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 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 speci-fied specified
fied 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, | Loads in-process debugging libraries |
|server=y,suspend=n | 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 \
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 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:
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 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 break-point breakpoint
point 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:
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 clearMyClass:45. Using the clear command
with no argument displays a list of all breakpoints currently set. The cont command continues execu-tion. execution.
tion.
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 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 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.
SEE ALSO
java(1), javac(1), javadoc(1), javah(1), javap(1)
12 Nov 2001 jdb(1)
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