springbot单机秒杀，锁与事务之间的大坑

Posted 2020-11-27 jwcc

一：

先上结论，如果把锁放在事务里面，会出现脏读数据，解决方案：锁上移。

1.有问题的代码：

@Service
public class SeckillServiceImpl extends ServiceImpl<SeckillMapper, Seckill> implements ISeckillService {

    private static Lock lock = new ReentrantLock(true);

    @Autowired
    private ISuccessKilledService successKilledService;

    @Override
    @Transactional
    public String startKill(String id) {

        lock.lock();
        try {
            Seckill seckill = this.getOne(new LambdaQueryWrapper<Seckill>().eq(Seckill::getSeckillId, id));
            int number = seckill.getNumber();
            if (number > 0) {

                Seckill seckill1 = seckill.setNumber(--number);
                this.update(seckill1, new LambdaQueryWrapper<Seckill>().eq(Seckill::getSeckillId, seckill.getSeckillId()));

                SuccessKilled successKilled = new SuccessKilled();
                successKilled.setUserId(1L);
                successKilled.setSeckillId(seckill.getSeckillId());
                successKilled.setState(0);
                successKilled.setCreateTime(new Date());
                successKilledService.save(successKilled);
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
        return "success";
    }
}

本例子中锁在事务里面，并发时就会出现锁已经解锁了单事务没有提交，另一个线程读到了没有 提交之前的数据。

解决方案：

AOP+锁。

@Target({ElementType.PARAMETER, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Servicelock {
    String description() default "";
}

@Component
@Aspect
public class LockAspect {
    private static Lock lock = new ReentrantLock(true);//互斥锁 参数默认false，不公平锁

    @Pointcut("@annotation(com.example.demo.Servicelock)")
    public void lockAspect() {

    }

    @Around("lockAspect()")
    public Object around(ProceedingJoinPoint joinPoint) {
        lock.lock();
        Object obj = null;
        try {
            obj = joinPoint.proceed();
        } catch (Throwable e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
        return obj;
    }
}

注意：通知类型一定要是环绕通知。

 @RequestMapping("start")
    @ResponseBody
    @Servicelock
    public String start(String seckillId) {

        return killService.startKill(seckillId);
    }