jdb(1) jdb(1)
NAME
jdb - Java debugger
SYNOPSIS
jdb [ 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(tm) debugger, jdb, is a simple command-line
debugger for Java classes. It is a demonstration of the
Java Platform Debugger Architecture that provides inspec-
tion 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 fre-
quently 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 instruc-
tion.
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
You can then attach jdb to the VM with the following comm-
mand:
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 appli-
cation after a breakpoint, exception, or
step.
dump For primitive values, this command is iden-
tical 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:
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 spec-
ified with the thread index described in
the threads command.
threads Lists the threads that are currently run-
ning. For each thread, its name and cur-
rent 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 cur-
rently running
run After starting jdb, and setting any neces-
sary 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 com-
mand), 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:
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 begin-
ning of the method
java.lang.String.length.
stop in MyClass.init init identifies the MyClass con-
structor.
stop in MyClass.clinit clinit identifies the static ini-
tialization code for MyClass.
If a method is overloaded, you must also specify its argu-
ment 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
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.
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 com-
mand. Notice that the ignore command does
not cause the debugged VM to ignore spe-
cific 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 immedi-
ately 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 appli-
cation class is loaded. At that point, you
can set any necessary breakpoints and use
the cont command to continue execution.
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 docu-
mentation on these connection alternatives.
-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.
SEE ALSO
java(1), javac(1), javadoc(1), javah(1), javap(1)
See (or search java.sun.com) for the following:
Java Language Debugging @
http://java.sun.com/j2se/1.3/debug-
ging/index.html
14 July 2000 jdb(1)