jdb(1)			    General Commands 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 Architec-
       ture @
       http://docs.oracle.com/javase/7/docs/technotes/guides/jpda/index.html
       that provides inspection and debugging of a local or remote Java Vir-
       tual 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://docs.oracle.com/javase/7/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 the 1.4.2 documentation @
       http://java.sun.com/j2se/1.4.2/docs/guide/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(1) -g option.
	     print supports many simple Java expressions including those with
	     method invocations, for example:

	     o print MyClass.myStaticField

	     o print myObj.myInstanceField

	     o print i + j + k (i, j, k are primities and either fields or
	       local variables)

	     o print myObj.myMethod() (if myMethod returns a non-null)

	     o 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 cur-
	     rently 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:

	  o stop at MyClass:22 (sets a breakpoint at the first instruction for
	    line 22 of the source file containing MyClass)

	  o stop in java.lang.String.length (sets a breakpoint at the beginnig
	    of the method java.lang.String.length)

	  o stop in MyClass.<init> (<init> identifies the MyClass constructor)

	  o 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 speci-
	     fied 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 Archi-
       tecture has additional documentation @
       http://docs.oracle.com/javase/7/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(1), java(1), javah(1), javap(1), javadoc(1).

				  16 Mar 2012				jdb(1)