Why Are @NonNull Annotation on FQN Not Allowed?

Question

Is it a bug in Eclipse or why I can't annotate parameters with fully qualified type names (FQN) with @NonNull?

    import org.eclipse.jdt.annotation.NonNull;

    public class Foo {
        // No problem
        public void bar(@NonNull String x) {
        }

        // Error: Type annotations are not allowed on type names used to access
        // static members
        public void baz(@NonNull java.lang.String x) {
        }
    }

Answer 1

This is not a bug in Eclipse. Instead, it has to do with the way annotation types are processed by the Java compiler. The explanation given below illustrates the issue.

First of all, the @NonNull annotation can apply to any use of a type (it is annotated with @Target(ElementType.TYPE_USE)). However, the type it applies to according to the Java Language Specification, is the type the annotation is closest to. This is not always what would be expected:

In the first case, the @NonNull annotation applies to the use of the type String, which is a valid type. As such, the annotation is valid.

In the second case, however, the annotation applies to java, which is a package name. As such, it is not a valid type, so the annotation cannot apply here. This will result in a compile-time error.

The Java Language Specification (paragraph 9.7.4) states:

For example, assume an annotation type TA which is meta-annotated with just @Target(ElementType.TYPE_USE). The terms @TA java.lang.Object and java.@TA lang.Object are illegal because the simple name to which @TA is closest is classified as a package name. On the other hand, java.lang.@TA Object is legal.

From this we can conclude that @NonNull java.lang.String x will not work, but java.lang.@NonNull String x will.

Also see The Java Language Specification, Chapter 9.7.4.