How do I pass a method to an annotation using Java 8?

Question

I would like to pass a method to an annotation. Is something like this possible?

@MyAnnotation(method = MyClass::myMethod)
private String myVariable;

No: https://stackoverflow.com/questions/1458535/which-types-can-be-used-for-java-annotation-members — Jorn Vernee, May 31 '17 at 17:26
Hopefully this becomes supported someday. It is exactly what I was trying to do. — wheeleruniverse, Jun 29 '18 at 11:07
According to Brian Goetz the implementation efforts are hindering progress on that feature http://mail.openjdk.java.net/pipermail/core-libs-dev/2018-November/056596.html — Mahatma_Fatal_Error, Sep 10 '21 at 15:36

Dan Grahn · Accepted Answer · 2017-06-01T11:20:41.283

17

Passing a method isn't an option. Instead, pass the following which should allow you to find the method using reflection.

@MyAnnotation(clazz=String.class, method="contains", params= {CharSequence.class})

@interface MyAnnotation {
   Class<?> clazz();
   String method();
   Class<?>[] params() default {};
}

MyAnnotation annotation = // get the annotation
annotation.clazz().getMethod(annotation.method(), annotation.params());

edited Jun 01 '17 at 11:20

answered May 31 '17 at 18:09

Dan Grahn

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Note that annotations allow to omit the braces for single elements arrays, so you may write `params=CharSequence.class` in your example use case. To simplify the no-argument case (even further), I’d use `Class>[] params() default {};` in the annotation type declaration, so you can write, e.g. `@MyAnnotation(clazz=String.class, method="isEmpty")`. – Holger Jun 01 '17 at 10:24
1

Alternatively, in order to avoid the risk of mistyping `"contains"`, `@interface MyAnnotation` can have a field `Class extends Function> value()` if you are willing to have one (potentially nested) class per function and use reflection to instantiate the function. – Johannes Brodwall Mar 11 '19 at 23:17

score 5 · Answer 2 · answered May 31 '17 at 18:44

JSL says:

the annotation attributes only can takes: byte, char, double, float, int, long, short, boolean, String, Enum type, Class, Annotation, 1 dimension array type[type.

but a method reference expression must be assigned to a functional interface. so you can't refer a method reference expression at present.

How do I pass a method to an annotation using Java 8?

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