Google’s Web Toolkit Release Candidate 1.5 is out.

That’s the stuff programmers can use to develop and debug web applications in Java and then deploy them as highly optimized JavaScript.

That way they’re supposed to be able to sidestep common AJAX headaches like browser compatibility, and enjoy significant performance and productivity gains.

Google Health, the widgetry Google released last week, uses Google Web Toolkit.

It includes Java 5 language support for the Java 5 syntax and capabilities like Java generics, enumerated types, annotations, auto-boxing and variable parameter lists.

Google says the compiler in Google Web Toolkit 1.5 produces code faster than ever – faster than developers could normally write by hand in JavaScript – and delivers performance gains big enough for end users to notice. It says the trick is in the deep inlining, better dead code elimination, and other forms of enhanced static analysis.

The widgetry offers a growing set of libraries for building AJAX, including thoroughly tested reusable libraries for implementing user interfaces, data structures, client/server communication, internationalization, testing and accessibility.