GWT Demystified

This will be a first in a series of tutorials and essays digging deeper into some of the more esoteric (but extremely important!) functionality of the Google Web Toolkit. (This article is written by Ray Cromwell, CTO) I will be talking about some of the nitty gritty details of the GWT compiler, how to create your own custom generators and the cool stuff you can do with them, and finally, how to plugin new optimization algorithms into the GWT compiler itself.

The Google Web Toolkit can be considered a package of three fundamental technologies: The Java to Javascript Compiler, the runtime library/APIs, and the Hosted Mode browser. All three are very important components of GWT in their own right, but because the APIs and the Hosted browser are the most visual pieces (you can SEE THEM operating) they tend to have more impact on people, while the real heavy duty stuff being done by the GWT compiler goes mostly unnoticed.

This is why I am not surprised that many language zealots engaged in language wars often appear grimaced when I tell them I am rewriting my Javascript code in GWT. "Huh!? Why!?" Then the inevitable signs of misunderstanding crop up: "Didn't applets show running Java in the browser is slow?", "Doesn't GWT produce bloated code?", etc. Now, I am by no means a zealot for the Java language, but there does appear to be frequent misunderstandings about what the GWT Compiler is, and does.

The GWT Compiler is a real compiler. Not a simple translator that walks an abstract syntax tree turning Java expressions and statements into the nearest Javascript equivalents. In fact, the GWT Compiler performs optimizations that are very hard to do statically on raw Javascript, and hard to do even in regular Java.

For example, static dead code elimination is very hard to do safely in Javascript, and limited in Java as well (at the bytecode level). You can never really be sure that a public method in Java won't be called, because of dynamic class loading, and frankly, you can't even be sure that private methods won't be invoked due to reflection and interception techniques, thus the only safe way to do dead code elimination is to defer it to runtime and let Hotspot deal with it.

In contrast, GWT has very good information on which methods are reachable, and is extremely aggressive at removing unused methods, saving both bandwidth and start up time. The closest equivalent in the Java world would be MIDLets, since the same sorts of closed-world assumptions can be made.

GWT does more than just remove dead code. It performs inlining, polymorphic-to-monomorphic call conversion (devirtualization), a form of type-inferencing which GWT calls Type-Tightening (GWT can infer that a field of type Animal is only ever assigned type Cat. In fact, it can infer that such a field is always null!), and lots of other little tricks. It doesn't appear to have common subexpression elimination, copy propagation, or in-block dead code elimination yet, but in my third tutorial, I will demonstrate a simple naive way to achieve this.

What this means is that in GWT, you don't pay much for what you don't use, and you don't have to worry about figuring out what's used. It means GWT's Javascript output is not bloated. It has a constant factor overhead related to the bootstrap process, but as your applications grow bigger, this code becomes smaller.

The GWT compiler already does a good job producing compact javascript code (Chronoscope is 30,000 lines of Java, 3.2 Megabytes of source, 1.9 Megabyte of compiled byte code, and 137k of Javascript after GWT 1.4 compiles it, and 45k after gzip -9), and there is a lot of headroom still left in terms of optimization that it can do.

So why do I use GWT? Besides the fact that we want to run in environments that don't have Javascript, GWT's Hosted Mode is extremely helpful providing full access to the universe of Java debuggers, and automatic code completion and popup Javadoc is very nice. (Even though I use IntelliJ IDEA, which already has good Javascript code completion and popup documentation)

GWT is a good platform for building AJAX/RIA applications and allows one to easily port or repurpose existing Java codebases and tools -- another good option for developers. It has a bright future ahead of it.

-Ray
Coming up: GWT Generators HOW-TO: generate classes on the fly at compile time.