Java custom annotations and instances of reading annotations using reflection

1. Custom annotations

Yuan comments:

@ES5en annotation: Define the annotation interface

@ES8en annotation: Used to restrict the scope of use of the described annotation. Compilation fails when the described annotation exceeds the scope of use. Such as: ElementType METHOD, ElementType. TYPE;

@ES15en annotation: Used to constrain the scope of a defined annotation. There are three scopes of scope:

1, RetentionPolicy.SOURCE: Scope is the source code, applied to the Java file, when javac is executed to remove the annotation.

2. RetentionPolicy. CLASS: The scope is base 2 code, which exists in the class file. When Java is executed, the annotation is removed.

3. RetentionPolicy. RUNTIME: The scope is runtime, which means we can get the comment dynamically.

Documented: Used to specify that javadoc generates the API document to display this comment.

Inherited: Used to specify that a described comment can be inherited by a subclass of the class it describes. By default, it cannot be inherited by a subclass.

Custom annotation interface:

package com.java.annotation;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Target({ElementType.METHOD,ElementType.TYPE})
@Inherited
@Documented
@Retention(RetentionPolicy.RUNTIME)
public @interface Annotation_my {
 
 String name() default " zhang 3";//defalt  Represents the default value 
 
 String say() default "hello world";
 
 int age() default 21;
 
}

Next, we define an interface:

package com.java.annotation;

@Annotation_my // Use the annotations that we just defined 
public interface Person {
 
 @Annotation_my
 public void name();
 
 @Annotation_my
 public void say();
 
 @Annotation_my
 public void age();

}

With the interface defined, we can write the implementation class of the interface (the interface cannot be instantiated)

package com.java.annotation;

@Annotation_my
@SuppressWarnings("unused")
public class Student implements Person {
 
 private String name;

 @Override
 @Annotation_my(name=" Rogue childe ") // Assigned to name  The default is zhang 3
// If no default value is given when defining an annotation, it is required here name initialise 
 public void name() {
  
 }


 @Override
 @Annotation_my(say=" hello world  ! ")
 public void say() {
  
 }

 @Override
 @Annotation_my(age=20)
 public void age() {
  
 }
}

Then we write a test class to test our annotations

package com.java.annotation;

import java.lang.annotation.Annotation;
import java.lang.reflect.Field;
import java.lang.reflect.Method;

public class Text {
 Annotation[] annotation = null;

 public static void main(String[] args) throws ClassNotFoundException {
  new Text().getAnnotation();
 }
 
 public void getAnnotation() throws ClassNotFoundException{
  Class<?> stu = Class.forName("com.java.annotation.Student");// Statically loaded class 
  boolean isEmpty = stu.isAnnotationPresent(com.java.annotation.Annotation_my.class);// judge stu Is the annotation interface we just defined used if(isEmpty){
   annotation = stu.getAnnotations();// Gets in the annotation interface 
   for(Annotation a:annotation){
    Annotation_my my = (Annotation_my)a;// Cast into Annotation_my type 
    System.out.println(stu+":\n"+my.name()+" say: "+my.say()+" my age: "+my.age());
   }
  }
  Method[] method = stu.getMethods();//
  System.out.println("Method");
  for(Method m:method){
   boolean ismEmpty = m.isAnnotationPresent(com.java.annotation.Annotation_my.class);
   if(ismEmpty){
    Annotation[] aa = m.getAnnotations();
    for(Annotation a:aa){
     Annotation_my an = (Annotation_my)a;
     System.out.println(m+":\n"+an.name()+" say: "+an.say()+" my age: "+an.age());
    }
   }
  }
  //get Fields by force
  System.out.println("get Fileds by force !");
  Field[] field = stu.getDeclaredFields();
  for(Field f:field){
   f.setAccessible(true);
   System.out.println(f.getName());
  }
  System.out.println("get methods in interfaces !");
  Class<?> interfaces[] = stu.getInterfaces();
  for(Class<?> c:interfaces){
   Method[] imethod = c.getMethods();
   for(Method m:imethod){
    System.out.println(m.getName());
   }
  }
 }

}