是否可以在Java 9中注释lambda表达式？

Question

This question is now over 3 years old and specifically addressed Java 8, with the accepted answer also citing the Java SE 8 Final Specification .

I would be interested if something regarding this question will change in Java 9: Is there any way to annotate a lambda expression similar to annotating a corresponding anonymous class?

---

Example:

Annotation:

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE_USE)
public @interface MyTypeAnnotation {
    public String value();
}

Working annotation of anonymous class:

Consumer<String> consumer = new @MyTypeAnnotation("Hello ") Consumer<String>() {
    @Override
    public void accept(String str) {
        System.out.println(str);
    }
};

Annotating a lamba expression, currently not working in Java 8:

Consumer<String> myAnnotatedConsumer = @MyTypeAnnotation("Hello") (p -> System.out.println(p));

Answer 1

The existence of a Stackoverflow question is not sufficient to indicate that such a feature is planned, not even that someone is ever considering it.

If you look at the list of JEPs , you will see that there is no such JEP, not even in a draft state, suggesting such a feature.

Also, if you look at the current state of Java 9's LambdaMetafactory , you will see that no change has been made to support passing the meta information that would be necessary to generate a runtime class with recorded annotation data.

There seems to be some desire to add plenty of meta-information to what actually should be a small piece of throw-away code, but I doubt that the language designer will follow it. Lambda expressions are meant to define a function which encapsulates behavior , not an alternative way to describe an anonymous class. The long term evolution will rather lead to lambda expression implementations which are even less of an ordinary class.

Answer 2

it's interesting that they do annotate the inner class that "represents" the lambda expression in InnerClassLambdaMetafactory.spinInnerClass via :

 mv.visitAnnotation("Ljava/lang/invoke/LambdaForm$Hidden;", true)

But that is annotating a class obviously, not a lambda per-se.

Annotating a lambda would require changes to the invokedynamic and implicitly the LambdaMetafactory as far as I can see - and what would happen when invokedynamic would not create a class for the lambda, but something else , what would happen to those annotations?

Answer 3

According to the What's new in Oracle JDK 9 page, No. This has not changed in Java 9.

Of course, that is not definitive, but the JLS for Java 9 has not been released yet.