Friday, April 18, 2014
What is an annotation ?
Annotation is a metadata that provides data or information about a program which is not a part of the program itself with no direct effect on the code it annotates.
Uses of annotation
- Information used by compiler : They can be used which carries special meaning to the compiler and helps in detecting any compile time errors.
- Deployment-time processing : They can be used by softwares, servers to generate codes, files and also to deploy codes as in case of servlets.
- Runtime processing : They can be used to carry information during runtime.
Annotations can be applied to declarations : declaration of classes, fields, methods etc. From Java 8, annotations can also be applied to use of types as below
- Class intance types
- Type-Cast
- implements clause
- throes exception declaration
Depending on the usage of annotation, the retention policy and target of an annotation declared is mentioned.Before we create our own annotaions, these two things must be kept in mind
- Retention Policy - There is an enum to define named RetentionPolicy. The constants are used to define how long the annotations should be retained. It has three values : SOURCE, CLASS, RUNTIME .
- Target - This is used to define which part of the program can be annotated with that particular annotation. There is an enum ElementType which has several constants PACKAGE, FIELD, METHOD, LOCAL_VARIABLE etc., to define which part should be targeted.
In our next tutorial we will deal with pre-defined annotations in java.
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