groovy.transform.stc

Annotation Type ClosureParams

@Target(value=PARAMETER)
@Retention(value=RUNTIME)
public @interface ClosureParams

Parameter annotation aimed at helping the 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 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 types
• options(), a set of options that are passed to the hint when the type is inferred

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 argument

hint to tell that the only expected argument type corresponds to the type of the first generic argument type of the first method parameter.

Author:

Cédric Champeau

Required Element Summary

Required Elements

Modifier and Type	Required Element and Description
Class<? extends ClosureSignatureHint>	value

Optional Element Summary

Optional Elements

Modifier and Type	Optional Element and Description
String[]	options

Element Detail

value

public abstract Class<? extends ClosureSignatureHint> value

options

public abstract String[] options

Default:

{}