jdb - The Java Debugger

jdb helps you find and fix bugs in Java language programs.

SYNOPSIS

jdb [ 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 Architecture that provides inspection and debugging of a local or remote Java Virtual Machine.
There are minor incompatibilites between the current version of jdb and previous versions in the areas of command line arguments and supported commands. A strictly compatible version of jdb, oldjdb, is available, but its use is discouraged since it is much less reliable than the new implementation.
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:
C:\> 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 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_shmem,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.
C:\> java -Xdebug -Xrunjdwp:transport=dt_shmem,address=jdbconn,server=y,suspend=n 
You can then attach jdb to the VM with the following commmand:
C:\> jdb -attach jdbconn 
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. The examples above assume that debugging will take place with the default VM. To debug with the classic VM, use the -tclassic option, for example:
C:\> jdb -tclassic MyClass 
If you want to attach jdb to a classic VM that is already running, the VM must be started with the following options:

option purpose

-Xdebug Enables debugging support in the VM

-Xrunjdwp:transport=dt_shmem,server=y,suspend=n Loads in-process debugging libraries and specifies the kind of connection to be made.

-Xnoagent Disables VM support for oldjdb

-Djava.compiler=NONE Disables the JIT compiler. This is required for debugging under the classic VM

For example, the following command will run the MyClass application with the classic VM, and allow jdb to connect to it at a later time.
C:\> java -Xdebug -Xnoagent -Djava.compiler=NONE -Xrunjdwp:transport=dt_shmem,address=jdbconn,server=y,suspend=n 
You can then attach jdb to the VM with the same commmand:
C:\> jdb -attach jdbconn 
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 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.
run
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 VM).
cont
Continues execution of the debugged application after a breakpoint, 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 specified 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 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 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 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 displays a list of all breakpoints currently set. The cont command continues 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 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.
Use the 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.
The ignore command negates the effect of a previous catch command.
NOTE: The 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 -X<option>.
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.
-tclassic
Launches the debugged application with the classic VM.
-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.
-Joption
Pass option to the Java virtual machine, where option is one of the options described on the reference page for the java application launcher. 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 documentation on these connection alternatives.

SEE ALSO

javac, java, javah, javap, javadoc, oldjdb.

option	purpose
`-Xdebug`	Enables debugging support in the VM
`-Xrunjdwp:transport=dt_shmem,server=y,suspend=n`	Loads in-process debugging libraries and specifies the kind of connection to be made.