kaolin fire presents :: software :: tutorial :: jdb/gjdb

Debugging java with jdb and gjdb

In this tutorial, we looked at two things. How to get started on a project, and how to use the gjdb. (actually, I was using the jdb in the tutorial and didn't notice til someone pointed it out that the reader went over the gnu version. there are slight differences.)

First, I'll go over the "how to get started with a project" bit, and then I'll use that code to give you a little kickstart with the jdb/gjdb. Part of Project 2 is to do some web crawling. So... what's the first thing you're going to have to get down? (well, besides the first part of the project which is supposed to be the data structure you use in the second).

The FIRST step (in my opinion) is to open a page... any page at all... and print the results to the screen. This ought to give you some idea of what's going on with the networking. (of course, the rest is a piece of cake for you, right? *just kidding, honest*) Now this first step may seem hard to some of you, and juvenile to others, but in any case it is a chunk, and it's a chunk you can do.

Or at least, with my tutorial's help we whipped it out in about 5 minutes, looking at a similar example in a book.... (note: copying other code is called "modularity" as long as you say where you got it... but if you modify it then you don't have to call it anything. :) )

import java.io.*;
import java.net.*;

public class Browse {

  public static void main(String[] argv) {
    if (argv.length != 1) {
      System.out.println("usage: java Browse ");
      System.exit(0);
    }
     
    String dest=argv[0];
    try {
      URL url=new URL(dest);
      BufferedReader in = 
        new BufferedReader(new InputStreamReader(url.openStream()));
      String line;
      while ((line=in.readLine()) != null) {
          System.out.println(line);
      }
    } catch (Exception e) {
      System.out.println("Whee! I got an exception.");
    }
  }

}

Next, we set up out input stream with some somewhat self-explanatory wrappers. url.openStream returns an InputStream for a new socket for the port we connected at. We give that to an InputStreamReader so that we have an easier time of reading the stream, and then we give that to a BufferedReader because well, buffering is good. (if you want me to explain WHY buffering, I'll try).

What the while loop does is simply milk the input stream for any information it can, and dump it straight to the screen. The exception will most likely only happen if the URL does not exist, although there are other possibilities. Then the program will end.

Now you know how to open a URL and see what the output looks like!

And now I'll go over the gjdb a bit. If you've ever used the gdb, it's somewhat similar, but if you haven't, DON'T PANIC. (pretend there's a green smiley-face here sticking its tongue out at you) The gjdb is really snifty at telling you what your program is doing when, and it comes with the jdk.... (well, the jdb does, so, close enough, and I'll give you both versions as best as I can remember).

First, a note: REMEMBER TO COMPILE WITH -g (i.e.: javac -g Browse.java)
This makes your class files bigger, and puts debugging and line number info into them.

Another note: gjdb seems to be somewhat slower in starting... but is more "standardized" in its usage...

The easiest way to start the debugger is:

     jdb

gjdb

    load Browse

load Browse

    stop in Browse.main

break Browse.main

    run http://erif.org/

run http://erif.org/

    stop at Browse:17

 break Browse:17

    cont

cont

    locals

info locals