Finds and fixes bugs in Java platform programs.
jdb [options] [classname] [arguments]
Command-line options. See Options.
Name of the main class to debug.
Arguments passed to the
main() method of the class.
The Java Debugger (JDB) is a simple command-line debugger for Java classes. The
jdb command and its options call the JDB. The
jdb command demonstrates the Java Platform Debugger Architecture (JDBA) and provides inspection and debugging of a local or remote Java Virtual Machine (JVM). See Java Platform Debugger Architecture (JDBA) at
There are many ways to start a JDB session. The most frequently used way is to have JDB launch a new JVM with the main class of the application to be debugged. Do this by substituting the
jdb command for the
java command in the command line. For example, if your application's main class is
MyClass, then use the following command to debug it under JDB:
When started this way, the
jdb command calls a second JVM with the specified parameters, loads the specified class, and stops the JVM before executing that class's first instruction.
Another way to use the
jdb command is by attaching it to a JVM that is already running. Syntax for starting a JVM to which the
jdb command attaches when the JVM is running is as follows. This loads in-process debugging libraries and specifies the kind of connection to be made.
java -agentlib:jdwp=transport=dt_socket,server=y,suspend=n MyClass
You can then attach the
jdb command to the JVM with the following command:
jdb -attach 8000
MyClass argument is not specified in the
jdb command line in this case because the
jdb command is connecting to an existing JVM instead of launching a new JVM.
There are many other ways to connect the debugger to a JVM, and all of them are supported by the
jdb command. The Java Platform Debugger Architecture has additional documentation on these connection options.
The following is a list of the basic
jdb commands. The JDB supports other commands that you can list with the
? commands display the list of recognized commands with a brief description.
After you start JDB and set breakpoints, you can use the
run command to execute the debugged application. The
run command is available only when the
jdb command starts the debugged application as opposed to attaching to an existing JVM.
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 to find out how to get more information about an object.
Note: To display local variables, the containing class must have been compiled with the
javac -g option.
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, the
dump command is identical to the
dump command 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
List the threads that are currently running. For each thread, its name and current status are printed and an index that can be used in other commands. 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.
where command with no arguments dumps the stack of the current thread. The
all command dumps the stack of all threads in the current thread group. The
threadindex command 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, then local variables and fields can be displayed with the
dump commands. The
down commands select which stack frame is the current stack frame.
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
stop in java.lang.String.length sets a breakpoint at the beginning of the method
stop in MyClass.<clinit> uses
<clinit> to identify the static initialization code for
When a method is overloaded, you must also specify its argument types so that the proper method can be selected for a breakpoint. For example,
clear command removes breakpoints using the following syntax:
clear MyClass:45. Using the
stop command with no argument displays a list of all breakpoints currently set. The
cont command continues execution.
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.
When an exception occurs for which there is not a
catch statement anywhere in the throwing thread's call stack, the JVM typically prints an exception trace and exits. When running under JDB, however, control returns to JDB at the offending throw. You can then use the
jdb command to diagnose the cause of the exception.
catch command to cause the debugged application to stop at other thrown exceptions, for example:
catch java.io.FileNotFoundException or
mypackage.BigTroubleException. Any exception that is an instance of the specified class or subclass stops the application at the point where it is thrown.
ignore command negates the effect of an earlier
catch command. The
ignore command does not cause the debugged JVM to ignore specific exceptions, but only to ignore the debugger.
When you use the
jdb command instead of the
java command on the command line, the
jdb command accepts many of the same options as the
java command, including
-X options. The following list contains additional options that are accepted by the
Other options are supported to provide alternate mechanisms for connecting the debugger to the JVM it is to debug. For additional documentation about these connection alternatives, see Java Platform Debugger Architecture (JPDA) at
Displays a help message.
Uses the specified path to search for source files in the specified path. If this option is not specified, then use the default path of dot (.).
Attaches the debugger to a running JVM with the default connection mechanism.
Waits for a running JVM to connect to the specified address with a standard connector.
Starts the debugged application immediately upon startup of JDB. The
-launch option removes the need for the
run command. The debugged 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 command to continue execution.
List the connectors available in this JVM.
Connects to the target JVM with the named connector and listed argument values.
Prints information for debugging the
Runs the application in the Java HotSpot VM client.
Runs the application in the Java HotSpot VM server.
option to the JVM, 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 MB. See java(1).
Turns on verbose mode.
Sets a system property.
Lists directories separated by colons in which to look for classes.
Nonstandard target JVM option.