[J2SE 5.0 topics] [2.7] Annotations

Annotations (Annotations) are a major new feature in J2SE 5.0. In the future, the EJB 3.0 specification will fully utilize this feature to simplify the definition and implementation of Session Bean, Message-Driven Bean, and EntityBean.
Java annotations are more or less affected by. NET. This is not to say that Java has no annotation. Previous annotations such as JavaDoc, especially @ deprecated, are annotations, but this update pushed the annotation to an unprecedented level in Java history. Through annotations, we can implement previously relatively complex advanced functions.
Let's take a look at an actual example:
/*
* Created on 2004-12-28
*
*/
Import java. lang. annotation .*;
/**
* @ Author Sean
*
*/
Public class MyAnnotations {
@ MyAnnotationForMethods (
Index = 1,
Info = "This is a method to test MyAnnotation .",
Developer = "Somebody else"
)
Public void testMethod1 (){
//...
}
@ MyEmptyAnnotation
@ MySingleElementAnnotation ("For instruction purpose only .")
@ MyAnnotationForMethods (
Index = 2,
Info = "This method is to show multiple annotations ."
)
Public void testMethod2 (){
//...
}

}
@ Target (ElementType. METHOD)
@ Interface MyAnnotationForMethods {
Int index ();
String info ();
String developer () default "Sean GAO ";
}
@ Interface MyEmptyAnnotation {
}
@ Interface MySingleElementAnnotation {
String value ();
}
In this example, I have defined three annotations: The first MyAnnotationForMethods contains three members: index, info, and developer. The developer has the default value "Sean GAO ", previous @ Target (ElementType. METHOD) is the annotation of this annotation, indicating that this annotation can only be used for methods; the second MyEmptyAnnotation is an empty annotation that can be used as a tag, such as the earlier Serializable interface; the third MySingleElementAnnotation contains a member, value, which is a rigid rule. When there is only one element, the name value must be used, it can be a value type, String, Class, enum, or an unary array of the preceding data type.