Question

Hibernate Search, Hibernate, Struts2… I see more examples… In same examples I see the annotation on the field.. Other on the get/set method.. There are differences? Or is casual..

I hope that is not a stupid question!

Saluti!

Luigi

Answer 1

The difference depends on the annotation and how it is used. For example, in Spring you can use the @Controller annotation only on a class. This tells Spring that the class is a controller.

As far as methods are concerned, @RequestMapping is an annotation that goes on a method. For properties, you can have validation annotations like @NotNull (in Hibernate validator).

Annotations are definitely not casual; they carry meaning and can affect the way the code behaves.

From the Java documentation regarding annotations:

Annotations provide data about a
program that is not part of the
program itself. They have no direct
effect on the operation of the code
they annotate.

Annotations have a number of uses,
among them:

• Information for the compiler — Annotations can be used by the
  compiler to detect errors or suppress
  warnings.

• Compiler-time and deployment-time processing — Software tools can
  process annotation information to
  generate code, XML files, and so
  forth.

• Runtime processing — Some annotations are available to be
  examined at runtime.

Annotations can be applied to a
program’s declarations of classes,
fields, methods, and other program
elements.

You can specify what an annotation can annotate by specifying the the elements (using a @Target annotation) when you define your own annotation.