes breakpoints using a syntax as
in "clear MyClass:45". Using the
clear or command with no argument displays a list of
all breakpoints currently set. The cont command
The step commands 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.
When an exception occurs for which there isn’t a 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. You can then use
jdb to diagnose the cause of the exception.
catch command to cause the debugged application to
stop at other thrown exceptions, for example: "catch
java.io.FileNotFoundException" or "catch
mypackage.BigTroubleException. Any exception which is an
instance of the specifield class (or of a subclass) will
stop the application at the point where it is thrown.
ignore command negates the effect of a previous
ignore command does not cause the debugged VM to ignore
specific exceptions, only the debugger.
command line options
When you use jdb in place of the Java application launcher
on the command line, jdb accepts many of the same options
as the java command, including -D, -classpath, and
The following additional options are accepted by jdb:
Displays a help message.
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.
Attaches the debugger to previously running VM using the default
Waits for a running VM to connect at the specified address using
Waits for a running VM to connect at any available address using
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 to continue execution.
List the connectors available in this VM
to target VM using named connector with listed argument values.
Prints info for debugging jdb.
Runs the application in the Java HotSpot(tm) VM (Client).
Runs the application in the Java HotSpot(tm) VM (Server).
Pass option to the Java virtual machine used to run jdb.
(Options for the application Java virtual machine are passed to
the run command.) For example, -J-Xms48m sets the
startup memory to 48 megabytes.
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
on these connection alternatives.
Options Forwarded to Debuggee Process
Turns on verbose mode.
Sets a system property.
-classpath <directories separated by
":"> Lists directories in which to look for classes.
Non-standard target VM option
javah, javap, javadoc.