jdb helps you find and fix bugs in Java language programs.
jdb [ options ] [ class ] [ arguments ]
- Command-line options, as specified below.
- Name of the class to begin debugging.
- Arguments passed to the
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 SessionThere 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 MyClassWhen 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=nYou can then attach jdb to the VM with the following commmand:C:\> jdb -attach jdbconnNote 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 MyClassIf 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=nYou 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 CommandsThe 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.
- 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).
- Continues execution of the debugged application after a breakpoint, exception, or step.
- 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()
- 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.
- 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: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,4. (java.lang.Thread)0x1 main running
- 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.
wherewith no arguments dumps the stack of the current thread.
where alldumps the stack of all threads in the current thread group.
wherethreadindex 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.
BreakpointsBreakpoints 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.
SteppingThe 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.
ExceptionsWhen 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 OptionsWhen 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:
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.
- -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.
- Launches the debugged application with the classic VM.
- 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.
- 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.
javac, java, javah, javap, javadoc, oldjdb.