java annotations (four meta-annotations: @Retention @Target @Document @Inherited)

Turn:

There are four meta-annotations in java: @Retention @Target @Document @Inherited;

　　 @Retention: the retention position of the annotation　　　　　　　　　

　　　　　　@Retention(RetentionPolicy. SOURCE )   //Annotations only exist in the source code, not in the class bytecode file

　　　　　　@Retention(RetentionPolicy. CLASS )  // The default retention policy, annotations will exist in the class bytecode file, but cannot be obtained at runtime,     

　　　　　　@Retention(RetentionPolicy.RUNTIME)   // The annotation will exist in the class bytecode file and can be obtained by reflection at runtime

　　

　　@Target: the target of the annotation

　　　　　　　　

　　　　　　　　@Target(ElementType.TYPE)   // Interface, class, enumeration, annotation

　　　　　　　　@Target(ElementType.FIELD)  // Field, enumeration constant

　　　　　　　　@Target(ElementType.METHOD)  // 方法

　　　　　　　　@Target(ElementType.PARAMETER) // Method parameters

　　　　　　　　@Target(ElementType.CONSTRUCTOR)  // Constructor

　　　　　　　　@Target(ElementType.LOCAL_VARIABLE) // local variable

　　　　　　　　@Target(ElementType.ANNOTATION_TYPE) // 注解

　　　　　　　　@Target(ElementType.PACKAGE) / // 包   

 

     @Document: indicates that the annotation will be included in the javadoc

 

　  @Inherited: Indicates that subclasses can inherit the annotation in the parent class

 

Example:

　　      @Retention(RetentionPolicy.RUNTIME)
　　　　@Target({ElementType.METHOD})
　　　　public @interface AnnatDemo{
　　　　　　　 public int value();
　　　　}

The above code defines the @AnnatDemo annotation, the purpose of which is to annotate the method and keep it in the runtime environment. We can use reflection to obtain the annotation on a method and call the defined method.

 

For example @AnnatDemo acts on the following methods:

public interface IClientProtocolEx extends IProtocol {

　　int METHOD_START=0;

　　@AnnatDemo(METHOD_START)
　　 public String say(String person);

}

 

Then you can use the following code for reflection:

        Class ipt=IClientProtocalEx.class;

　　　Method[] mts=ipt.getMethod();

         for(Method mt:mts)

　　　{

　　　　AnnatDemo ad=mt.getAnnotation(AnnatDemo.class);//If there is no such annotation on the method, return null

           int value=ad.value();

　　     System.out.println("value:"+value);

　　　}

 

Annotations are part of building a base jar. Projects have their own framework, and if used properly, annotations are a good part of it!