元注解详解

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface CompanyDocument {
    String name();
    String date();
}

@Inherited
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface InheritedAnnotation {
    String value();
}

// 容器注解
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface Schedules {
    Schedule[] value();  // 必须返回重复注解的数组
}

// 重复注解
@Repeatable(Schedules.class)  // 指定容器注解
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface Schedule {
    String cron();
    String desc() default "";
}

public @interface Employee {
    int id();
    String name() default "新员工";
    double salary() default 8888.88;
    String[] skills() default {};
}

import java.lang.annotation.*;
import java.lang.reflect.Field;

/**
 * 飞翔科技自定义校验注解 —— 配合元注解配置
 */

// 1. 标记注解：标识字段需要校验
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
@Documented
@interface NotNull {
    String message() default "字段不能为 null";
}

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
@Documented
@interface Min {
    int value() default 0;
    String message() default "值太小";
}

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
@Documented
@interface Max {
    int value() default 99999;
    String message() default "值太大";
}

// 2. 组合校验注解
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
@Documented
@interface Validate {
    boolean notNull() default false;
    int min() default Integer.MIN_VALUE;
    int max() default Integer.MAX_VALUE;
}

// 3. 飞翔科技员工类
class FeixiangEmployee {
    @NotNull(message = "员工ID不能为空")
    private Integer id;

    @NotNull(message = "员工姓名不能为空")
    private String name;

    @Min(value = 3000, message = "薪资不能低于3000")
    @Max(value = 50000, message = "薪资不能超过50000")
    private double salary;

    @Validate(notNull = true, min = 18, max = 65)
    private int age;

    public FeixiangEmployee(Integer id, String name, double salary, int age) {
        this.id = id;
        this.name = name;
        this.salary = salary;
        this.age = age;
    }

    @Override
    public String toString() {
        return "员工[id=" + id + ", name=" + name
                + ", salary=" + salary + ", age=" + age + "]";
    }
}

// 4. 校验引擎
class Validator {
    public static void validate(Object obj) throws Exception {
        Class<?> clazz = obj.getClass();
        for (Field field : clazz.getDeclaredFields()) {
            field.setAccessible(true);
            Object value = field.get();

            // 处理 @NotNull
            NotNull notNull = field.getAnnotation(NotNull.class);
            if (notNull != null && value == null) {
                throw new RuntimeException(notNull.message());
            }

            // 处理 @Min
            Min min = field.getAnnotation(Min.class);
            if (min != null && value instanceof Number) {
                double numVal = ((Number) value).doubleValue();
                if (numVal < min.value()) {
                    throw new RuntimeException(min.message() + "，当前值: " + numVal);
                }
            }

            // 处理 @Max
            Max max = field.getAnnotation(Max.class);
            if (max != null && value instanceof Number) {
                double numVal = ((Number) value).doubleValue();
                if (numVal > max.value()) {
                    throw new RuntimeException(max.message() + "，当前值: " + numVal);
                }
            }
        }
        System.out.println("✓ " + obj + " 校验通过！");
    }
}

// 5. 测试主类
public class FeixiangAnnotationDemo {
    public static void main(String[] args) {
        System.out.println("========== 飞翔科技注解校验系统 ==========\n");

        // 测试1：合法员工
        FeixiangEmployee emp1 = new FeixiangEmployee(1001, "大翔", 18888.88, 28);
        try {
            Validator.validate(emp1);
        } catch (Exception e) {
            System.out.println("✗ 校验失败: " + e.getMessage());
        }

        // 测试2：薪资过低
        FeixiangEmployee emp2 = new FeixiangEmployee(1002, "小崔", 2500.0, 22);
        try {
            Validator.validate(emp2);
        } catch (Exception e) {
            System.out.println("✗ 校验失败: " + e.getMessage());
        }

        // 测试3：姓名为空
        FeixiangEmployee emp3 = new FeixiangEmployee(1003, null, 8888.88, 30);
        try {
            Validator.validate(emp3);
        } catch (Exception e) {
            System.out.println("✗ 校验失败: " + e.getMessage());
        }

        // 测试4：年龄超限
        FeixiangEmployee emp4 = new FeixiangEmployee(1004, "朱璐", 12000.0, 70);
        try {
            Validator.validate(emp4);
        } catch (Exception e) {
            System.out.println("✗ 校验失败: " + e.getMessage());
        }

        System.out.println("\n========== End ==========");
    }
}

import java.lang.annotation.*;
import java.lang.reflect.Method;
import java.util.*;

/**
 * 飞翔科技定时任务调度系统 —— 演示 @Repeatable 用法
 */

// 1. 容器注解
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
@interface TaskSchedules {
    TaskSchedule[] value();
}

// 2. 可重复的定时任务注解
@Repeatable(TaskSchedules.class)
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
@interface TaskSchedule {
    String cron();          // cron 表达式
    String desc() default ""; // 任务描述
    int priority() default 5; // 优先级 1-10
}

// 3. 任务处理器
class TaskProcessor {

    @TaskSchedule(cron = "0 0 9 * * MON-FRI", desc = "早晨站会提醒", priority = 8)
    @TaskSchedule(cron = "0 30 12 * * MON-FRI", desc = "午休提醒", priority = 3)
    public void dailyMeetingReminder() {
        System.out.println("  [执行] 日常会议提醒任务");
    }

    @TaskSchedule(cron = "0 0 0 1 * *", desc = "月初财务报表生成", priority = 10)
    @TaskSchedule(cron = "0 0 0 15 * *", desc = "月中财务复核", priority = 9)
    @TaskSchedule(cron = "0 0 0 L * *", desc = "月末财务结算", priority = 10)
    public void financialReport() {
        System.out.println("  [执行] 财务报告生成任务");
    }

    @TaskSchedule(cron = "0 0 2 * * SUN", desc = "数据库备份", priority = 7)
    public void databaseBackup() {
        System.out.println("  [执行] 数据库备份任务");
    }

    @TaskSchedule(cron = "0 */5 * * * *", desc = "系统健康检查", priority = 6)
    @TaskSchedule(cron = "0 */1 * * * *", desc = "用户活跃度统计", priority = 4)
    public void systemMonitor() {
        System.out.println("  [执行] 系统监控任务");
    }
}

// 4. 调度器
class Scheduler {
    public static void scanAndPrintTasks(Object obj) {
        Class<?> clazz = obj.getClass();
        System.out.println("扫描类: " + clazz.getSimpleName());
        System.out.println("=".repeat(60));

        for (Method method : clazz.getDeclaredMethods()) {
            // 使用 getAnnotationsByType 读取 @Repeatable 注解（JDK 8 新增方法）
            TaskSchedule[] schedules = method.getAnnotationsByType(TaskSchedule.class);

            if (schedules.length > 0) {
                System.out.println("\n方法: " + method.getName());
                System.out.println("-".repeat(40));
                for (TaskSchedule schedule : schedules) {
                    System.out.printf("  ├─ Cron: %-25s | 描述: %-15s | 优先级: %d\n",
                            schedule.cron(), schedule.desc(), schedule.priority());
                }
            }
        }

        // 也展示 getAnnotation 获取容器注解的方式
        System.out.println("\n" + "=".repeat(60));
        System.out.println("（以下展示通过容器注解 TaskSchedules 获取）\n");
        for (Method method : clazz.getDeclaredMethods()) {
            TaskSchedules container = method.getAnnotation(TaskSchedules.class);
            if (container != null) {
                System.out.println("方法 " + method.getName() + " 有 "
                        + container.value().length + " 个定时任务");
            }
        }
    }
}

// 5. 主程序
public class FeixiangScheduleDemo {
    public static void main(String[] args) {
        System.out.println("========== 飞翔科技定时任务调度系统 ==========\n");

        TaskProcessor processor = new TaskProcessor();
        Scheduler.scanAndPrintTasks(processor);

        System.out.println("\n" + "=".repeat(60));
        System.out.println("飞翔科技的总监 Frank 说道：");
        System.out.println("\"有了 @Repeatable，一个方法可以挂多个定时任务，代码清爽多了！\"");
        System.out.println("\n========== End ==========");
    }
}

// 错误示例
@Retention(RetentionPolicy.CLASS)   // ← 编译后保留但运行时不可反射访问！
@Target(ElementType.METHOD)
@interface MyLog {}

public class ErrorDemo1 {
    @MyLog
    public void doWork() {}

    public static void main(String[] args) throws Exception {
        Method m = ErrorDemo1.class.getMethod("doWork");
        MyLog anno = m.getAnnotation(MyLog.class);
        System.out.println(anno);  // 输出: null ← 注解在运行时消失了！
    }
}

@Retention(RetentionPolicy.RUNTIME)  // ← 正确：运行时可见
@Target(ElementType.METHOD)
@interface MyLog {}

@Inherited
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@interface FeixiangAnnotation {}

@FeixiangAnnotation
interface BaseService {}

class UserService implements BaseService {}  // UserService 不会继承 @FeixiangAnnotation

// 验证
public class ErrorDemo2 {
    public static void main(String[] args) {
        // 接口继承：输出 null
        System.out.println(UserService.class.getAnnotation(FeixiangAnnotation.class)); // null!
        // @Inherited 只对类继承生效，对接口继承无效
    }
}

// 错误示例
@interface BugReports {
    String[] value();  // ← 错误！必须返回 BugReport[]，不能是 String[]
}

@Repeatable(BugReports.class)
@interface BugReport {
    String value();
}
// 编译错误：containing annotation type must declare value() element with return type BugReport[]

public @interface WrongAnnotation {
    Integer count();       // ← 编译错误！不能用包装类型，只能用 int
    List<String> tags();   // ← 编译错误！不能用集合类型
    Date updateDate();     // ← 编译错误！只能用 String、Class、枚举、注解等基础类型
}

// 正确做法
public @interface CorrectAnnotation {
    int count();                 // ✓ 基本类型
    String[] tags();             // ✓ String 数组
    Class<?> entityClass();      // ✓ Class 类型
    ElementType type();          // ✓ 枚举类型
}