最近学习测试mybatis,单个增删改查都没问题,最后使用mvn test的时候发现了几个问题:
- update失败,原因是数据库死锁
- select等待,原因是connection连接池被用光了,需要等待
get:
- 要勇于探索,坚持就是胜利。刚看到错误的时候直接懵逼,因为错误完全看不出来,属于框架内部报错,在犹豫是不是直接睡觉得了,毕竟也快12点了。最后还是给我一点点找到问题所在了。
- 同上,要敢于去深入你不了解的代码,敢于研究不懂的代码。
- 距离一个合格的码农越来越远了,因为越学越觉得漏洞百出,自己的代码到处都是坑。所以,一定要记录下来。
下面记录这两个问题。
1.mysql数据库死锁
这里,感谢http://www.cnblogs.com/lin-xuan/p/5280614.html,我找到了答案。在这里,我还是重现一下:
数据库死锁是事务性数据库 (如SQL Server, MySql等)经常遇到的问题。除非数据库死锁问题频繁出现导致用户无法操作,一般情况下数据库死锁问题不严重。在应用程序中进行try-catch就可以。那么数据死锁是如何产生的呢?
InnoDB实现的是行锁 (row level lock),分为共享锁 (S) 和 互斥锁 (X)。
- 共享锁用于事务read一行。
- 互斥锁用于事务update或delete一行。
当客户A持有共享锁S,并请求互斥锁X;同时客户B持有互斥锁X,并请求共享锁S。以上情况,会发生数据库死锁。
如果还不够清楚,请看下面的例子。
双开两个mysql客户端
客户端A:
开启事务,并锁定共享锁S 在id=12的时候:
mysql> START TRANSACTION; Query OK, 0 rows affected (0.00 sec) mysql> SELECT * FROM blog WHERE id = 12 LOCK IN SHARE MODE; +----+-------+-----------+ | id | name | author_id | +----+-------+-----------+ | 12 | testA | 50 | +----+-------+-----------+ 1 row in set (0.00 sec)
客户端B:
开启事务,尝试删除id=12:
mysql> START TRANSACTION; Query OK, 0 rows affected (0.00 sec) mysql> DELETE FROM blog WHERE id = 12;
删除操作需要互斥锁 (X),但是互斥锁X和共享锁S是不能相容的。所以删除事务被放到锁请求队列中,客户B阻塞。
这时候客户端A也想要删除12:
mysql> DELETE FROM blog WHERE id = 12; Query OK, 1 row affected (0.00 sec)
和参考文章不同的是,居然删除成功了,但客户端B出错了:
ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transaction
于是,我尝试删除13,这下都阻塞了:
我的mybatis测试代码中,因为上一个测试没有commit导致死锁,commit后就ok了。在这里,我想说,数据库的东西全还给老师了,关于锁以及事务需要重新温习一下了。
2.Mybatis中datasource的数据库连接数
当我mvn test的时候,我发现有个查询的test打印日志:
2016-07-21 23:43:53,356 DEBUG [org.apache.ibatis.transaction.jdbc.JdbcTransaction] - Opening JDBC Connection 2016-07-21 23:43:53,356 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] - Waiting as long as 20000 milliseconds for connection.
于是,果然等了一段时间后才执行成功。跟踪源码,找到这处日志就明白了。首先,我这里使用的数据库连接配置是mybatis默认的:
<environment id="development"> <transactionManager type="JDBC"/> <dataSource type="POOLED"> <property name="driver" value="${jdbc.driver}"/> <property name="url" value="${jdbc.url}"/> <property name="username" value="${jdbc.username}"/> <property name="password" value="${jdbc.password}"/> </dataSource> </environment>
当数据库连接池的连接数用光了之后就要等20s再去获取:
![](https://yqfile.alicdn.com/img_1c53668bcee393edac0d7b3b3daff1ae.gif?x-oss-process=image/resize,w_1400/format,webp)
![](https://yqfile.alicdn.com/img_405b18b4b6584ae338e0f6ecaf736533.gif?x-oss-process=image/resize,w_1400/format,webp)
while (conn == null) { synchronized (state) { if (!state.idleConnections.isEmpty()) { // Pool has available connection conn = state.idleConnections.remove(0); if (log.isDebugEnabled()) { log.debug("Checked out connection " + conn.getRealHashCode() + " from pool."); } } else { // Pool does not have available connection if (state.activeConnections.size() < poolMaximumActiveConnections) { // Can create new connection conn = new PooledConnection(dataSource.getConnection(), this); if (log.isDebugEnabled()) { log.debug("Created connection " + conn.getRealHashCode() + "."); } } else { // Cannot create new connection PooledConnection oldestActiveConnection = state.activeConnections.get(0); long longestCheckoutTime = oldestActiveConnection.getCheckoutTime(); if (longestCheckoutTime > poolMaximumCheckoutTime) { // Can claim overdue connection state.claimedOverdueConnectionCount++; state.accumulatedCheckoutTimeOfOverdueConnections += longestCheckoutTime; state.accumulatedCheckoutTime += longestCheckoutTime; state.activeConnections.remove(oldestActiveConnection); if (!oldestActiveConnection.getRealConnection().getAutoCommit()) { try { oldestActiveConnection.getRealConnection().rollback(); } catch (SQLException e) { log.debug("Bad connection. Could not roll back"); } } conn = new PooledConnection(oldestActiveConnection.getRealConnection(), this); oldestActiveConnection.invalidate(); if (log.isDebugEnabled()) { log.debug("Claimed overdue connection " + conn.getRealHashCode() + "."); } } else { // Must wait try { if (!countedWait) { state.hadToWaitCount++; countedWait = true; } if (log.isDebugEnabled()) { log.debug("Waiting as long as " + poolTimeToWait + " milliseconds for connection."); } long wt = System.currentTimeMillis(); state.wait(poolTimeToWait); state.accumulatedWaitTime += System.currentTimeMillis() - wt; } catch (InterruptedException e) { break; } } } } if (conn != null) { if (conn.isValid()) { if (!conn.getRealConnection().getAutoCommit()) { conn.getRealConnection().rollback(); } conn.setConnectionTypeCode(assembleConnectionTypeCode(dataSource.getUrl(), username, password)); conn.setCheckoutTimestamp(System.currentTimeMillis()); conn.setLastUsedTimestamp(System.currentTimeMillis()); state.activeConnections.add(conn); state.requestCount++; state.accumulatedRequestTime += System.currentTimeMillis() - t; } else { if (log.isDebugEnabled()) { log.debug("A bad connection (" + conn.getRealHashCode() + ") was returned from the pool, getting another connection."); } state.badConnectionCount++; localBadConnectionCount++; conn = null; if (localBadConnectionCount > (poolMaximumIdleConnections + 3)) { if (log.isDebugEnabled()) { log.debug("PooledDataSource: Could not get a good connection to the database."); } throw new SQLException("PooledDataSource: Could not get a good connection to the database."); } } } } }
当连接数少于10个的时候回创建,超过10个就会等待,不然就报错。
唯有不断学习方能改变! -- Ryan Miao