How to create your own annotation in Java and get the value

I will introduce a sample program that creates your own annotation in Java and gets the value of the annotation.

environment

• jdk1.8.0_231

C:\>java -version
java version "1.8.0_231"
Java(TM) SE Runtime Environment (build 1.8.0_231-b11)
Java HotSpot(TM) 64-Bit Server VM (build 25.231-b11, mixed mode)

C:\>

1. Create your own annotation

This is a sample of self-made annotation. Here, the members have "name" and "value".

MyAnnotation.java

package test01;

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

@Retention(RetentionPolicy.RUNTIME)     // (1) @Control how much annotation information is retained in Retantion
@Target({                               // (2) @Target Controls what type of element the annotation can be added to
    ElementType.TYPE,
    ElementType.FIELD,
    ElementType.CONSTRUCTOR,
    ElementType.METHOD
})
public @interface MyAnnotation {        // (3)Create MyAnnotation
    String name();
    int value() default 0;
}

Specify to what stage the annotation information is retained by @ Retention in (1).

The following can be mainly set for @Retention.

Here, specify RetentionPolicy.RUNTIME.

Specify the location where the annotation can be applied with @ Target in (2).

The following can be mainly set for @ Target.

Here, specify ʻElementType.TYPE, ʻElementType.FIELD, ʻElementType.CONSTRUCTOR, and ʻElementType.METHOD in @ Target.

In (3), create an annotation with the name MyAnnotation. Set the members to name and value.

2. Annotate the class

Add your own MyAnnotation to the classes, fields, constructors and methods of the Sample class.

Sample.java

package test01;

@MyAnnotation(name="class", value=100)              // (1)Grant MyAnnotation to class
public class Sample {

    @MyAnnotation(name="field", value=200)          // (2)Grant MyAnnotation to field
    private String name;

    @MyAnnotation(name="constructor", value=300)    // (3)Add MyAnnotation to constructor
    public Sample() {}

    @MyAnnotation(name="method", value=400)         // (4)Give method MyAnnotation
    public void execute() {
        System.out.println("execute");
    }
}

(1) Add your own annotation (MyAnnotation) to the class. (2) Add your own annotation (MyAnnotation) to the field. (3) Add your own annotation (MyAnnotation) to the constructor. (4) Add your own annotation (MyAnnotation) to the method.

3. Get annotation and refer to

Main.java

package test01;

import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.Method;

public class Main {

    public static void main(String[] args)
            throws ClassNotFoundException, NoSuchMethodException, NoSuchFieldException, SecurityException {

        // (1)Get the annotation (MyAnnotation) attached to the class
        Class<?> clazz = Class.forName("test01.Sample");
        MyAnnotation annoClass = (MyAnnotation) clazz.getAnnotation(MyAnnotation.class);
        System.out.println("class annotation : name=" + annoClass.name() + ", value=" + annoClass.value());

        // (2)Get the annotation (MyAnnotation) attached to the field
        Field field = clazz.getDeclaredField("name");
        MyAnnotation annoField = (MyAnnotation) field.getAnnotation(MyAnnotation.class);
        System.out.println("field annotation : name=" + annoField.name() + ", value=" + annoField.value());

        // (3)Get the annotation (MyAnnotation) attached to the constructor
        Constructor<?> cons = clazz.getConstructor();
        MyAnnotation annoCons = (MyAnnotation) cons.getAnnotation(MyAnnotation.class);
        System.out.println("constructor annotation : name=" + annoCons.name() + ", value=" + annoCons.value());

        // (4)Get the annotation (MyAnnotation) attached to the method
        Method method = clazz.getMethod("execute");
        MyAnnotation annoMethod = (MyAnnotation) method.getAnnotation(MyAnnotation.class);
        System.out.println("method annotation : name=" + annoMethod.name() + ", value=" + annoMethod.value());

    }
}

(1) Acquires and displays the contents of the annotation (MyAnnotation) attached to the Sample class. (2) Acquires and displays the contents of the annotation (MyAnnotation) attached to the field name of the Sample class. (3) Acquires and displays the contents of the annotation (MyAnnotation) attached to the constructor of the Sample class. (4) Acquires and displays the contents of the annotation (MyAnnotation) attached to the ʻexecute method of the Sample` class.

reference

Lesson: Annotations (The Java™ Tutorials < Learning the Java Language)

that's all