Java中的注解2_自定义注解实现非空判断

1.项目结构如下

2.YangNotNull

package cn.yang37.annotation.p20210425;

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


/**
 * @Description: 自定义注解 实现非空判断
 * @Author: Yiang37
 * @Date: 2021/04/25 21:45:45
 * @Version: 1.0
 */@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface YangNotNull {
     String value();
}

3.YangNotNullExplain

package cn.yang37.annotation.p20210425;


import java.lang.reflect.Field;
import java.util.ArrayList;
import java.util.List;

/**
 * @Description:
 * @Class: YangNotNullExplain
 * @Author: Yiang37
 * @Date: 2021/4/25 21:47
 * @Version: 1.0
 */
public class YangNotNullExplain {
    public static <T> List<String> validate(T t) throws IllegalAccessException {
        ArrayList<String> resList = new ArrayList<>();
        //获取类中声明的Field
        Field[] declaredFields = t.getClass().getDeclaredFields();

        for (Field declaredField : declaredFields) {
            // 判断YangNotNull类型注解是否在declaredField类型上
            if (declaredField.isAnnotationPresent(YangNotNull.class)) {
                // 反射获取的字段值 设置不检查访问权限
                declaredField.setAccessible(true);
                Object value = declaredField.get(t);
                if (value == null) {
                    YangNotNull yangNotNull = declaredField.getAnnotation(YangNotNull.class);
                    resList.add(yangNotNull.value() + "不能为空!");
                }
            }

        }
        return resList;
    }
}

4.User

package cn.yang37.annotation.p20210425;

/**
 * @Description:
 * @Class: User
 * @Author: Yiang37
 * @Date: 2021/4/25 21:56
 * @Version: 1.0
 */
public class User {

    @YangNotNull("inputUsername")
    private String username;

    @YangNotNull("inputAge")
    private int age;

    @YangNotNull("inputSex")
    private String sex;
}

5.UserTest

package cn.yang37.annotation.p20210425;

import java.util.List;

/**
 * @Description:
 * @Class: UserTest
 * @Author: Yiang37
 * @Date: 2021/4/25 21:59
 * @Version: 1.0
 */
public class UserTest {
    public static void main(String[] args) throws IllegalAccessException {
        User user = new User();

        List<String> validateList = YangNotNullExplain.validate(user);

        validateList.forEach(System.out::println);
    }
}

结果

inputUsername不能为空!
inputSex不能为空!