Finding Issue or a root cause of a bug by adding system.out.println() is very boring task: as you need to add debug line, compile, run, test and at end delete bedug line, when u added. Even more if you run ur Java program via some Environment ( Like Amazon ) , it;s simple waste your time., as it need significant activation time.
Java provide a easier way to debug your java program on fly.
The Java Debugger, jdb, is a simple command-line debugger 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.
Most Useful command is as below :
Table 1. Step Commands
| Jdb Item | Emacs Command | jdb Command | Description |
|---|
| Step Over | jde-bug-step-over | next | Advance to the next line in the current method, stepping over any lines that invoke other methods. |
| Step Into | jde-debug-step-into | step | Advance to the next line in the program. |
| Step Out | jde-debug-step-out | step up | Advance to the next line in the method that invoked the current method. |
| Continue | jde-debug-cont | cont | Advance to the next breakpoint or to the end of the program, whichever comes first. |
Example :
stop at com.dipankar.kharbandhi.midnapur.services.codebase.impl.HelloWorld:72
After that Hit homepage or any other page
1) See the current Locals value - "dump this"
2) Set to new value - "print setWeblabOverride("Pass Value")"
3) See the new value - "dump this"
4) Continue - "cont"
========================================================================
Full list of commend as below:
dipankar@superstar] ./jdb -attach edipankar.com:8000
/jdk1.6.0/bin
Set uncaught java.lang.Throwable
Set deferred uncaught java.lang.Throwable
Initializing jdb ...
> help
** command list **
connectors -- list available connectors and transports in this VM
run [class [args]] -- start execution of application's main class
threads [threadgroup] -- list threads
thread <thread id> -- set default thread
suspend [thread id(s)] -- suspend threads (default: all)
resume [thread id(s)] -- resume threads (default: all)
where [<thread id> | all] -- dump a thread's stack
wherei [<thread id> | all]-- dump a thread's stack, with pc info
up [n frames] -- move up a thread's stack
down [n frames] -- move down a thread's stack
kill <thread id> <expr> -- kill a thread with the given exception object
interrupt <thread id> -- interrupt a thread
print <expr> -- print value of expression
dump <expr> -- print all object information
eval <expr> -- evaluate expression (same as print)
set <lvalue> = <expr> -- assign new value to field/variable/array element
locals -- print all local variables in current stack frame
classes -- list currently known classes
class <class id> -- show details of named class
methods <class id> -- list a class's methods
fields <class id> -- list a class's fields
threadgroups -- list threadgroups
threadgroup <name> -- set current threadgroup
stop in <class id>.<method>[(argument_type,...)]
-- set a breakpoint in a method
stop at <class id>:<line> -- set a breakpoint at a line
clear <class id>.<method>[(argument_type,...)]
-- clear a breakpoint in a method
clear <class id>:<line> -- clear a breakpoint at a line
clear -- list breakpoints
catch [uncaught|caught|all] <class id>|<class pattern>
-- break when specified exception occurs
ignore [uncaught|caught|all] <class id>|<class pattern>
-- cancel 'catch' for the specified exception
watch [access|all] <class id>.<field name>
-- watch access/modifications to a field
unwatch [access|all] <class id>.<field name>
-- discontinue watching access/modifications to a field
trace [go] methods [thread]
-- trace method entries and exits.
-- All threads are suspended unless 'go' is specified
trace [go] method exit | exits [thread]
-- trace the current method's exit, or all methods' exits
-- All threads are suspended unless 'go' is specified
untrace [methods] -- stop tracing method entrys and/or exits
step -- execute current line
step up -- execute until the current method returns to its caller
stepi -- execute current instruction
next -- step one line (step OVER calls)
cont -- continue execution from breakpoint
list [line number|method] -- print source code
use (or sourcepath) [source file path]
-- display or change the source path
exclude [<class pattern>, ... | "none"]
-- do not report step or method events for specified classes
classpath -- print classpath info from target VM
monitor <command> -- execute command each time the program stops
monitor -- list monitors
unmonitor <monitor#> -- delete a monitor
read <filename> -- read and execute a command file
lock <expr> -- print lock info for an object
threadlocks [thread id] -- print lock info for a thread
pop -- pop the stack through and including the current frame
reenter -- same as pop, but current frame is reentered
redefine <class id> <class file name>
-- redefine the code for a class
disablegc <expr> -- prevent garbage collection of an object
enablegc <expr> -- permit garbage collection of an object
!! -- repeat last command
<n> <command> -- repeat command n times
# <command> -- discard (no-op)
help (or ?) -- list commands
version -- print version information
exit (or quit) -- exit debugger
<class id>: a full class name with package qualifiers
<class pattern>: a class name with a leading or trailing wildcard ('*')
<thread id>: thread number as reported in the 'threads' command
<expr>: a Java(tm) Programming Language expression.
Most common syntax is supported.
Startup commands can be placed in either "jdb.ini" or ".jdbrc"
in user.home or user.dir
Njoy -- *DD*
