Java Annotations Annotation Learning notes

Tag: string representing value equal retention type document SRC round

One, custom Note 1. Use keyword @interface2. By default, annotations can be decorated with classes, methods, interfaces, and so on. 3. The following is an example of a basic annotation:

  

   
//注解中的成员变量非常像定义接口
   
public @interface MyAnnotation {
   
    //不带有默认值
   
    String name();
   

   
    //带有默认值
   
    int age() default 20;
   
}
  

4. Interpretation of 3 common methods in reflect

• Getannotion (class<t> annotationclass) returns an annotation of the specified type that exists on the element, if no null is returned
  
• Annotation[] Getannotations () returns all annotations on this element.
  
• Boolean isannotationpersent (class<? extends Annotation> Annotationclass) determines whether the court contains notes on the type of formulation.

Two, Yuan annotation

  

   
@Target(ElementType.METHOD)
   
@Retention(RetentionPolicy.RUNTIME)
   
public @interface ValueBind {
   

   
    filedType type();
   

   
    String value() default "SUSU";
   

   
    enum filedType {
   
        STRING, INT
   
    }
   
}
  

Where @target and @Retention are the meta annotations.

• There are several values @Retention:
  
• 
  
• These are usually retentionpolicy.runtime.
  

There are several values for @Target:
@Inherited indicates that the annotation is inherited. Third, Demo
1. Valuebind annotations

  

   
@Target(ElementType.METHOD)
   
@Retention(RetentionPolicy.RUNTIME)
   
@Documented
   
public @interface ValueBind {
   

   
    filedType type();
   

   
    String value() default "SUSU";
   

   
    enum filedType {
   
        STRING, INT
   
    }
   
}
  

2. Person class

  

   
public class Person implements Serializable {
   

   
    private String name;
   

   
    private int age;
   

   
    public String getName() {
   
        return name;
   
    }
   

   
    @ValueBind(type = ValueBind.filedType.STRING, value = "LCF")
   
    public void setName(String name) {
   
        this.name = name;
   
    }
   

   
    public int getAge() {
   
        return age;
   
    }
   

   
    @ValueBind(type = ValueBind.filedType.INT, value = "21")
   
    public void setAge(int age) {
   
        this.age = age;
   
    }
   

   
    @Override
   
    public String toString() {
   
        return "Person{" +
   
                "name=‘" + name + ‘\‘‘ +
   
                ", age=" + age +
   
                ‘}‘;
   
    }
   
}
  

3. Main method

  

   
import java.lang.reflect.Method;
   

   
public class Main {
   

   
    public static void main(String[] args) throws Exception {
   
        Object person = Class.forName("Person").newInstance();
   
        //以下获取的是：类的注解
   
        // Annotation[] annotations = person.getClass().getAnnotations();
   
        Method[] methods = person.getClass().getMethods();
   
        for (Method method : methods) {
   
            //如果该方法上面具有ValueBind这个注解
   
            if (method.isAnnotationPresent(ValueBind.class)) {
   
                //通过这个注解可以获取到注解中定义的
   
  valuebind  
      
        annotation  
       =  
        method  
        getannotation   valuebind   class   
       
   
                if (annotation.type().equals(ValueBind.filedType.STRING)) {
   
                    method.invoke(person, annotation.value());
   
                } else {
   
                    method.invoke(person, Integer.parseInt(annotation.value()));
   
                }
   
            }
   
        }
   
        System.out.println(person);
   
    }
   
}
  

Output: Person{name= ' LCF ', age=21}

Java Annotations Annotation Learning notes