grails metaprogramming

Question

Hi,

My understanding is that there are two obvious places in a Grails app where one can do meta-programming:

1. The init closure of Bootstrap.groovy
2. The doWithDynamicMethods closure of a plugin

The meta-programming I'm referring to here should be visible throughout the metaprogramming, typical examples include adding (or replacing) methods of 3rd party classes.

String.metaClass.myCustomMethod = { /* implementation omitted */ }

The disadvantage of (1), is that the metaprogramming won't be applied when the application is dynamically reloaded. The disadvantage of (2) is that I need to create and maintain an entire plugin just for the sake of a little metaprogramming.

Is there a better place to do this kind of metaprogramming?

Thanks, Don

Answer 1

Depending on your use case, Groovy AST transformations are a third option. AST transformation are modifications of the bytecode at compile time. They are available since Groovy 1.6 and have been improved a lot in Groovy 1.7. Esp. ASTBuilder is a very elegant way.

Be aware that using AST within Grails might require some modifications to the build. The classes performing the AST must be compiled before the classes that are subject to AST. This could be easily done by hooking into the "CompileStart" event in scripts/_Events.groovy and precompile the Transformation first.

Answer 2

Check out the Delegating MetaClass, it's part of groovy and makes your metaclass changes part of the metaclass that's used on every instance of that class right from the start. It operates on a simple convention and even works outside of grails.