How to supply Enum value to an annotation from a Constant in Java

Question

I'm unable to use an Enum taken from a Constant as a parameter in an annotation. I get this compilation error: "The value for annotation attribute [attribute] must be an enum constant expression".

This is a simplified version of the code for the Enum:

public enum MyEnum {
    APPLE, ORANGE
}

For the Annotation:

@Retention(RetentionPolicy.RUNTIME)
@Target({ ElementType.METHOD })
public @interface MyAnnotation {
    String theString();

    int theInt();

    MyEnum theEnum();
}

And the class:

public class Sample {
    public static final String STRING_CONSTANT = "hello";
    public static final int INT_CONSTANT = 1;
    public static final MyEnum MYENUM_CONSTANT = MyEnum.APPLE;

    @MyAnnotation(theEnum = MyEnum.APPLE, theInt = 1, theString = "hello")
    public void methodA() {

    }

    @MyAnnotation(theEnum = MYENUM_CONSTANT, theInt = INT_CONSTANT, theString = STRING_CONSTANT)
    public void methodB() {

    }

}

The error shows up only in "theEnum = MYENUM_CONSTANT" over methodB. String and int constants are ok with the compiler, the Enum constant is not, even though it's the exact same value as the one over methodA. Looks to me like this is a missing feature in the compiler, because all three are obviously constants. There are no method calls, no strange use of classes, etc.

What I want to achieve is:

• To use the MYENUM_CONSTANT in both the annotation and later in the code.
• To stay type safe.

Any way to achieve these goals would be fine.

Edit:

Thanks all. As you say, it cannot be done. The JLS should be updated. I decided to forget about enums in annotations this time, and use regular int constants. As long as the int is assigned from a named constant, the values are bounded and it's "sort of" type safe.

It looks like this:

public interface MyEnumSimulation {
    public static final int APPLE = 0;
    public static final int ORANGE = 1;
}
...
public static final int MYENUMSIMUL_CONSTANT = MyEnumSimulation.APPLE;
...
@MyAnnotation(theEnumSimulation = MYENUMSIMUL_CONSTANT, theInt = INT_CONSTANT, theString = STRING_CONSTANT)
public void methodB() {
...

And I can use MYENUMSIMUL_CONSTANT anywhere else in the code.

Answer 1

"All problems in computer science can be solved by another level of indirection" --- David Wheeler

Here it is:

Enum class:

public enum Gender {
    MALE(Constants.MALE_VALUE), FEMALE(Constants.FEMALE_VALUE);

    Gender(String genderString) {
    }

    public static class Constants {
        public static final String MALE_VALUE = "MALE";
        public static final String FEMALE_VALUE = "FEMALE";
    }
}

Person class:

import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.annotation.JsonSubTypes;
import com.fasterxml.jackson.annotation.JsonTypeInfo;
import static com.fasterxml.jackson.annotation.JsonTypeInfo.As;
import static com.fasterxml.jackson.annotation.JsonTypeInfo.Id;

@JsonTypeInfo(use = Id.NAME, include = As.PROPERTY, property = Person.GENDER)
@JsonSubTypes({
    @JsonSubTypes.Type(value = Woman.class, name = Gender.Constants.FEMALE_VALUE),
    @JsonSubTypes.Type(value = Man.class, name = Gender.Constants.MALE_VALUE)
})
public abstract class Person {
...
}

Answer 2

I think that the most voted answer is incomplete, since it does not guarantee at all that the enum value is coupled with the underlying constant String value. With that solution, one should just decouple the two classes.

Instead, I rather suggest to strengthen the coupling shown in that answer by enforcing the correlation between the enum name and the constant value as follows:

public enum Gender {
    MALE(Constants.MALE_VALUE), FEMALE(Constants.FEMALE_VALUE);

    Gender(String genderString) {
      if(!genderString.equals(this.name()))
        throw new IllegalArgumentException();
    }

    public static class Constants {
        public static final String MALE_VALUE = "MALE";
        public static final String FEMALE_VALUE = "FEMALE";
    }
}

As pointed out by @GhostCat in a comment, proper unit tests must be put in place to ensure the coupling.

Answer 3

It seems to be defined in the JLS #9.7.1:

[...] The type of V is assignment compatible (§5.2) with T, and furthermore:

• [...]
• If T is an enum type, and V is an enum constant.

And an enum constant is defined as the actual enum constant (JLS #8.9.1), not a variable that points to that constant.

Bottom line: if you want to use an enum as a parameter for your annotation, you will need to give it an explicit MyEnum.XXXX value. If you want to use a variable, you will need to pick another type (not an enum).

One possible workaround is to use a String or int that you can then map to your enum - you will loose the type safety but the errors can be spotted easily at runtime (= during tests).

Answer 4

The controlling rule seems to be "If T is an enum type, and V is an enum constant.", 9.7.1. Normal Annotations. From the text, it appears the JLS is aiming for extremely simple evaluation of the expressions in annotations. An enum constant is specifically the identifier used inside the enum declaration.

Even in other contexts, a final initialized with an enum constant does not seem to be a constant expression. 4.12.4. final Variables says "A variable of primitive type or type String, that is final and initialized with a compile-time constant expression (§15.28), is called a constant variable.", but does not include a final of enum type initialized with an enum constant.

I also tested a simple case in which it matters whether an expression is a constant expression - an if surrounding an assignment to an unassigned variable. The variable did not become assigned. An alternative version of the same code that tested a final int instead did make the variable definitely assigned:

  public class Bad {

    public static final MyEnum x = MyEnum.AAA;
    public static final int z = 3;
    public static void main(String[] args) {
      int y;
      if(x == MyEnum.AAA) {
        y = 3;
      }
  //    if(z == 3) {
  //      y = 3;
  //    }
      System.out.println(y);
    }

    enum MyEnum {
      AAA, BBB, CCC
    }
  }

Answer 5

I quote from the last line in the question

Any way to achieve these goals would be fine.

So i tried this

1. Added a enumType parameter to the annotation as a placeholder

   @Retention(RetentionPolicy.RUNTIME)
   @Target({ ElementType.METHOD })
   public @interface MyAnnotation {
   
       String theString();
       int theInt();
       MyAnnotationEnum theEnum() default MyAnnotationEnum.APPLE;
       int theEnumType() default 1;
   }
   

2. Added a getType method in the implementation

   public enum MyAnnotationEnum {
       APPLE(1), ORANGE(2);
       public final int type;
   
       private MyAnnotationEnum(int type) {
           this.type = type;
       }
   
       public final int getType() {
           return type;
       }
   
       public static MyAnnotationEnum getType(int type) {
           if (type == APPLE.getType()) {
               return APPLE;
           } else if (type == ORANGE.getType()) {
               return ORANGE;
           }
           return APPLE;
       }
   }
   

3. Made a change to use an int constant instead of the enum

   public class MySample {
       public static final String STRING_CONSTANT = "hello";
       public static final int INT_CONSTANT = 1;
       public static final int MYENUM_TYPE = 1;//MyAnnotationEnum.APPLE.type;
       public static final MyAnnotationEnum MYENUM_CONSTANT = MyAnnotationEnum.getType(MYENUM_TYPE);
   
       @MyAnnotation(theEnum = MyAnnotationEnum.APPLE, theInt = 1, theString = "hello")
       public void methodA() {
       }
   
       @MyAnnotation(theEnumType = MYENUM_TYPE, theInt = INT_CONSTANT, theString = STRING_CONSTANT)
       public void methodB() {
       }
   
   }
   

I derive the MYENUM constant from MYENUM_TYPE int, so if you change MYENUM you just need to change the int value to the corresponding enum type value.

Its not the most elegant solution, But i'm giving it because of the last line in the question.

Any way to achieve these goals would be fine.

Just a side note, if you try using

public static final int MYENUM_TYPE = MyAnnotationEnum.APPLE.type;

The compiler says at the annotation- MyAnnotation.theEnumType must be a constant

Answer 6

My solution was

public enum MyEnum {

    FOO,
    BAR;

    // element value must be a constant expression
    // so we needs this hack in order to use enums as
    // annotation values
    public static final String _FOO = FOO.name();
    public static final String _BAR = BAR.name();
}

I thought this was the cleanest way. This meets couple of requirements:

• If you want the enums to be numeric
• If you want the enums to be of some other type
• Compiler notifies you if a refactor references a different value
• Cleanest use-case (minus one character): @Annotation(foo = MyEnum._FOO)

EDIT

This leads occasionally to compilation error, which leads to the reason of the original element value must be a constant expression

So this is apparently not an option!