Question

Lets say I have a custom Annotation like below:

public @interface myLimits {

    int MIN_LIMIT = 400;

    int stockLimit() default MIN_LIMIT;
}

Instead of hard-coding the value of MIN_LIMIT in the annotation definition, can we externalize it to a properties file?

Something like below – Assuming “min.limit” is defined in a properties file.

public @interface myLimits {

    @value("${min.limit}")
    int MIN_LIMIT;

    int stockLimit() default MIN_LIMIT;
}

How do I externalize MIN_LIMIT instead of hard-coding?

Answer 1

Strictly speaking: you can’t. Annotations are processed in a separate compile round before the rest of the code, which means that everything that appears anywhere in an annotation must be a compile-time constant.

Here’s what the JLS has to say about it:

An element type T is commensurate with an element value V if and only
if one of the following conditions is true:

• T is an array type E[] and either:
  • V is an
    ElementValueArrayInitializer and each ElementValueInitializer
    (analogous to a variable initializer in an array initializer) in V is
    commensurate with E.
  • Or V is an ElementValue that is commensurate with
    T.
• The type of V is assignment compatible (§5.2) with T and,
  furthermore:
  • If T is a primitive type or String, V is a constant
    expression (§15.28).
  • V is not null.
  • if T is Class, or an invocation of Class, and V is a class literal (§15.8.2).
  • If T is an enum type, and V
    is an enum constant.

It is a compile-time error if the element type is
not commensurate with the ElementValue.