groovy.transform.stc.ClosureParams Maven / Gradle / Ivy
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* Licensed to the Apache Software Foundation (ASF) under one
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* distributed with this work for additional information
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* to you under the Apache License, Version 2.0 (the
* "License"); you may not use this file except in compliance
* with the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing,
* software distributed under the License is distributed on an
* "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
* KIND, either express or implied. See the License for the
* specific language governing permissions and limitations
* under the License.
*/
package groovy.transform.stc;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
/**
* 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 {@link java.util.Map.Entry}
* or a (key,value) pair, like the {@link org.codehaus.groovy.runtime.DefaultGroovyMethods#each(java.util.Map, groovy.lang.Closure)}
* method.
* For those reasons, the {@link ClosureParams} annotation only takes two arguments:
*
* - {@link ClosureParams#value()} defines a {@link groovy.transform.stc.ClosureSignatureHint} hint class
* that the compiler will use to infer the parameter types
* - {@link ClosureParams#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 {@link 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
*/
@Target(ElementType.PARAMETER)
@Retention(RetentionPolicy.RUNTIME)
public @interface ClosureParams {
Class value();
String[] options() default {};
}