jdb(1)jdb(1)NAMEjdb - The Java Debugger
jdb helps you find and fix bugs in Java language programs.
SYNOPSISjdb [ options ] [ class ] [ arguments ]
options
Command-line options, as specified below.
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 @
http://java.sun.com/javase/6/docs/technotes/guides/jpda/index.html 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 in the command line. For example, if your
application's main class is MyClass, you use the following command to
debug it under JDB:
% 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. 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.
-agentlib:jdwp=transport=dt_socket,server=y,suspend=n
For example, the following command will run the MyClass application,
and allow jdb to connect to it at a later time.
% java -agentlib:jdwp=transport=dt_socket,address=8000,server=y,suspend=n MyClass
You can then attach jdb to the VM with the following commmand:
% 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 @
http://java.sun.com/javase/6/docs/technotes/guides/jpda/conninv.html on
these connection options. For information on starting a J2SE 1.4.2 or
early VM for use with jdb see 1.4.2 documentation @
http://java.sun.com/j2se/1.4.2/docs/technotes/guides/jpda/conninv.html
Basic jdb Commands
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 recog‐
nized commands with a brief description.
run
After starting jdb, and setting any necessary breakpoints, you
can use this command to start the execution the debugged applica‐
tion. This command is available only when jdb launches the
debugged application (as opposed to attaching to an existing VM).
cont
Continues execution of the debugged application after a break‐
point, exception, or step.
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
below for getting more information about an object.
NOTE: To display local variables, the containing class must have
been compiled with the javac -g option.
print supports many simple Java expressions including those with
method invocations, for example:
* print MyClass.myStaticField
* print myObj.myInstanceField
* print i + j + k (i, j, k are primities and either fields or
local variables)
* print myObj.myMethod() (if myMethod returns a non-null)
* print new java.lang.String("Hello").length()
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.
threads
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:
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,
thread
Select a thread to be the current thread. Many jdb commands are
based on the setting of the current thread. The thread is speci‐
fied with the thread index described in the threads command
above.
where
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 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.
Breakpoints
Breakpoints can be set in jdb at line numbers or at the first instruc‐
tion 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 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 initial‐
ization 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
"clear MyClass:45". Using the clear or command with no argument dis‐
plays a list of all breakpoints currently set. The cont command contin‐
ues execution.
Stepping
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.
Exceptions
When an exception occurs for which there isn't a catch statement any‐
where 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.
Use the catch command to cause the debugged application to stop at
other thrown exceptions, for example: "catch java.io.FileNotFoundExcep‐
tion" 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.
The ignore command negates the effect of a previous catch command.
NOTE: The ignore command does not cause the debugged VM to ignore spe‐
cific exceptions, only the debugger.
Command Line Options
When you use jdb in place of the Java application launcher on the com‐
mand line, jdb accepts many of the same options as the java command,
including -D, -classpath, and -X<option>.
The following additional options are accepted by jdb:
-help
Displays a help message.
-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.
-listen <address>
Waits for a running VM to connect at the specified address using
standard connector.
-listenany
Waits for a running VM to connect at any available address using
standard connector.
-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 to continue execution.
-listconnectors
List the connectors available in this VM
-connect
<connector-name>:<name1>=<value1>,... Connects to target VM using
named connector with listed argument values.
-dbgtrace [flags]
Prints info for debugging jdb.
-tclient
Runs the application in the Java HotSpot(tm) VM (Client).
-tserver
Runs the application in the Java HotSpot(tm) VM (Server).
-Joption
Pass option to the Java virtual machine used to run jdb. (Options
for the application Java virtual machine are passed to the run com‐
mand.) 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 Archi‐
tecture has additional documentation @
http://java.sun.com/javase/6/docs/technotes/guides/jpda/conninv.html on
these connection alternatives.
Options Forwarded to Debuggee Process
-v -verbose[:class|gc|jni]
Turns on verbose mode.
-D<name>=<value>
Sets a system property.
-classpath <directories separated by
":"> Lists directories in which to look for classes.
-X<option>
Non-standard target VM option
SEE ALSO
javac, java, javah, javap, javadoc.
05 Aug 2006 jdb(1)
