Is there a way to pass a constant to an annotation in Groovy?

Question

In Java, it's possible to pass a constant String as a parameter to an annotation, but I can't figure out how to do the same in Groovy.

For example:

    @Retention(RetentionPolicy.RUNTIME)
    @Target(value=[ElementType.METHOD])
    public @interface MyGroovyAnnotation {
        String value()
    }

    class MyGroovyClass {

        public static final String VALUE = "Something"

        @MyGroovyAnnotation(value=VALUE)
        public String myMethod(String value) {
            return value    
        }
    }

Here, where the method myMethod is annotated with @MyGroovyAnnotation, if I pass a String literal like @MyGroovyAnnotation(value="Something"), it works perfectly, but if I try to pass VALUE like in the example above, I get:

From Eclipse:

Groovy:Expected 'VALUE' to be an inline constant of type java.lang.String in @MyGroovyAnnotation

Running from GroovyConsole:

expected 'VALUE' to be an inline constant of type java.lang.String not a field expression in @MyGroovyAnnotation
 at line: 20, column: 31

Attribute 'value' should have type 'java.lang.String'; but found type 'java.lang.Object' in @MyGroovyAnnotation
 at line: -1, column: -1

Does anybody have any idea what I need to do to get this to work, or if it's even possible? Thanks for any help or insight you can provide.

Solution 1

I ran into this same issue and Gerard's answer works, but I didn't need to make a new Constants class, just refer to the existing class.

For example:

@Retention(RetentionPolicy.RUNTIME)
@Target(value=[ElementType.METHOD])
public @interface MyGroovyAnnotation {
    String value()
}

class MyGroovyClass {

    public static final String VALUE = "Something"

    @MyGroovyAnnotation(value=MyGroovyClass.VALUE)
    public String myMethod(String value) {
        return value    
    }
}

I wanted to leave a comment on the accepted answer, but I didn't have 50 reputation.

Solution 2

The question that's suggested as similar here (accessing-static-field-in-annotation), is different to this, as the answer there was to make the String final, which is already the case here. I've gotten a way to make this work, so I guess it's best that I answer here for others with the same question! :)

The above code doesn't work, but specifying the String to pass to the annotation as a field in another class works fine, oddly enough:

    Retention(RetentionPolicy.RUNTIME)
    @Target(value=[ElementType.METHOD])
    public @interface MyGroovyAnnotation {
        String value()
    }

    class Constants {
        public static final String VALUE = "Something"
    }

    public class MyGroovyClass {

        @MyGroovyAnnotation(value=Constants.VALUE)
        public String myMethod(String value) {
            return value
        }
    }

I'm not sure exactly why one of these situations works and the other doesn't. Reading the comments in the bug mentioned in the aforementioned similar question, it seems that the Groovy developers ran into problems covering all cases that Java covers with respect to passing constant String references as annotation parameters.

Comments

[tim_yates]

possible duplicate of Accessing static field in annotation

[grdryn]

Thanks @tim_yates, I hadn't spotted that one. The question is similar there but the chosen answer isn't really a solution since it doesn't make any difference. I'd upvote the answer that references the groovy bug link, but I don't have enough reputation to do that yet. I'll go through the bug and see if I can provide more information to both this question and that older one. Thanks again!

[grdryn]

Thanks for the feedback Jon. I think this is better than what I had. I've changed the accepted answer to be this one...so now you have 50 rep! :)

[Eric Majerus]

A year and a half later now Jon has 1,130 rep. Quite the increase!