The Mobile Developer

Waba: Run Java programs on your handheld

Waba is an open source product developed by Wabasoft, a small company whose tagline is "we have big ideas for small devices". Waba is not a licensed Java runtime environment officially sanctioned by Sun Microsystems. It's a clean-room implementation of a stripped-down Java interpreter and runtime library. Wabasoft takes great pains to point out that Waba is not Java.

If Waba is not Java, then how can you use it to run Java programs? As it turns out, the Waba interpreter accepts a subset of the bytecodes accepted by a Java interpreter and can read the class files produced by a Java compiler. As long as you avoid operations that produce unsupported bytecodes (anything involving long or double types, threads, threads or exceptions) you can use your favorite Java development tool to write a program that the Waba VM can run.

But what about the runtime library? Just as important as the interpreter are all those classes for doing basic things like string manipulation and displaying a user interface. Licensing concerns aside, porting the standard Java runtime classes to a handheld device could easily exceed the amount of memory available on those devices. So how does Waba do it?

The answer is simple: Waba provides its own runtime library, but with a twist. If you know anything about compiling Java, you know that Java bytecodes will include hard-coded references to a few classes like java.lang.Object, java.lang.String and java.lang.StringBuffer. The Waba VM simply replaces these with references to equivalent classes (waba.lang.Object, etc.) in its own runtime library. Except for these "parallel" classes, the rest of the Waba runtime library is unique to Waba and includes basic functionality for building graphical user interfaces and reading/writing data. However, included in the Waba SDK is a second runtime library that implements the classes in the Waba library in terms of their equivalents in the core Java library. This lets you run a Waba program on a machine with a standard Java runtime environment, which is particularly useful when you're developing and debugging a Waba application. Waba thus lets you write programs that can run unchanged in handheld devices or in web browsers, which is a neat trick.

The Waba VM is free and runs on Palm OS or Windows CE. As I write this, Waba is about to be released, having been in beta for a number of months now and garnered quite a bit of interest from Java enthusiasts. It's being released with an open source license, which could lead to it being ported to other devices. If you're at all interested in running Java programs on small devices you should definitely check it out.