@Target(value=PARAMETER) @Retention(value=RUNTIME) public @interface ClosureParams
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 Map.Entry
or a (key,value) pair, like the DefaultGroovyMethods.each(java.util.Map, groovy.lang.Closure)
method.
For those reasons, the ClosureParams
annotation only takes two arguments:
value()
defines a ClosureSignatureHint
hint class
that the compiler will use to infer the parameter typesoptions()
, a set of options that are passed to the hint when the type is inferredAs 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 argument
Modifier and Type | Required Element and Description |
---|---|
Class<? extends ClosureSignatureHint> |
value |
public abstract Class<? extends ClosureSignatureHint> value
public abstract String[] options