Using a default class literal value on an annotation

Question

I want to annotate some of the fields of a given bean class with the following annotation:

@Target({FIELD})
@Retention(RUNTIME)
public @interface Process {

    Class<? extends ProcessingStrategy> using() default DefaultImplStrategy.class;

}

Without going into the domain too much, each annotated property needs to have a ProcessingStrategy defined on it, hence the using() property on the annotation. That's fine and works the way I'd like it to.

I also want to specify a default implementation of the strategy, to be used most of the time (the default defined below). This works fine in Eclipse.

However, when I try to compile this using a regular JDK (invoked through maven) I get the following error:

found   : java.lang.Class<DefaultImplStrategy>
required: java.lang.Class<? extends ProcessingStrategy>

I'm guessing it's some combination of generics, annotations, class literals and defaulting that are at fault here but I honestly do not know why. I've had a look at the rules around default values in the JLS and I don't seem to be violating anything.

Given that DefaultImplStrategy definitely implements ProcessingStrategy, what am I doing wrong here?

Answer 1

The short version of this is that some combination of maven, Lombok and default annotations don't play nicely together. The longer version is on the Lombok mailing list .

The solution is relatively simple: fully qualify the default Type ie

@Target({FIELD})
@Retention(RUNTIME)
public @interface Process {

    Class<? extends ProcessingStrategy> using() default com.example.processing.DefaultImplStrategy.class;

}

Answer 2

不知道为什么，但是如果你给出了DefaultImplStrategy的完整类路径，它可能会起作用