Detailed explanation of the usage of @Target in JDK 5 Annotation

Preface

At present, more and more architectural designs are using annotations, such as spring3.0, struts2 and other frameworks. Let's first look at the definition of the annotation. The following is a code that uses JDK 5 Annotation @Target:

 @Target({ElementType.METHOD})@Retention(RetentionPolicy.RUNTIME)@Inherited@Documentedpublic @interface AsynLog { }

1. Usage of @Target

java.lang.annotation.Target

Used to set the scope of annotation usage

java.lang.annotation.ElementType

Target specifies annotation scopes to use enumeration collections

2. Usage of ElementType

Here is a highlight: ElementType. PACKAGE. It is not used in general classes, but in the fixed file package-info.java. It should be emphasized here that the naming must be "package-info". Since package-info.java is not a legal class, using eclipse to create a class will prompt that it is illegal, so you need to create package-info.java in the way of creating a file.

For example, a range PACKAGE can be used to define:

 @Target({ElementType.PACKAGE,ElementType.METHOD})@Retention(RetentionPolicy.RUNTIME)@Inherited@Documentedpublic @interface AsynLog { }

Then, create the file: package-info.java, with the content as follows:

The code copy is as follows:
@AsynLog
package org.my.commons.logs.annotation;

Keynote: Annotations can only be used within the range set by ElementType, otherwise an error will be compiled. For example: the scope only contains ElementType.METHOD, which means that the annotation can only be used on the class method, and the exception will be compiled outside the scope of use.