Java Annotation 101

JDK 5.0 introduced a few new features including Generics, For/in loop, Autoboxing/Unboxing, Typesafe Enums, Varargs, Static Import and Annotation. This is about 15 years ago, but it is still valuable to do a quick review of this release, esp. the annotation.

Annotations are like meta-tags that you can add to your code and apply to package declarations, type declarations, constructors, methods, fields, parameters and variables.

Simply speaking, an annotation is a mechanism for associating a meta-tag with program elements and allowing the compiler or the VM to extract program behaviors from these annotated elements and generate interdependent code when necessary.

Usually you need use @interface to define an annotation type, then annotate your codes using @MyAnnotation.

There are three annotation types:

• Marker type annotation has no elements, except the annotation name itself.
• Single-element, or single-value type, annotation provides a single piece of data only. This can be represented with a data=value pair or, simply with the value (a shortcut syntax) only, within parenthesis.
• Full-value type annotation has multiple data members. Therefore, you must use a full data=value parameter syntax for each member.

There are two types of annotations available with JDK5:

• Simple
• Meta

There are only three types of simple annotations provided by JDK5. They are:

• Override
• Deprecated
• SuppressWarnings

Meta-annotations, which are actually known as the annotations of annotations, contain four types.

• Target
• Retention
• Documented
• Inherited