jdb
jdb(1) jdb(1)NAME
jdb - Java debuggerSYNOPSIS
jdb [ options ] [ class ] [ arguments ] PARAMETERS options Command-line options. class Name of the class to begin debugging. arguments Arguments passed to the main() method of class.DESCRIPTION
The Javatm debugger, jdb, is a simple command-line debug ger 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. Starting a jdb Session There are many ways to start a jdb session. The most fre quently 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(1) in the command line. For example, if your application's main class is MyClass, you use the following command to debug it under jdb: example% 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 instruc tion. 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:trans | Loads in-process debugging libraries | |port=dt_socket, | and specifies the kind of connection | |server=y,suspend=n | to be made. | +-------------------+--------------------------------------+ For example, the following command will run the MyClass application and allow jdb to connect to it at a later time: example% jdb -Xdebug \ -Xrunjdwp:transport=dt_socket,address=8000,server=y,suspend=n You can then attach jdb to the VM with the following comm mand: example% 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 on these connection options. Basic jdb Commands The following is a list of the basic jdb commands. The Java debugger supports other commands listed with the help command. Notice that to display local (stack) variables, the class must have been compiled with the javac -g option. cont Continues execution of the debugged appli cation after a breakpoint, exception, or step. dump For primitive values, this command is iden tical 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. help, or ? As the most important jdb command, help displays the list of recognized commands with a brief description. 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 for getting more information about an object. print supports many simple Java expressions including those with method invocations. For example: · print MyClass.myStaticField · print myObj.myInstanceField · print i + j + k ... where i, j, and k are primitives and either fields or local variables. · print myObj.myMethod() ... if myMethod returns a non-null. · print new java.lang.String("Hello").length() thread Selects a thread to be the current thread. Many jdb commands are based on the setting of the current thread. The thread is spec ified with the thread index described in the threads command. threads Lists the threads that are currently run ning. For each thread, its name and cur rent 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 run After starting jdb, and setting any neces sary breakpoints, use this command to start the execution of the debugged application. This command is available only when jdb launches the debugged application (as opposed to attaching to an existing VM). where The where subcommand with no arguments dumps the stack of the current thread (which is set with the thread command). Using where all dumps the stack of all threads in the current thread group. Using 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 com mand), local variables and fields can be displayed with the print and dump commands. The up and down commands select which stack frame is current. Breakpoint Commands Breakpoints are 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 begin ning of the method java.lang.String.length. stop in MyClass.init init identifies the MyClass con structor. stop in MyClass.clinit clinit identifies the static ini tialization code for MyClass. If a method is overloaded, also specify its argu ment 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 clearMyClass:45. Using the clear command with no argument displays a list of all breakpoints currently set. The cont command continues execution. Stepping Commands The step command 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. Exception Commands When an exception occurs for which there is no 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. Use jdb to determine the cause of the exception. catch Causes 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 specified class (or of a subclass) will stop the application at the point where it is thrown. ignore Negates the effect of a previous catch com mand. Notice that the ignore command does not cause the debugged VM to ignore spe cific exceptions, only the debugger.OPTIONS
When using jdb in place of the Java application launcher on the command line, jdb accepts many of the same options as the java(1) command, including -D, -classpath, and -Xoption. 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. -launch Launches the debugged application immedi ately 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 appli cation class is loaded. At that point, you can set any necessary breakpoints and use the cont command to continue execution. 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 docu mentation on these connection alternatives.SEE ALSO
java(1), javac(1), javadoc(1), javah(1), javap(1) See (or search java.sun.com) for the following: Java Language Debugging @ http://java.sun.com/j2se/1.3/debug ging/index.html 13 June 2000 jdb(1)