Explain the usage of JDK 5 Annotation annotation @target _java

Objective

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

@Target ({Elementtype.method})
@Retention (retentionpolicy.runtime)
@Inherited
@Documented
Public @interface Asynlog {
 

I. The use of @Target

Java.lang.annotation.Target

Used to set the annotation usage range

Java.lang.annotation.ElementType

Target uses ElementType to specify the enumeration collection that the annotation can be scoped to

second, the use of ElementType

Take value	Annotation usage Scope
Method	Can be used on method
TYPE	Can be used on a class or interface
Annotation_type	Available on the annotation type (type modified by @interface)
Constructor	can be used to construct a method on
FIELD	Can be used on domain
Local_variable	Can be used on local variables
PACKAGE	Package information for recording Java files
PARAMETER	Can be used on parameters

Here is the key note: ElementType. PACKAGE. It is not used in a generic class, but is used in a fixed file Package-info.java. Here you need to emphasize that naming must be "Package-info". Since Package-info.java is not a legitimate class, using Eclipse to create a class prompts you to be illegal, so you need to create the Package-info.java in the form of a file.

For example, define the available scope package:

@Target ({Elementtype.package,elementtype.method})
@Retention (retentionpolicy.runtime)
@Inherited
@ Documented public
@interface Asynlog {
 

So, create the file: Package-info.java, which reads as follows:

Copy Code code as follows:

@AsynLog
Package org.my.commons.logs.annotation;

Note: Annotations can only be used within the scope of the ElementType set, otherwise the error will be compiled. For example, a range that contains only elementtype.method indicates that the annotation can only be used on the method of the class and that the exception will be compiled beyond the scope of use.