Rémi Forax's Blog

March 2007 Archives

@nnotation type or type @nnotation

The purpose of JSR 308 is to allow to define annotation on types.
Currently, the JLS 3 only allows to annotate language elements than accept modifiers so it's not possible to annotate types.

Why allowing this is a good idea ?

There is real interest to allow annotation on Java types, it enables to write safer code by performing static source code analysis. Imagine annotations like:

1. JetBrain's IDEA @NonNull, @Nullable to avoid NullPointerException and helps database mapper
2. @Scoped to denote objects allocated in scoped memory of RTSJ
3. Java Concurrent In Practice @Immutable or @ThreadSafe that allows to write safer code in a multi-threaded environment.

There are two possible syntax, the first one is to allow annotation of type at the left of the type like any other Java annotations. But it introduce an ambiguity because a reader doesn't see clearly if an annotation belong to a variable, a method etc. or its type. In the following sample, @NonNull refers to String and @Column refers to name.

  @Local
  class MyBean {
      @NonNull @Column(name="_NAME") String name;
      @Deprecated @NonNull Dimension getSize() { ... }
  }

The second one is to allow annotation on type at the right of a type, in that case there is no ambiguity.

  @Local
  class MyBean {
      @Column(name="_NAME") String @NonNull name;
      @Deprecated Dimension @NonNull getSize() { ... }
  }

The JSR308 expert group seems to think that the first syntax is better, more Java-like even if it introduce an ambiguity.
I am not totally convinced.

What do you think ?

Cheers, Rémi

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