Annotations are often used in the spring framework.

Java supports annotations since 1.5.

Annotation understanding:

What are annotations?

Starting from JDK5, Java has added support for metadata, that is, annotations. There is a certain difference between annotations and annotations. Annotations can be understood as special marks in the code.These tags can be read during compilation, class loading, and runtime, and corresponding processing is performed. Through annotations, developers can embed supplementary information in the source code without changing the original code and logic.

How to use annotations?

@XXXXX

When defining classes and methods, add @XXX

@MyFirstAnnotation(value = "test")
    public  void test() throws NoSuchMethodException {
                 //Get the annotations above the method
        MyFirstAnnotation annotation = this.getClass().getMethod("test").getAnnotation(MyFirstAnnotation.class);
 // Use of annotation objects
        System.out.println("method="+annotation.method());
        System.out.println("value="+annotation.value());
    }

How to write a comment yourself?

/**
* Customize a comment
* @Documented Define whether the annotation appears in the javadoc, optional
* @Target(ElementType.TYPE) Specify the type attribute of Annotation, specify that it is a class, interface, or method that it modifies, optional, if not specified, it can be used everywhere
* @Retention(RetentionPolicy.RUNTIME) Specify the policy attributes of the annotation, optional runtime or class loading
 * @interface defines annotations, which means that the Annotation interface is implemented
 *
 */

The underlying implementation is a dynamic proxy.

package com.wang.annotation;


import java.lang.annotation.Documented;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.reflect.Method;

public class AnnotationTest {


    public static void main(String[] args) throws NoSuchMethodException {
        AnnotationTest annotationTest = new AnnotationTest();
        annotationTest.test();
    }

    /**
     * Use of annotations
           * The test method has an annotation MyFirstAnnotation
     * @throws NoSuchMethodException
     */

    @MyFirstAnnotation(value = "test")
    public  void test() throws NoSuchMethodException {
                 //Get the annotations above the method
        MyFirstAnnotation annotation = this.getClass().getMethod("test").getAnnotation(MyFirstAnnotation.class);
 // Use of annotation objects
        System.out.println("method="+annotation.method());
        System.out.println("value="+annotation.value());
    }
}

/**
   * Customize a comment
   * @Documented defines whether the annotation appears in javadoc, optional
   * @Target(ElementType.TYPE) Specify the type attribute of Annotation, specify that it is modified by the class, interface, or method, optional, not specified, it can be used everywhere
   * @Retention(RetentionPolicy.RUNTIME) Specify the policy attributes of the annotation, optional
   * @interface defines annotations, meaning that the Annotation interface is implemented
 *
 */

@Documented
//@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@interface MyFirstAnnotation {
    String value() default "";
    String method() default "POST";
}