Parameter annotation aimed at helping IDEs or the static type checker to infer the parameter types of a closure. Without this annotation, a method signature may look like this:
public <T,R> List<R> doSomething(List<T> source, Closure<R> consumer)
The problem this annotation tries to solve is to define the expected parameter types of the
consumer closure. The generics type defined in
Closure<R> correspond to the
result type of the closure, but tell nothing about what the closure must accept as arguments.
There's no way in Java or Groovy to express the type signature of the expected closure call method from outside the closure itself, so we rely on an annotation here. Unfortunately, annotations also have limitations (like not being able to use generics placeholder as annotation values) that prevent us from expressing the type directly.
Additionally, closures are polymorphic. This means that a single closure can be used with different, valid, parameter signatures. A typical use case can be found when a closure accepts either a Entry or a (key,value) pair, like the org.codehaus.groovy.runtime.DefaultGroovyMethods#each(java.util.Map, groovy.lang.Closure) method.
For those reasons, the ClosureParams annotation takes these arguments:
As a result, the previous signature can be written like this:
public <T,R> List<R> doSomething(List<T> source, @ClosureParams(FirstParam.FirstGenericType.class) Closure<R> consumer)
Which uses the FirstParam.FirstGenericType first generic type of the first argumenthint to tell that the only expected argument type corresponds to the type of the first generic argument type of the first method parameter.