Six month ago we started a new project with GWT, Guice and MyBatis.
Our previous experience was very intensive, we used Spring, old-Struts and classic implemented view tier (jsp, html, jquery).
Our two goals: deliver the business requirements (obviously) – produce a GWT framework reusable in other projects.

We reached the first objective, our application is deployed in production.
But on the other side, until two days ago, we had a big problem: GWT complile time was near to 30 mins and needed almost 4Gb ram.

We were pretty sure that the problem derived from our mental form, and we started to analyze it trying to get rid of any preconceptions.

As you know, the key feature of GWT is the Deferred Binding.
Quote …/doc/latest/DevGuideCodingBasicsDeferred.html
>>There are two ways in which types can be replaced via deferred binding:
Replacement: A type is replaced with another depending on a set of configurable rules.
Code generation: A type is substituted by the result of invoking a code generator at compile time.<<

We develop in java, filled by java paradigms, but GWT generates javascript… was not easy for us to keep in mind this postulate, mostly at the start of the project.
At permutation time GWT “solves” generic types (interfaces or superclasses) used in RPC calls with every possible concrete type that implements/extends them.
So, if you write java you are right, but permutations are slow.

We instantiated MyBatis Criteria in the client side, passing them to the business via RPC… and also we passed though RPC too much superclasses instead of their specific type.
It wasn’t a real mistake, nor a java mistake. But surely was a bad approach to the GWT logic.

Also we learned you should never be scared of the too much code needed in order to let GWT do its job as best as it can. Every time we thought it would be better to replace a class with its superclass or interface, just for refactoring purposes, we did bad. In other words, try to be always as much detailed as you can when writing methods signatures and defining variable types.

So… after changing MyBatis generator to write specific types that extend the Criteria superclass, and after a brief refactoring, now the GWT complation takes 2 mins and 1Gb ram.

You can find a more specific explanation here: http://stackoverflow.com/a/3060233, wrote by Ashwin Prabhu.

Antonio & Federico