浅谈Spring自定义注解从入门到精通

@Target({ ElementType.FIELD}) 
//只允许用在类的字段上
@Retention(RetentionPolicy.RUNTIME) 
//注解保留在程序运行期间，此时可以通过反射获得定义在某个类上的所有注解
@Constraint(validatedBy = ParamConstraintValidated.class)
public @interface Check {
  /**
   * 合法的参数值
   **/
  String[] paramValues();
 
  /**
   * 提示信息
   **/
  String message() default "参数不为指定值";
 
  Class<?>[] groups() default {};
 
  Class<? extends Payload>[] payload() default {};
}

public class ParamConstraintValidated implements ConstraintValidator<Check, Object> {
  /**
   * 合法的参数值，从注解中获取
   * */
  private List<String> paramValues;
 
  @Override
  public void initialize(Check constraintAnnotation) {
    //初始化时获取注解上的值
    paramValues = Arrays.asList(constraintAnnotation.paramValues());
  }
 
  public boolean isValid(Object o, ConstraintValidatorContext constraintValidatorContext) {
    if (paramValues.contains(o)) {
      return true;
    }
 
    //不在指定的参数列表中
    return false;
  }
}

@Data
public class User {
  /**
   * 姓名
   * */
  private String name;
 
  /**
   * 性别 man or women
   * */
  @Check(paramValues = {"man", "woman"})
  private String sex;
}

@RestController("/api/test")
public class TestController {
  @PostMapping
  public Object test(@Validated @RequestBody User user) {
    return "hello world";
  }
}

@Target({ ElementType.METHOD, ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
public @interface PermissionCheck {
  /**
   * 资源key
   * */
  String resourceKey();
}

public class PermissionCheckInterceptor extends HandlerInterceptorAdapter {
  /**
   * 处理器处理之前调用
   */
  @Override
  public boolean preHandle(HttpServletRequest request, HttpServletResponse response,
               Object handler) throws Exception {
    HandlerMethod handlerMethod = (HandlerMethod)handler;
    PermissionCheck permission = findPermissionCheck(handlerMethod);
 
    //如果没有添加权限注解则直接跳过允许访问
    if (permission == null) {
      return true;
    }
 
    //获取注解中的值
    String resourceKey = permission.resourceKey();
 
    //TODO 权限校验一般需要获取用户信息，通过查询数据库进行权限校验
    //TODO 这里只进行简单演示，如果resourceKey为testKey则校验通过，否则不通过
    if ("testKey".equals(resourceKey)) {
      return true;
    }
 
    return false;
  }
 
  /**
   * 根据handlerMethod返回注解信息
   *
   * @param handlerMethod 方法对象
   * @return PermissionCheck注解
   */
  private PermissionCheck findPermissionCheck(HandlerMethod handlerMethod) {
    //在方法上寻找注解
    PermissionCheck permission = handlerMethod.getMethodAnnotation(PermissionCheck.class);
    if (permission == null) {
      //在类上寻找注解
      permission = handlerMethod.getBeanType().getAnnotation(PermissionCheck.class);
    }
 
    return permission;
  }
}

 @GetMapping("/api/test")
 @PermissionCheck(resourceKey = "test")
 public Object testPermissionCheck() {
   return "hello world";
 }

@Target({ ElementType.METHOD, ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
public @interface CustomCache {
  /**
   * 缓存的key值
   * */
  String key();
}

@Aspect
@Component
public class CustomCacheAspect {
  /**
   * 在方法执行之前对注解进行处理
   *
   * @param pjd
   * @param customCache 注解
   * @return 返回中的值
   * */
  @Around("@annotation(com.cqupt.annotation.CustomCache) && @annotation(customCache)")
  public Object dealProcess(ProceedingJoinPoint pjd, CustomCache customCache) {
    Object result = null;
 
    if (customCache.key() == null) {
      //TODO throw error
    }
 
    //TODO 业务场景会比这个复杂的多，会涉及参数的解析如key可能是#{id}这些，数据查询
    //TODO 这里做简单演示，如果key为testKey则返回hello world
    if ("testKey".equals(customCache.key())) {
      return "hello word";
    }
 
    //执行目标方法
    try {
      result = pjd.proceed();
    } catch (Throwable throwable) {
      throwable.printStackTrace();
    }
 
    return result;
  }
}

@GetMapping("/api/cache")
@CustomCache(key = "test")
public Object testCustomCache() {
  return "don't hit cache";
}