jdb(1) | Basic Tools | jdb(1) |
jdb - Finds and fixes bugs in Java platform programs.
jdb [options] [classname] [arguments]
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 http://docs.oracle.com/javase/8/docs/technotes/guides/jpda/index.html
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:
jdb MyClass
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
The 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 -help option.
Note: To display local variables, the containing class must have been compiled with the javac -g option.
The print command 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()
4. (java.lang.Thread)0x1 main running
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 print and dump commands. The up and 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:
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, MyClass.myMethod(int,java.lang.String) or MyClass.myMethod().
The clear command removes breakpoints using the following syntax: clear MyClass:45. Using the clear or stop command with no argument displays a list of all breakpoints currently set. The cont command continues execution.
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.
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.
Use the catch command to cause the debugged application to stop at other thrown exceptions, for example: catch java.io.FileNotFoundException or catchmypackage.BigTroubleException. Any exception that is an instance of the specified class or subclass stops the application at the point where it is thrown.
The 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 -D, -classpath, and -X options. The following list contains additional options that are accepted by the jdb command.
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 http://docs.oracle.com/javase/8/docs/technotes/guides/jpda/index.html
21 November 2013 | JDK 8 |