In this article we are going to elaborate the jdb commands.

Syntax

jdb [options] [class] [arguments]

What is jdb?

JDB is Java Debugger. JDB helps to find and fix bugs of JAVA code. Mainly it is a command line debugger of java code.

So, if we find any error when compiling , we can use this tool to get the problem and solve that. We can start jdb in many ways.

Ways to start jdb

  • Start jdb to a new JVM with main class of the application needed debugging. In this way, jdb invokes a new JVM with any specific parameters(we select), loads the specified class, and stops the VM before executing that class’s first instruction.
  • Start jdb with a already running JVM(attaching). The JVM must have started in a debug supporting mode( Loads in process debug libraries and connection type to connect).

Example

  • JVM start command

      java -agentlib:jdwp=transport=dt_shmem,address=jdbconn,server=y,suspend=n MyClass

  • Attaching jdb to the VM

      jdb -attach jdbconn

  • We can directly attach JDB with current VM and start debugging session. In this way we have to run every command step by step. To start jdb session, just type

      jdb

It will initiate jdb.

JDB options :

  • sourcepath (directories separated by “;”) : Directories to search source files
  • launch: Launch VM immediately
  • tclient : Runs the application in client Hotspot(tm) Performance Engine
  • tserver : Run the application in server Hotspot(tm) Performance Engine
  • listconnectors :List the connectors available in this VM
  • connect (connector-name):(name1)=(value1).. : Connect to target VM using named connector with listed argument values
  • attach (address) : Attach to a running VM at the specified address using connector.
  • listen (address) : Wait for a running VM to connect at the specified address using connector
  • listenany : Wait for a running VM to connect at any available address using connector
  • dbgtrace (flags) : print info for debugging jdb
  • J(option) : Pass option to the Java virtual machine by jdb.
  • v or -verbose(:class:gc:jni) : Turn on verbose mode
  • D(name)=(value) : Sets a system property
  • classpath (directories separated by “;”) : list directories in which to look for classes
  • X(option) :Non-standard target VM option. See java commands for more details.

JDB commands (in a JDB Session)

  • connectors : list available connectors and transports in this VM
  • run (class (args)): Start execution of application’s main class

Thread info

  • threads (thread group) : Lists threads
  • thread (thread id) : Sets default thread
  • suspend (thread id(s)) : Suspend threads (default: all)
  • resume (thread id(s)) : Resumes threads (default: all)
  • where ((thread id) : all) : Dumps a thread’s stack
  • wherei ((thread id) : all) : Dumps a thread’s stack, with PC info
  • up (n frames): Moves up a thread’s stack
  • down (n frames): Moves down a thread’s stack
  • kill (thread id) (expr) : Kills a thread with the given exception object
  • interrupt (thread id): Interrupts a thread
  • threadgroups: Lists thread groups
  • threadgroup (name): Sets current thread group
  • threadlocks (thread id) : Prints lock info for a thread
  • lock (expr): Prints lock info for an object

Object & Class Info

  • print (expr): Prints value of expression
  • dump (expr): Prints all object information
  • eval (expr): Evaluates expression (same as print)
  • set (lvalue) = (expr): Assigns new value to field/variable/array element
  • locals : Prints all local variables in current stack frame
  • classes : Lists currently known classes
  • class (class id): Shows details of named class
  • methods (class id): Lists a class’s methods
  • fields (class id): Lists a class’s fields

Debug & Breakpoints

  • stop in (class id).(method) ((argument_type,…)): Sets a break point in a method
  • stop at (class id):(line) : Sets a breakpoint at a line
  • clear : list breakpoints
  • clear (class id).(method)((argument_type,…)) : Clears a break point in a method
  • clear (class id):(line) : Clears a break point at a line
  • step : Executes current line
  • step up : Executes until the current method returns to its caller
  • stepi : Executes current instruction
  • next : step one line (step OVER calls)
  • cont : Continue execution from break point
  • catch (uncaught:caught:all) (class id):(class pattern): Breaks when specified exception occurs
  • ignore (uncaught:caught:all) (class id):(class pattern): Cancels ‘catch’ for the specified exception
  • redefine (class id) (class file name) : Redefines the code for a class
  • watch (access:all) (class id).(field name) : Watches access/modifications to a field
  • unwatch (access:all) (class id).(field name) : Discontinue watching access/modifications to a field
  • trace (go) methods (thread) : Traces method entries and exits. All threads are suspended unless ‘go’ is specified
  • trace (go) method exit : exits (thread) : Traces the current method’s exit, or all methods’ exits. All threads are suspended unless ‘go’ is specified
  • untrace (methods): Stops tracing method entries and/or exits
  • list (line number:method) : print source code
  • use (source file path): Displays /changes the source path
  • sourcepath (source file path): Displays /changes the source path
  • exclude ((class pattern), … : “none”) : Does not report step or method events for specified classes
  • classpath : print classpath info from target VM

Monitors

  • monitor : Lists monitors
  • monitor (command): Executes command each time the program stops
  • unmonitor (monitor#) : Deletes a monitor
  • read (filename): Reads and executes a command file
  • !! : Repeats last command(n) (command) : Repeat command n times
  • # (command) : Discard (no-op)
  • exit (or quit) : Exit debugger
  • pop : Pops the stack through and including the current frame
  • reenter : Same as pop, but current frame is reentered

GC behavior

  • disablegc (expr): Prevents garbage collection(GC) of an object
  • enablegc (expr): Permits garbage collection(GC) of an object

Though all IDEs have build in debugger, but jdb is very useful while debugging remotely to server. This is very useful when you need to work in CLI. If you are new at this, i would prefer to use any GUI based tool.

Thanks….:)