Composing Java annotations

Biju KunjummenDecember 11th, 2012Last Updated: November 8th, 2014

0 513 1 minute read

The allowed attribute types of a Java annotations are deliberately very restrictive, however some clean composite annotation types are possible with the allowed types.

Consider a sample annotation from the tutorial site:

package annotation;
@interface ClassPreamble {
   String author();
   String[] reviewers();
}

Here the author and reviewers are of String and array types which is in keeping with the allowed types of annotation attributes. The following is a comprehensive list of allowed types(as of Java 7):

String
Class
any parameterized invocation of Class
an enum type
an annotation type, do note that cycles are not allowed, the annotated type cannot refer to itself
an array type whose element type is one of the preceding types.

Now, to make a richer ClassPreable consider two more annotation types defined this way:

package annotation;
 
public @interface Author {
 String first() default '';
 String last() default '';
}
 
package annotation;
 
public @interface Reviewer {
 String first() default '';
 String last() default '';
}

With these, the ClassPreamble can be composed from the richer Author and Reviewer annotation types, this way:

package annotation;
@interface ClassPreamble {
   Author author();
   Reviewer[] reviewers();
}

Now an annotation applied on a class looks like this:

package annotation;
 
@ClassPreamble(author = @Author(first = 'John', last = 'Doe')
    , reviewers = {@Reviewer(first = 'first1', last = 'last1'), @Reviewer(last = 'last2') }
)
public class MyClass {
....
}

This is a contrived example just to demonstrate composition of annotations, however this approach is used extensively for real world annotations, for eg, to define a many to many relationship between two JPA entities:

@ManyToMany
@JoinTable(name='Employee_Project',
      joinColumns=@JoinColumn(name='Employee_ID'),
      inverseJoinColumns=@JoinColumn(name='Project_ID'))
private Collection<Project> projects;