jdb(1)                     OpenBSD Reference Manual                     jdb(1)



NAME
       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 @
       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
       executing 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
             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:

       -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
          command.) 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 @
       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)
