1 问题描述
最近有小伙伴在做商品抽奖活动时,在对奖品库存进行扣减,有线程安全的问题,遂加锁synchronized进行同步,
但发现加锁后并没有控制住库存线程安全的问题,导致库存仍被超发。
先简单介绍下,各层的技术
架构
:
中间层框架:
spring
4.1.0
持久层:MyBatis 3.2.6
MVC框架:Spring MVC 4.1.0
存在问题的代码:
- @Override
- public void saveMemberTicket(ApplyTicketReq applyTicketReq) throws ServiceException {
- synchronized (this.class) {
- // 检查库存是否有剩余
- preCheck(applyTicketReq);
- // 扣减库存
- modifyTicketAmount(applyTicketReq);
- }
- }
@Override
public void saveMemberTicket(ApplyTicketReq applyTicketReq) throws ServiceException {
synchronized (this.class) {
// 检查库存是否有剩余
preCheck(applyTicketReq);
// 扣减库存
modifyTicketAmount(applyTicketReq);
}
}
库存扣减超发问题具体描述:
当库存剩余为1时,线程1拿到锁进入同步代码块,扣减库存,线程2等待锁;
当线程1执行完同步代码块时,线程2拿到锁,执行同步代码块,检查到的库存剩余仍为1;【此时,库存应该为0,产生库存扣减超发问题】
2 排查问题
排查问题开始之前,简单说下自己排查问题的几个原则(仅供参考):
问题重现:一定要先重现问题,任何重现不了的问题,都不是问题。同理,任何存在的问题,都必然能再次重现。
由近及远:先确认自己的代码无问题,然后再去确认外部代码无问题(如:框架代码,第三方代码等)。
由外到内:程序就是一个IPO,有输入Input(如:参数、环境等)也有输出Out(如:结果、异常等),输出Out是问题的表象,先确定外部因素Input无问题,再确认程序代码逻辑无问题。
由浅入深:其实就是由易到难、自上向下,先从上层应用排查问题,如:上层API、应用层、HTTP传输等,然后再确认底层应用排查问题,如:底层API、网络层、系统层、字节码、JVM等;
确定synchronized关键字是否起作用;
【建议:尽量慎用synchronized关键字,非常影响程序性能】根据多线程并发
测试
,
可以确认多线程之间是同步执行synchronized代码块,确认synchronized同步执行没问题。
确定Spring事务是否提交成功;查看Spring 事务配置:
-
<!– Transaction Support –>
-
<
tx:advice
id
=
“useTxAdvice”
transaction-manager
=
“txManager”
>
-
<
tx:attributes
>
-
<
tx:method
name
=
“*remove*”
propagation
=
“REQUIRED”
read-only
=
“false”
rollback-for
=
“java.lang.Exception”
no-rollback-for
=
“com.xxx.exception.ServiceException”
/>
-
<
tx:method
name
=
“*save*”
propagation
=
“REQUIRED”
read-only
=
“false”
rollback-for
=
“java.lang.Exception”
no-rollback-for
=
“com.xxx.exception.ServiceException”
/>
-
<
tx:method
name
=
“*modify*”
propagation
=
“REQUIRED”
read-only
=
“false”
rollback-for
=
“java.lang.Exception”
no-rollback-for
=
“com.xxx.exception.ServiceException”
/>
-
<
tx:method
name
=
“*update*”
propagation
=
“REQUIRED”
read-only
=
“false”
rollback-for
=
“java.lang.Exception”
no-rollback-for
=
“com.xxx.exception.ServiceException”
/>
-
<
tx:method
name
=
“create*”
propagation
=
“REQUIRED”
read-only
=
“false”
rollback-for
=
“java.lang.Exception”
no-rollback-for
=
“com.xxx.exception.ServiceException”
/>
-
<
tx:method
name
=
“find*”
propagation
=
“SUPPORTS”
/>
-
<
tx:method
name
=
“get*”
propagation
=
“SUPPORTS”
/>
-
<
tx:method
name
=
“query*”
propagation
=
“SUPPORTS”
/>
-
<
tx:method
name
=
“page*”
propagation
=
“SUPPORTS”
/>
-
<
tx:method
name
=
“count*”
propagation
=
“SUPPORTS”
/>
-
</
tx:attributes
>
-
</
tx:advice
>
-
<!–把事务控制在Service层–>
-
<
aop:config
>
-
<
aop:pointcut
id
=
“pc”
expression
=
“execution(public * com.xxx..service.*.*(..))”
/>
-
<
aop:advisor
pointcut-ref
=
“pc”
advice-ref
=
“useTxAdvice”
/>
-
</
aop:config
>
<!-- Transaction Support -->
<tx:advice id="useTxAdvice" transaction-manager="txManager">
<tx:attributes>
<tx:method name="*remove*" propagation="REQUIRED" read-only="false" rollback-for="java.lang.Exception" no-rollback-for="com.xxx.exception.ServiceException"/>
<tx:method name="*save*" propagation="REQUIRED" read-only="false" rollback-for="java.lang.Exception" no-rollback-for="com.xxx.exception.ServiceException"/>
<tx:method name="*modify*" propagation="REQUIRED" read-only="false" rollback-for="java.lang.Exception" no-rollback-for="com.xxx.exception.ServiceException"/>
<tx:method name="*update*" propagation="REQUIRED" read-only="false" rollback-for="java.lang.Exception" no-rollback-for="com.xxx.exception.ServiceException"/>
<tx:method name="create*" propagation="REQUIRED" read-only="false" rollback-for="java.lang.Exception" no-rollback-for="com.xxx.exception.ServiceException"/>
<tx:method name="find*" propagation="SUPPORTS"/>
<tx:method name="get*" propagation="SUPPORTS"/>
<tx:method name="query*" propagation="SUPPORTS"/>
<tx:method name="page*" propagation="SUPPORTS"/>
<tx:method name="count*" propagation="SUPPORTS"/>
</tx:attributes>
</tx:advice>
<!--把事务控制在Service层-->
<aop:config>
<aop:pointcut id="pc" expression="execution(public * com.xxx..service.*.*(..))" />
<aop:advisor pointcut-ref="pc" advice-ref="useTxAdvice" />
</aop:config>
由于Spring事务是通过AOP实现的,所以在saveMemberTicket方法执行之前会有开启事务,之后会有提交事务逻辑。而synchronized代码块执行是在事务之内执行的,可以推断在synchronized代码块执行完时,事务还未提交,其他线程进入synchronized代码块后,读取的库存数据不是最新的。
3 解决问题
将synchronized关键字加入到Controller层,使synchronized锁的范围大于事务控制的范围。
-
@RequestMapping(
value
=
“applyTicket”
) - @ResponseBody
- public void applyTicket(@FromJson ApplyTicketReq applyTicketReq) throws Exception {
- synchronized (String.valueOf(applyTicketReq.getMemberRoomId()).intern()) {
- synchronized (String.valueOf(applyTicketReq.getTicketId()).intern()) {
- service.saveMemberTicket(applyTicketReq);
- }
- }
- responseMessage(ModelResult.CODE_200,ModelResult.SUCCESS);
- }
@RequestMapping(value = "applyTicket")
@ResponseBody
public void applyTicket(@FromJson ApplyTicketReq applyTicketReq) throws Exception {
synchronized (String.valueOf(applyTicketReq.getMemberRoomId()).intern()) {
synchronized (String.valueOf(applyTicketReq.getTicketId()).intern()) {
service.saveMemberTicket(applyTicketReq);
}
}
responseMessage(ModelResult.CODE_200,ModelResult.SUCCESS);
}
4 总结问题
根据以上的排查过程,已经很清楚的确认了事务与锁之间存在的问题。由于事务范围大于锁代码块范围,在锁代码块执行完成后,此时事务还未提交,导致此时进入锁代码块的其他线程,读到的仍是原有的库存数据。
关于程序加锁自己的一点见解:
建议程序中尽量不要加锁;
尽量在业务和代码层,解决线程安全的问题,实现无锁的线程安全;
如果以上两点都做不到,一定要加锁,尽量使用
Java
.util.concurrent包下的锁(因为是非阻塞锁,基于CAS
算法
实现,具体可以查看AQS类的实现);
如果以上三点仍然都做不到,一定要加阻塞锁:synchronized锁,两个原则:
(1)尽量减小锁粒度;
(2)尽量减小锁的代码范围;