Which types can be used for Java annotation members?

The concept of annotations fits really well with the design of my project, until I realized you can't have complex datatypes in the annotation. I got around it by using the class of what I wanted to instantiate rather than an instantiated object of that class. It's not perfect, but java rarely is.

@interface Decorated { Class<? extends PropertyDecorator> decorator() }

interface PropertyDecorator { String decorate(String value) }

class TitleCaseDecorator implements PropertyDecorator {
    String decorate(String value)
}

class Person {
    @Decorated(decorator = TitleCaseDecorator.class)
    String name
}

It's specified by section 9.6.1 of the JLS. The annotation member types must be one of:

• primitive
• String
• an Enum
• another Annotation
• Class
• an array of any of the above

It does seem restrictive, but no doubt there are reasons for it.

Also note that multidimensional arrays (e.g. String[][]) are implicitly forbidden by the above rule.

Arrays of Class are not allowed as described in this answer.

I agree with Skaffman for the Types available.

Additional restriction : it has to be a compile-time constant.

For example, the following are forbidden:

@MyAnnot("a" + myConstantStringMethod())
@MyAnnot(1 + myConstantIntMethod())

Also don't forget that annotations themselves can be part of an annotation definition. This allows some simple annotation nesting - handy in cases where you would like to have one annotation present many times.

For example:

@ComplexAnnotation({
    @SimpleAnnotation(a="...", b=3),
    @SimpleAnnotation(a="...", b=3),
    @SimpleAnnotation(a="...", b=3)
})
public Object foo() {...}

where SimpleAnnotation is

@Target(ElementType.METHOD)
public @interface SimpleAnnotation {
    public String a();
    public int b();
)

and ComplexAnnotation is

@Target(ElementType.METHOD)
public @interface ComplexAnnotation {
    public SimpleAnnotation[] value() default {};
)

Examples taken from: http://web.archive.org/web/20131216093805/https://blogs.oracle.com/toddfast/entry/creating_nested_complex_java_annotations

(original URL: https://blogs.oracle.com/toddfast/entry/creating_nested_complex_java_annotations)