48| 项目中的ThreadLocal、事务、锁问题讨论与解决

小滴课堂讲师 2024年09月21日预计阅读 2 分钟原文

### 常见的几个坑之ThreadLocal和锁粒度

* ThreadLoacl的问题
  * set之后建议remove

```java
@Slf4j
@Component
public class LoginInterceptor implements HandlerInterceptor {
    public static ThreadLocal<LoginUser> threadLocal = new ThreadLocal<>();
    
    @Override
    public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception {
        threadLocal.remove();
    }
 }
```

* 优惠券锁粒度
  *  "lock:coupon:"+couponId+userid，锁粒度更细化。如果不加userId，当多个人添加优惠券会被锁住。

* mq监听加锁的逻辑bug
  * 口误，应该是先加锁，然后操作业务逻辑

```java
@RabbitHandler
public void releaseCouponRecord(CouponRecordMessage recordMessage, Message message, Channel channel) throws IOException {
 //防止同个解锁任务并发进入；如果是串行消费不用加锁；加锁有利也有弊，看项目业务逻辑而定
Lock lock = redissonClient.getLock("lock:coupon_record_release:"+recordMessage.getTaskId());
        lock.lock();
        log.info("监听到消息：releaseCouponRecord消息内容：{}", recordMessage);
          // ...
        finally {
            lock.unlock();
        }
}
```

### 常见的几个坑之事务和锁的先后顺序

* 问题：

* 分布式锁放在事务内部，并不能解决超领问题，锁释放后，事务没有及时的提交，还是会超领
  * 模拟方式：释放锁后-线程睡眠

```java
      public JsonData addCoupon(Long couponId, CouponCategoryEnum category) {
          LoginUser loginUser = LoginInterceptor.threadLocal.get();
          String lockKey = "lock:coupon:" + couponId+":"+loginUser.getId();
          RLock rLock = redissonClient.getLock(lockKey);
          //多个线程进入，会阻塞等待释放锁
          rLock.lock();
          log.info("领劵接口加锁成功:{}", Thread.currentThread().getId());
          try {
             // ...
          } finally {
              rLock.unlock();
              log.info("解锁成功");
          }
          // 线程睡眠
          try {
              TimeUnit.SECONDS.sleep(10L);
          } catch (InterruptedException e) {
              throw new RuntimeException(e);
          }
  
          return JsonData.buildSuccess();
      }    
  ```

* 解决方式

* 将加锁的的操作放在事务的外层，保证事务提交成功后，才能进行锁的释放

```java
    public JsonData addPromotionCoupon(@ApiParam(value = "优惠券id", required = true) @PathVariable("coupon_id") long couponId) {
        LoginUser loginUser = LoginInterceptor.threadLocal.get();
        String lockKey = "lock:coupon:" + couponId+":"+loginUser.getId();
        RLock rLock = redissonClient.getLock(lockKey);
        //多个线程进入，会阻塞等待释放锁
        rLock.lock();
        try {
           return couponService.addCoupon(couponId, CouponCategoryEnum.NEW_USER)
        } finally {
            rLock.unlock();
            log.info("解锁成功");
        }
        return null;
    }
```

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