Troubleshooting blues…

“Why won’t my MIDlet connect with my Servlet?”

It’s such a simple question, isn’t it?

Unfortunately, there are so many layers of communication and so many places to make a little (yet fatal) error, that there’s no simple answer that solves the problem every time.

In my experience, establishing the initial “hello world”-level back-and-forth communication between the client and the server is one of the most frustrating steps in client-server programming. Once you’ve established a complete and correct communication circuit, you’ve got your foundation to build on. Before that — even if you know your networking and protocols well — you’re kind of tossing your message into the black void and hoping it comes out the other side…

But troubleshooting — tracking the problem down wherever it may be — is something every good engineer should be ready and able to do. So let’s talk a little bit about strategies for narrowing down where the problem may lie in a simple case such as a MIDlet that fails to connect with a Servlet via HTTP.

1. Does it work locally, on the simulator?

Getting it running on the WTK simulator is the first step to making sure it isn’t your MIDlet or Servlet that are the problem. Unfortunately, even if it works on the simulator, it still may be that your MIDlet is wrong. Sometimes the WTK is excessively lenient about things that your handset might not like such as reading the body of the message before getting the response code.

If it’s not even working on the WTK, what can you do?

As a general rule, at this point, I would strip my example down to the simplest possible example — a servlet that returns “hello world” and a MIDlet that displays the response code and the message recieved — to reduce the number of places where things can go wrong.

Be sure you know where your server logs are, and check them for clues. Try your URL in your browser and see what the browser returns. If you’re running the client and server on the same machine, you can substitute 127.0.0.1 for the IP address to see if it’s some sort of routing issue. If none of that yields any response from the server, try your server’s “Hello World Servlet” tutorial.

On the client side, note that the WTK has a built-in profiler. You can find it under Edit > Preferences > Monitor > Enable Profiling. This will display for you the raw data exchanged between the client and the server. I’m a little wary of this profiler because it starts some processes that sometimes interfere with installing a MIDlet using Project > Run via OTA for some unexplained reason, but when you really need it, the profiler is useful.

Now, let’s suppose it’s working on the WTK simulator but not on the handset.

2. Can the handset successfully make any HTTP communications from Java?

Generally a handset has a configurable profile which gives information regarding proxy servers and whatnot that tells how to get from the operator’s network to the open Internet. Sometimes the handset has a separate profile for Java and for WAP. They’re usually careful to make sure the WAP profile is right, so if they’re separate, one thing to do is to copy the data from the WAP profile to the Java profile, if you can find the right menus to do it…

One way to check if your Java profile is correct is to try some other working MIDlet that makes a connection via HTTP. For example, I ran the MobileZoo MIDlet the other day, and it succeeded in contacting MobileZoo, so I know that my handset is correctly configured to use HTTP from Java.

3. Is the server accepting requests from the outside?

You know the old story: internal routing is different from external routing, and those lovely firewalls… Try your URL on a computer outside of your local network.

Now once you’ve done all that, it should work!!!

Unfortunately for my “Hello World” MIDlet/Servlet it doesn’t…

I think we’ve narrowed it down to the fact that I’ve set up my server to use a non-standard port for HTTP, and some step in the network chain doesn’t like it.

Hopefully it will be up and running this weekend. I’ll keep you posted!!! 😀