How much of Java can I really use with GWT?

Question

I'd like to learn GWT, and I like the fact that it compiles to Javascript. My question is, how much of Java I can really use with GWT? My guess would be that limitations apply mostly for client side, while on the server side I should be able to make use of any existing Java library, right? Or, will I be only able to use a small subset , because of the compilation to Javascript thing?

What are it's limitations? I am interested in what it's not able to do, or things that require too many workarounds to implement. I need to know if learning GWT is a good choice for a possible freelance carrier in web development.

Answer 1

The GWT website has this documentation exactly to answer that question.

Answer 2

See the JRE emulation docs. Those are the supported out of the box emulated classes that you can use.

"Google Web Toolkit includes a library that emulates a subset of the Java runtime library. The list below shows the set of JRE packages, types and methods that GWT can translate automatically. Note that in some cases, only a subset of methods is supported for a given type."

You can also provide your own emulation for other classes using <super-source/> in your gwt.xml to point to a package that will provide replacement Java classes for those that can't be directly compiled to JavaScript.