What do you do when you need to debug a Java
program and your IDE isn’t around to help? You turn to jdb, which
is the Java Debugger provided by Sun. It’s console-based and kind
of a throwback to the good old days of programming, but it works,
and (when you don’t have other options) it’s a great tool to have
in your programming kit.

With jdb, you can do most of the things you’re
used to doing in your normal debugger: set break points, evaluate
variables, step over and into code, etc. Using jdb will require you
to learn some of the jdb commands. To get the most out of your jdb
debugging sessions, you’ll want to compile your code with the -g
switch so that debug information is placed in your class files.

Here’s the output of a simple debug session. To
start the session, we invoke the jdb command with our
application’s class name.

/usr/home/reas>jdb DebugTip

Initializing jdb …

Next, we set a breakpoint in the bar method of
our DebugTip class.

> stop in DebugTip.bar
Deferring breakpoint DebugTip.bar.
It will be set after the class is loaded.

Now, we use the run command to start program

> run
run DebugTip
Set uncaught java.lang.Throwable
Set deferred uncaught java.lang.Throwable
VM Started: Set deferred breakpoint DebugTip.bar

Breakpoint hit: “thread=main”, DebugTip.bar(), line=15 bci=0
15            Date
date = new Date();

The jdb has loaded the class and executed
statements until our breakpoint is hit. It displays the next line
of code to be executed. We use the next command to execute the
next statement.

main[1] next

Step completed: “thread=main”, DebugTip.bar(), line=17 bci=8
System.out.println(“the time is now: ” + date);

After executing the line, jdb stops again. Now
we’ll use the locals
command to display local variables. As you can see by the output,
there is only one: date.

main[1] locals
Method arguments:
Local variables:
date = instance of java.util.Date(id=297)

Using the cont command causes jdb to
continue executing code until the next breakpoint is reached. Since
there are no more breakpoints, execution proceeds until the main
method exits.

main[1] cont
> the time is now: Tue Nov 18 11:46:19 EST 2003

The application exited


Jdb has the features you need for a productive
debugging session. If you’re used to IDE debuggers, the console
interface may be intimidating, but don’t let that stop you from
learning more about it. When you’re trying to identify a problem on
an OS with no GUI or without an IDE, jdb is a useful tool to have
at your disposal.

There are lots of commands, the most important
one being help, which
will list all of your available commands and options. Check out the
documentation and give it a try with the sample class. It will be
time well spent.

import java.util.Date;

public class DebugTip  {
    public static void main(String args[])
        DebugTip dt = new


    public void foo() {


    private void bar() {
        Date date = new

time is now: ” + date);

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