jdb(1)jdb(1)NAMEjdb - Java debugger
SYNOPSISjdb [ options ] [ class ] [ arguments ]
PARAMETERS
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 Architec‐
ture 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 execut‐
ing 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:trans‐ │ Loads in-process debugging libraries │
│port=dt_socket, │ and specifies the kind of connection │
│server=y,suspend=n │ 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 \
Class
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.
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 vari‐
ables 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:
· print MyClass.myStaticField
· print myObj.myInstanceField
· print i + j + k ... where i, j, and k are primitives
and either fields or local variables.
· print myObj.myMethod() ... if myMethod returns a
non-null.
· 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:
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 break‐
points, 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 com‐
mand). 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 vari‐
ables 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:
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)
or
MyClass.myMethod()
The clear command removes breakpoints using a syntax as in clearMy‐
Class: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 excep‐
tion 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.
catch Causes 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 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:
-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 neces‐
sary 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 underly‐
ing virtual machine.
Other options are supported for alternate mechanisms for connecting the
debugger and the VM it is to debug. The Java Platform Debugger Archi‐
tecture has additional documentation on these connection alternatives.
SEE ALSOjava(1), javac(1), javadoc(1), javah(1), javap(1)
12 Nov 2001 jdb(1)
