Sunday, December 9, 2012

Composing Java annotations

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;

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