Java反射与自定义注解实战指南详解

// 1. 类名.class（最安全高效）
Class<String> clazz1 = String.class;
// 2. 对象.getClass()
String s = "";
Class<?> clazz2 = s.getClass();
// 3. Class.forName()（最灵活）
Class<?> clazz3 = Class.forName("java.lang.String");

// 无参构造
Class<?> clazz = User.class;
User user = clazz.getDeclaredConstructor().newInstance();
// 有参构造
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
// 设置值
field.set(user, "Bob");
// 获取值
String name = (String) field.get(user);

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

// 示例：方法耗时监控注解
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface TimeMonitor {
    String taskName() default ""; // 属性带默认值
    int threshold() default 1000; // 超时阈值(毫秒)
}

@Service
public class OrderService {
    @TimeMonitor(taskName = "订单处理", threshold = 500)
    public void processOrder() {
        // 业务代码...
    }
}

@Aspect // 声明为切面
@Component // 纳入Spring容器
public class TimeMonitorAspect {
    /**
     * 环绕通知：拦截带@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(); // 执行目标方法
        long cost = System.currentTimeMillis() - start;
        // 超过阈值打印警告
        if (cost > monitor.threshold()) {
            System.err.printf("[%s] 执行耗时 %dms (超过阈值 %dms)%n",
                monitor.taskName(), cost, monitor.threshold());
        }
        return result;
    }
}

// 定义注解
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface Auth {
    String[] roles() default {"USER"}; // 允许的角色
}
// 使用
@RestController
public class AdminController {
    @Auth(roles = {"ADMIN"})
    @DeleteMapping("/users/{id}")
    public void deleteUser(@PathVariable Long id) { ... }
}
// AOP权限校验
@Around("@annotation(auth)")
public Object checkAuth(ProceedingJoinPoint pjp, Auth auth) {
    // 获取当前用户角色
    Set<String> userRoles = getCurrentUserRoles();
    // 检查权限
    if (!hasRequiredRole(userRoles, auth.roles())) {
        throw new AccessDeniedException("权限不足");
    }
    return pjp.proceed();
}

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

// 反射方式读取注解信息
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("监控方法: " + monitor.taskName());
        }
    }
}