How to Use a Constant String Array for Annotation Values in Java?

Is it possible to use a constant string array as an annotation value in Java?

public interface FieldValues {
   String[] FIELD1 = new String[]{"value1", "value2"};
}

In Java, annotations are a form of metadata that provide data about a program but are not part of the program itself. The parameters of an annotation must be resolvable at compile time, making it seem impossible to use a constant variable as an annotation value directly. This article explains why this is the case and how you can work with constant values more effectively in your Java annotations.

// Define the annotation
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface SomeAnnotation {
    String[] locations();
}

// Use an enum to define constants
public enum FieldValues {
    VALUE1("value1"),
    VALUE2("value2");

    private final String value;
    FieldValues(String value) {
        this.value = value;
    }
    public String getValue() {
        return value;
    }
}

// Apply the annotation using enum values
@SomeAnnotation(locations = {FieldValues.VALUE1.getValue(), FieldValues.VALUE2.getValue()})
public class MyClass {
  // class body
}

Causes

• Annotations require their parameters to be constant expressions that can be resolved during compilation.
• Using an array variable such as a class field to initialize annotation values is not allowed in Java, since the array itself does not qualify as a compile-time constant.

Solutions

• You should define your annotation values directly in the annotation's parameter, such as `@SomeAnnotation(locations = {"value1", "value2"})`.
• If you want to use constants, one workaround is to use an enum in combination with the annotation.

Mistake: Trying to use a variable or a non-final array as an annotation value.

Solution: Always use compile-time constants directly in the annotation parameters.

Mistake: Assuming annotation values can be initialized with non-constant expressions.

Solution: Make sure annotation values are constant expressions, such as string literals or enums.