Debug with jdb

原文地址： http://www.javaworld.com/article/2077445/testing-debugging/debug-with-jdb.html

Q: How do you use jdb (included in the JDK 1.2 package)
effectively to debug Java programs?

I‘ve tried many times, but I am successful only in loading a class file
to jdb; I can‘t debug it.
The help command isn‘t much use.

A: You ask an interesting question. To be honest,
I‘ve neverused jdb. I have always used the
debugger provided by my IDE environment. So to answer your question I had to do
a little research of my own.

It turns out that Sun considers jdb a proof of concept
for the Java Debugger API. The Java Debugger API allows us to actually peek into
the runtime and debug our code. The jdb is just one
implementation of a debugger that uses the API. Compared to the visual debuggers
with which I‘m familiar (yes, I guess I‘m a wimp), it‘s not the easiest debugger
to use -- though it is similar to other command-line debuggers, such
asgdb.

Anyhow, on to your question. Before attempting to debug your code, be sure to
use the -g option while compiling your classes. This
option tells the compiler to include debugging information in your class
file.

Let‘s define a contrived class for testing:

publicclassTestMe{privateint int_value;privateString string_value;publicstaticvoid main(String[] args){TestMe testMe =newTestMe();

    testMe.setInt_value(1);

    testMe.setString_value("test");int integer = testMe.getInt_value();Stringstring= testMe.getString_value();String toString = testMe.toString();}publicTestMe(){}publicint getInt_value(){return int_value;}publicString getString_value(){return string_value;}publicvoid setInt_value(int value){

    int_value = value;}publicvoid setString_value(String value){

    string_value = value;}publicString toString(){return"String value: "+ string_value +" int value: "+ int_value;}}

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Start the debugger:

> jdb TestMe

You should see:

>Initializing jdb...>0xaa:class

Let‘s take a look at some basic commands. In order to set breakpoints, we
need to know the line numbers or the method names of the places where we would
like to break. To obtain a list of methods, simply use
the methods command:

> methods TestMevoid main(java.lang.String[])void()int getInt_value()

   java.lang.String getString_value()void setInt_value(int)void setString_value(java.lang.String)

   java.lang.String toString()

Setting a breakpoint is simple. Use the following syntax:

stop in.[<argument_type,...>]

Or:

stop at :

We should start debugging at the beginning of the main method:

> stop inTestMe.main

   Breakpointsetin javaworld.TestMe.main

Now that we have a breakpoint, we can begin execution. To run up to the
breakpoint, simply use the run command:

> run

run javaworld.TestMe

running ...

main[1]Breakpoint hit: javaworld.TestMe.main (TestMe:10)

At this point, the debugger halts execution at the first line of the main
method. Notice that the cursor has changed to reflect the method that we are
currently in.

The list command will display the code at the
breakpoint. An arrow indicates the spot where the debugger has halted
execution.

main[1] list

6privateString string_value;78publicstaticvoid main(String[] args)9{10=>TestMe testMe =newTestMe();11             testMe.setInt_value(1);12             testMe.setString_value("test");1314int integer = testMe.getInt_value();

main[1]

Next, we‘ll want to step through a few lines of code
and see what‘s changed:

main[1] step

main[1]Breakpoint hit: javaworld.TestMe.(TestMe:20)

main[1] locals

Method arguments:Local variables:this=String value:nullint value:0

main[1] list

1617String toString = testMe.toString();18}1920=>publicTestMe()21{22}2324publicint getInt_value()

main[1] step

main[1]Breakpoint hit: java.lang.Object.(Object:27)

main[1] list

Unable to find Object.java

main[1] step

main[1]Breakpoint hit: javaworld.TestMe.(TestMe:22)

main[1] list

18}1920publicTestMe()21{22=>}2324publicint getInt_value()25{26return int_value;

main[1] step

main[1]Breakpoint hit: javaworld.TestMe.main (TestMe:10)

main[1] list

6privateString string_value;78publicstaticvoid main(String[] args)9{10=>TestMe testMe =newTestMe();11             testMe.setInt_value(1);12             testMe.setString_value("test");1314int integer = testMe.getInt_value();

main[1] step

main[1]Breakpoint hit: javaworld.TestMe.main (TestMe:11)

main[1] list

78publicstaticvoid main(String[] args)9{10TestMe testMe =newTestMe();11=>     testMe.setInt_value(1);12             testMe.setString_value("test");1314int integer = testMe.getInt_value();15Stringstring= testMe.getString_value();

main[1] locals

Method arguments:Local variables:

  args =

  testMe =String value:nullint value:0

After each step, I called
the list command to see where I was in the code. The
return value from the command listed the line number, but somehow that didn‘t
really help me very much.

As we step, we see that the main method is constructing
a TestMe instance. Each step takes us through the
constructor and finally back into the main method.
The localscommand lists all of the local variables visible in
the current stack. We see that at this point in the main method there are only
two local
variables: args and testMe.

By using step, we can get inside any of the methods to see
what is going on. When we combine step with
the locals command we can see our variables:

main[1] step

main[1]Breakpoint hit: javaworld.TestMe.setInt_value (TestMe:36)

main[1] list

32}3334publicvoid setInt_value(int value)35{36=>     int_value = value;37}3839publicvoid setString_value(String value)40{

main[1] locals

Method arguments:Local variables:

  value =1this=String value:nullint value:0

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If we step one more time, we end up in
the setInt_value()method. If
we step two more times, the method will set
the int_value member to 1 and
return. (To check to see that the method set the value, use
the locals command.)

Of course, when we step, we won‘t always want to trace into
each method we encounter. Some method calls can nest very deeply. If we were
forced to trace through an entire hierarchy, we might never finish.
Luckily, jdb has a way to execute a
method without tracing into that method:
the next command.

jdb also provides a few
other step commands.
The stepi command executes the current instruction. In
other words, the code at the => will execute but the
current line will not advance to the next instruction. You can
call stepi a million times, but
the => displayed from
the list command will not move.

jdb also provides the step
up command. The step up call executes until
the current method returns to its caller. Simply put, this stepper executes a
method and nothing else. Take the following code segment as an example:

int integer = testMe.getInt_value();

If this is our current line and we run step up,
the getInt_value() method will execute. However, that‘s
all that will happen. The return value will not get set
to integer.

jdb also allows us to set multiple breakpoints. To go from
one breakpoint directly to the next, jdb provides
the cont command.

Finally, there are times when we want to look at all the members of an
instance or class. Luckily, jdb provides
the dump and print commands:

main[1]dumpTestMeTestMe=0xa9:class(javaworld.TestMe){

    superclass =0x2:class(java.lang.Object)

    loader =(sun.misc.Launcher$AppClassLoader)0xaa}

main[1]printTestMeTestMe=0xa9:class(javaworld.TestMe)

main[1]dump testMe

testMe =(javaworld.TestMe)0xec{private java.lang.String string_value = test

    privateint int_value =1}

main[1]print testMe

testMe =String value: test int value:1

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When you run dump or print on a
class, you get class information, which includes superclass and loader
information. When you
run dump and print on an instance,
you get instance information, such as data members and their current values.

jdb also provides commands for getting down and dirty in
the threads and stacks. However, these commands are really beyond the scope of
a jdb intro.

One final point: you may ask, "How do you effectively usejdb?"
The effectiveness of use will depend on your comfort level
with jdb. When you first use jdb, the most
important command is help.
The help command lists each command and provides some
basic information to help you get started. Once you have
the help command mastered, you‘ll find yourself using the
commands that set breakpoints, along
with step and list. Any combination of
those commands will allow you to get started
using jdb. step, list, step, list...
should help you quickly locate code that is bombing out on you.

Learn more about this topic

• "Java Language Debugging," from the Postech ME
  Website
  http://mech.postech.ac.kr/Java/java.sun.com/products/JDK/debugging/


• "jdbThe Java Debugger," from Java Developer‘s
  Reference, Mike Cohen, et al. (Sams.net Publishing, 1996)
  http://docs.online.bg/PROGRAMMING/JAVA_Developers_Reference/ch15.htm