自定义注解的简单用法

自定义注解

@Target({ElementType.METHOD, ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Inherited
@Documented
public @interface MyAnnotation {

    /**
     * 只使用一个变量是默认为value
     */
    String value() default "";

    /**
     * 注解的成员变量的类型是受限的，
     * 合法的类型包括：原始类型、String、Class、Annotation、Enum
     */
    int num() default 1;

}

元注解（对注解进行注解的注解）：

• @Target

ElementType.PACKAGE 包声明
ElementType.TYPE 类、接口声明
ElementType.CONSTRUCTOR 构造方法声明
ElementType.METHOD 方法声明
ElementType.FIELD 字段声明
ElementType.PARAMETER 参数声明

• @Retention

RetentionPolicy.RUNTIME 运行时存在 可以通过反射获取
RetentionPolicy.CLASS 编译时存在 运行时忽略
RetentionPolicy.SOURCE 只在源码中显示 编译时丢弃

• @Inherited
  允许子类继承

• @Documented
  生成javadoc时会包含该注解

注解有传递性

解析

1. 定义一个类

@MyAnnotation("userClass")
public class User {

    @MyAnnotation(value = "id annotation", num = 66)
    private Long id;

    private String name;

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    @MyAnnotation(value = "getName annotation")
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

2. 解析

package morning.cat.annotation;

import org.junit.Test;

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

public class AnnotationTest {


    @Test
    public void test1() {
        Class<User> clazz = User.class;
        // 获取类上注解
        Annotation[] annotationArr = clazz.getAnnotations();
        for (Annotation annotation : annotationArr) {
            if (annotation instanceof MyAnnotation) {
                MyAnnotation mya = (MyAnnotation) annotation;
                System.out.println(mya.value() + " - " + mya.num());
            }

        }
    }

    @Test
    public void test2() {
        Class<User> clazz = User.class;
        Method[] methods = clazz.getMethods();
        // 查找方法上的MyAnnotation注解
        // 方法1
        for (Method method : methods) {
            boolean flag = method.isAnnotationPresent(MyAnnotation.class);
            if (flag) {
                MyAnnotation mya = method.getAnnotation(MyAnnotation.class);
                System.out.println(mya.value() + " - " + mya.num());
                System.out.println();
            }
        }
        // 方法2
        for (Method method : methods) {
            Annotation[] annotations = method.getAnnotations();
            for (Annotation annotation : annotations) {
                if (annotation instanceof MyAnnotation) {
                    MyAnnotation mya = (MyAnnotation) annotation;
                    System.out.println(mya.value() + " - " + mya.num());
                }

            }
        }

    }

    @Test
    public void test3() {
        Class<User> clazz = User.class;

        // 获取私有变量
        Field[] fields = clazz.getDeclaredFields();
        for (Field field : fields) {
            MyAnnotation mya = field.getAnnotation(MyAnnotation.class);
            if (mya != null) {
                System.out.println(mya.value() + " - " + mya.num());
            }
        }
    }
}