// 1. 類名.class（最安全高效）
Class<String> clazz1 = String.class;
// 2. 對象.getClass()
String s = "";
Class<?> clazz2 = s.getClass();
// 3. Class.forName()（最靈活）
Class<?> clazz3 = Class.forName("java.lang.String");

// 無參構造
Class<?> clazz = User.class;
User user = clazz.getDeclaredConstructor().newInstance();
// 有參構造
Constructor<?> cons = clazz.getConstructor(String.class, int.class);
User user = (User) cons.newInstance("Alice", 25);

Field field = clazz.getDeclaredField("name");
field.setAccessible(true); // 突破私有限制
// 設置值
field.set(user, "Bob");
// 獲取值
String name = (String) field.get(user);

Method method = clazz.getMethod("setName", String.class);
method.invoke(user, "Charlie"); // 調用方法

// 示例：方法耗時監(jiān)控注解
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface TimeMonitor {
    String taskName() default ""; // 屬性帶默認值
    int threshold() default 1000; // 超時閾值(毫秒)
}

@Service
public class OrderService {
    @TimeMonitor(taskName = "訂單處理", threshold = 500)
    public void processOrder() {
        // 業(yè)務代碼...
    }
}

@Aspect // 聲明為切面
@Component // 納入Spring容器
public class TimeMonitorAspect {
    /**
     * 環(huán)繞通知：攔截帶@TimeMonitor注解的方法
     */
    @Around("@annotation(monitor)")
    public Object monitorTime(ProceedingJoinPoint pjp, TimeMonitor monitor) throws Throwable {
        long start = System.currentTimeMillis();
        Object result = pjp.proceed(); // 執(zhí)行目標方法
        long cost = System.currentTimeMillis() - start;
        // 超過閾值打印警告
        if (cost > monitor.threshold()) {
            System.err.printf("[%s] 執(zhí)行耗時 %dms (超過閾值 %dms)%n",
                monitor.taskName(), cost, monitor.threshold());
        }
        return result;
    }
}

// 定義注解
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface Auth {
    String[] roles() default {"USER"}; // 允許的角色
}
// 使用
@RestController
public class AdminController {
    @Auth(roles = {"ADMIN"})
    @DeleteMapping("/users/{id}")
    public void deleteUser(@PathVariable Long id) { ... }
}
// AOP權限校驗
@Around("@annotation(auth)")
public Object checkAuth(ProceedingJoinPoint pjp, Auth auth) {
    // 獲取當前用戶角色
    Set<String> userRoles = getCurrentUserRoles();
    // 檢查權限
    if (!hasRequiredRole(userRoles, auth.roles())) {
        throw new AccessDeniedException("權限不足");
    }
    return pjp.proceed();
}

// 定義
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface AutoLog {
    String value() default "";
}
// AOP處理
@Around("@annotation(log)")
public Object logMethod(ProceedingJoinPoint pjp, AutoLog log) throws Throwable {
    String methodName = pjp.getSignature().getName();
    System.out.println("【" + log.value() + "】開始執(zhí)行: " + methodName);
    Object result = pjp.proceed();
    System.out.println("【" + log.value() + "】執(zhí)行完成");
    return result;
}

// 反射方式讀取注解信息
public void scanAnnotations() {
    Class<?> clazz = OrderService.class;
    for (Method method : clazz.getDeclaredMethods()) {
        if (method.isAnnotationPresent(TimeMonitor.class)) {
            TimeMonitor monitor = method.getAnnotation(TimeMonitor.class);
            System.out.println("監(jiān)控方法: " + monitor.taskName());
        }
    }
}