MySQL数据库连接池泄露导致MySQL Server超时关闭连接

前言

最近做项目，发现老项目出现xxx，这个错误其实很简单，出现在MySQL数据库Server端对长时间没有使用的client连接执行清楚处理，因为是druid数据库，且在github也出现这样的issue：The last packet successfully received from the server was 6,011 milliseconds ago. The last packet sent successfully to the server was 6,011 milliseconds ago.，所以怀疑是不是druid配置的问题，毕竟连接池会定时的检查连接，但是实际排查发现不是那么回事，是连接池泄露了，只不过并发不大，表现为{conn-10002} discard错误而已。

准备

按照笔者上次的文章mybatis plus相同Id与xml配置错误时，mybatis plus解决逻辑_mybatis plus xml文件查询id名字相同-CSDN博客

准备数据库，且通过开启root权限

demo

demo简单写一下，实际项目肯定复杂很多

   <dependencies><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-web</artifactId><version>2.7.17</version></dependency><dependency><groupId>com.alibaba</groupId><artifactId>druid-spring-boot-starter</artifactId><version>1.2.22</version></dependency><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-jdbc</artifactId><version>2.7.17</version></dependency><dependency><groupId>org.mybatis.spring.boot</groupId><artifactId>mybatis-spring-boot-starter</artifactId><version>2.3.2</version></dependency><dependency><groupId>mysql</groupId><artifactId>mysql-connector-java</artifactId><version>8.0.33</version></dependency></dependencies>

写一个main controller dao

@SpringBootApplication
@MapperScan("org.example.druid.demo.dao")
public class DruidMain {public static void main(String[] args) {SpringApplication.run(DruidMain.class, args);}
}@RestController
public class DemoController {@Autowiredprivate UserDao userDao;@RequestMapping(value = "/demo", method = RequestMethod.GET)public String demo(){int num = userDao.updateUser();return "{\"update\" : " + num + "}";}
}@Mapper
public interface UserDao {@Update("update User set age = 30 where id = 1")int updateUser();
}

配置文件

spring.datasource.druid.url= jdbc:mysql://127.0.0.1:3306/demo?useUnicode=true&characterEncoding=UTF-8&allowMultiQueries=true&autoReconnect=true&useSSL=false&serverTimezone=Asia/Shanghai
spring.datasource.druid.username= root
spring.datasource.druid.password= 123456i
spring.datasource.druid.driver-class-name= com.mysql.cj.jdbc.Driver
spring.datasource.druid.initialSize=2
spring.datasource.druid.minIdle=2
spring.datasource.druid.maxActive=2
spring.datasource.druid.maxWait=20000
spring.datasource.druid.timeBetweenEvictionRunsMillis=1000
spring.datasource.druid.minEvictableIdleTimeMillis=30000
spring.datasource.druid.validationQuery=SELECT 1
spring.datasource.druid.validationQueryTimeout=1
spring.datasource.druid.testOnBorrow=false
spring.datasource.druid.testOnReturn=false
spring.datasource.druid.testWhileIdle=true
spring.datasource.druid.keepAlive=true
spring.datasource.druid.socketTimeout=6000
spring.datasource.druid.poolPreparedStatements=true

访问 http://localhost:8080/demo

这一步实际上没什么问题，但是如果涉及编程式事务，那么可能出现代码逻辑漏洞

@Configuration
public class DemoConfiguration {@Beanpublic PlatformTransactionManager initPlatformTransactionManager(DruidDataSource dataSource) {return new DataSourceTransactionManager(dataSource);}
}@RestController
public class DemoController {@Autowiredprivate UserDao userDao;@Autowiredprivate PlatformTransactionManager transactionManager;@RequestMapping(value = "/demo", method = RequestMethod.GET)public String demo(){TransactionStatus transactionStatus = transactionManager.getTransaction(new DefaultTransactionDefinition());int num = 0;try {num = userDao.updateUser();transactionManager.commit(transactionStatus);} catch (TransactionException e) {transactionManager.rollback(transactionStatus);}return "{\"update\" : " + num + "}";}
}

这样写也没问题，但是，如果我们在事务拿到后，因为一些逻辑return呢

设置MySQL的超时时间

通过临时设置或者永久设置，--来自bing AI搜索

比如我们临时设置1分钟试试，问题复现

表现为{conn-10002} discard

2025-02-25 21:52:26.692 DEBUG 2129 --- [nio-8080-exec-1] c.a.druid.pool.PreparedStatementPool     : {conn-10002, pstmt-20000} exit cache
2025-02-25 21:52:26.695 DEBUG 2129 --- [nio-8080-exec-1] com.alibaba.druid.util.JdbcUtils         : close connection error

排查过程

The last packet successfully received from the server was 6,006 milliseconds ago. The last packet sent successfully to the server was 6,006 milliseconds ago.

2025-02-25 21:52:26.690 ERROR 2129 --- [nio-8080-exec-1] druid.sql.Statement                      : {conn-10002, pstmt-20002} execute error. select * from Usercom.mysql.cj.jdbc.exceptions.CommunicationsException: The last packet successfully received from the server was 183,457 milliseconds ago. The last packet sent successfully to the server was 183,458 milliseconds ago. is longer than the server configured value of 'wait_timeout'. You should consider either expiring and/or testing connection validity before use in your application, increasing the server configured values for client timeouts, or using the Connector/J connection property 'autoReconnect=true' to avoid this problem.at com.mysql.cj.jdbc.exceptions.SQLError.createCommunicationsException(SQLError.java:175) ~[mysql-connector-j-8.0.33.jar:8.0.33]at com.mysql.cj.jdbc.exceptions.SQLExceptionsMapping.translateException(SQLExceptionsMapping.java:64) ~[mysql-connector-j-8.0.33.jar:8.0.33]at com.mysql.cj.jdbc.ClientPreparedStatement.executeInternal(ClientPreparedStatement.java:916) ~[mysql-connector-j-8.0.33.jar:8.0.33]at com.mysql.cj.jdbc.ClientPreparedStatement.execute(ClientPreparedStatement.java:354) ~[mysql-connector-j-8.0.33.jar:8.0.33]at com.alibaba.druid.filter.FilterChainImpl.preparedStatement_execute(FilterChainImpl.java:3462) [druid-1.2.22.jar:na]at com.alibaba.druid.filter.FilterEventAdapter.preparedStatement_execute(FilterEventAdapter.java:434) [druid-1.2.22.jar:na]at com.alibaba.druid.filter.FilterChainImpl.preparedStatement_execute(FilterChainImpl.java:3460) [druid-1.2.22.jar:na]at com.alibaba.druid.wall.WallFilter.preparedStatement_execute(WallFilter.java:686) [druid-1.2.22.jar:na]at com.alibaba.druid.filter.FilterChainImpl.preparedStatement_execute(FilterChainImpl.java:3460) [druid-1.2.22.jar:na]at com.alibaba.druid.filter.FilterEventAdapter.preparedStatement_execute(FilterEventAdapter.java:434) [druid-1.2.22.jar:na]at com.alibaba.druid.filter.FilterChainImpl.preparedStatement_execute(FilterChainImpl.java:3460) [druid-1.2.22.jar:na]at com.alibaba.druid.proxy.jdbc.PreparedStatementProxyImpl.execute(PreparedStatementProxyImpl.java:158) [druid-1.2.22.jar:na]at com.alibaba.druid.pool.DruidPooledPreparedStatement.execute(DruidPooledPreparedStatement.java:483) [druid-1.2.22.jar:na]at org.apache.ibatis.executor.statement.PreparedStatementHandler.query(PreparedStatementHandler.java:65) [mybatis-3.5.14.jar:3.5.14]at org.apache.ibatis.executor.statement.RoutingStatementHandler.query(RoutingStatementHandler.java:80) [mybatis-3.5.14.jar:3.5.14]at org.apache.ibatis.executor.SimpleExecutor.doQuery(SimpleExecutor.java:65) [mybatis-3.5.14.jar:3.5.14]at org.apache.ibatis.executor.BaseExecutor.queryFromDatabase(BaseExecutor.java:336) [mybatis-3.5.14.jar:3.5.14]at org.apache.ibatis.executor.BaseExecutor.query(BaseExecutor.java:158) [mybatis-3.5.14.jar:3.5.14]at org.apache.ibatis.executor.CachingExecutor.query(CachingExecutor.java:110) [mybatis-3.5.14.jar:3.5.14]at org.apache.ibatis.executor.CachingExecutor.query(CachingExecutor.java:90) [mybatis-3.5.14.jar:3.5.14]at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:154) [mybatis-3.5.14.jar:3.5.14]at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:147) [mybatis-3.5.14.jar:3.5.14]at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:142) [mybatis-3.5.14.jar:3.5.14]at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_431]at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_431]at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_431]at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_431]at org.mybatis.spring.SqlSessionTemplate$SqlSessionInterceptor.invoke(SqlSessionTemplate.java:425) [mybatis-spring-2.1.2.jar:2.1.2]at com.sun.proxy.$Proxy57.selectList(Unknown Source) [na:na]at org.mybatis.spring.SqlSessionTemplate.selectList(SqlSessionTemplate.java:224) [mybatis-spring-2.1.2.jar:2.1.2]at org.apache.ibatis.binding.MapperMethod.executeForMany(MapperMethod.java:147) [mybatis-3.5.14.jar:3.5.14]at org.apache.ibatis.binding.MapperMethod.execute(MapperMethod.java:80) [mybatis-3.5.14.jar:3.5.14]at org.apache.ibatis.binding.MapperProxy$PlainMethodInvoker.invoke(MapperProxy.java:141) [mybatis-3.5.14.jar:3.5.14]at org.apache.ibatis.binding.MapperProxy.invoke(MapperProxy.java:86) [mybatis-3.5.14.jar:3.5.14]at com.sun.proxy.$Proxy61.selectUsers(Unknown Source) [na:na]at org.example.druid.demo.controller.DemoController.getUsers(DemoController.java:51) [classes/:na]at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_431]at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_431]at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_431]at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_431]at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:205) [spring-web-5.3.30.jar:5.3.30]at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:150) [spring-web-5.3.30.jar:5.3.30]at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:117) [spring-webmvc-5.3.30.jar:5.3.30]at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:895) [spring-webmvc-5.3.30.jar:5.3.30]at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:808) [spring-webmvc-5.3.30.jar:5.3.30]at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87) [spring-webmvc-5.3.30.jar:5.3.30]at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1072) [spring-webmvc-5.3.30.jar:5.3.30]at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:965) [spring-webmvc-5.3.30.jar:5.3.30]at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1006) [spring-webmvc-5.3.30.jar:5.3.30]at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:898) [spring-webmvc-5.3.30.jar:5.3.30]at javax.servlet.http.HttpServlet.service(HttpServlet.java:529) [tomcat-embed-core-9.0.82.jar:4.0.FR]at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:883) [spring-webmvc-5.3.30.jar:5.3.30]at javax.servlet.http.HttpServlet.service(HttpServlet.java:623) [tomcat-embed-core-9.0.82.jar:4.0.FR]at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:209) [tomcat-embed-core-9.0.82.jar:9.0.82]at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:153) [tomcat-embed-core-9.0.82.jar:9.0.82]at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:51) [tomcat-embed-websocket-9.0.82.jar:9.0.82]at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:178) [tomcat-embed-core-9.0.82.jar:9.0.82]at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:153) [tomcat-embed-core-9.0.82.jar:9.0.82]at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:100) [spring-web-5.3.30.jar:5.3.30]at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:117) [spring-web-5.3.30.jar:5.3.30]at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:178) [tomcat-embed-core-9.0.82.jar:9.0.82]at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:153) [tomcat-embed-core-9.0.82.jar:9.0.82]at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:93) [spring-web-5.3.30.jar:5.3.30]at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:117) [spring-web-5.3.30.jar:5.3.30]at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:178) [tomcat-embed-core-9.0.82.jar:9.0.82]at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:153) [tomcat-embed-core-9.0.82.jar:9.0.82]at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201) [spring-web-5.3.30.jar:5.3.30]at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:117) [spring-web-5.3.30.jar:5.3.30]at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:178) [tomcat-embed-core-9.0.82.jar:9.0.82]at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:153) [tomcat-embed-core-9.0.82.jar:9.0.82]at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:168) [tomcat-embed-core-9.0.82.jar:9.0.82]at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:90) [tomcat-embed-core-9.0.82.jar:9.0.82]at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:481) [tomcat-embed-core-9.0.82.jar:9.0.82]at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:130) [tomcat-embed-core-9.0.82.jar:9.0.82]at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:93) [tomcat-embed-core-9.0.82.jar:9.0.82]at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:74) [tomcat-embed-core-9.0.82.jar:9.0.82]at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342) [tomcat-embed-core-9.0.82.jar:9.0.82]at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:390) [tomcat-embed-core-9.0.82.jar:9.0.82]at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:63) [tomcat-embed-core-9.0.82.jar:9.0.82]at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:928) [tomcat-embed-core-9.0.82.jar:9.0.82]at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1794) [tomcat-embed-core-9.0.82.jar:9.0.82]at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:52) [tomcat-embed-core-9.0.82.jar:9.0.82]at org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1191) [tomcat-embed-core-9.0.82.jar:9.0.82]at org.apache.tomcat.util.threads.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:659) [tomcat-embed-core-9.0.82.jar:9.0.82]at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-9.0.82.jar:9.0.82]at java.lang.Thread.run(Thread.java:750) [na:1.8.0_431]
Caused by: com.mysql.cj.exceptions.CJCommunicationsException: The last packet successfully received from the server was 183,457 milliseconds ago. The last packet sent successfully to the server was 183,458 milliseconds ago. is longer than the server configured value of 'wait_timeout'. You should consider either expiring and/or testing connection validity before use in your application, increasing the server configured values for client timeouts, or using the Connector/J connection property 'autoReconnect=true' to avoid this problem.at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[na:1.8.0_431]at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) ~[na:1.8.0_431]at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[na:1.8.0_431]at java.lang.reflect.Constructor.newInstance(Constructor.java:423) ~[na:1.8.0_431]at com.mysql.cj.exceptions.ExceptionFactory.createException(ExceptionFactory.java:62) ~[mysql-connector-j-8.0.33.jar:8.0.33]at com.mysql.cj.exceptions.ExceptionFactory.createException(ExceptionFactory.java:105) ~[mysql-connector-j-8.0.33.jar:8.0.33]at com.mysql.cj.exceptions.ExceptionFactory.createException(ExceptionFactory.java:150) ~[mysql-connector-j-8.0.33.jar:8.0.33]at com.mysql.cj.exceptions.ExceptionFactory.createCommunicationsException(ExceptionFactory.java:166) ~[mysql-connector-j-8.0.33.jar:8.0.33]at com.mysql.cj.protocol.a.NativeProtocol.readMessage(NativeProtocol.java:582) ~[mysql-connector-j-8.0.33.jar:8.0.33]at com.mysql.cj.protocol.a.NativeProtocol.checkErrorMessage(NativeProtocol.java:762) ~[mysql-connector-j-8.0.33.jar:8.0.33]at com.mysql.cj.protocol.a.NativeProtocol.sendCommand(NativeProtocol.java:701) ~[mysql-connector-j-8.0.33.jar:8.0.33]at com.mysql.cj.protocol.a.NativeProtocol.sendQueryPacket(NativeProtocol.java:1052) ~[mysql-connector-j-8.0.33.jar:8.0.33]at com.mysql.cj.NativeSession.execSQL(NativeSession.java:657) ~[mysql-connector-j-8.0.33.jar:8.0.33]at com.mysql.cj.jdbc.ClientPreparedStatement.executeInternal(ClientPreparedStatement.java:893) ~[mysql-connector-j-8.0.33.jar:8.0.33]... 83 common frames omitted
Caused by: java.io.EOFException: Can not read response from server. Expected to read 4 bytes, read 0 bytes before connection was unexpectedly lost.at com.mysql.cj.protocol.FullReadInputStream.readFully(FullReadInputStream.java:67) ~[mysql-connector-j-8.0.33.jar:8.0.33]at com.mysql.cj.protocol.a.SimplePacketReader.readHeaderLocal(SimplePacketReader.java:81) ~[mysql-connector-j-8.0.33.jar:8.0.33]at com.mysql.cj.protocol.a.SimplePacketReader.readHeader(SimplePacketReader.java:63) ~[mysql-connector-j-8.0.33.jar:8.0.33]at com.mysql.cj.protocol.a.SimplePacketReader.readHeader(SimplePacketReader.java:45) ~[mysql-connector-j-8.0.33.jar:8.0.33]at com.mysql.cj.protocol.a.TimeTrackingPacketReader.readHeader(TimeTrackingPacketReader.java:52) ~[mysql-connector-j-8.0.33.jar:8.0.33]at com.mysql.cj.protocol.a.TimeTrackingPacketReader.readHeader(TimeTrackingPacketReader.java:41) ~[mysql-connector-j-8.0.33.jar:8.0.33]at com.mysql.cj.protocol.a.MultiPacketReader.readHeader(MultiPacketReader.java:54) ~[mysql-connector-j-8.0.33.jar:8.0.33]at com.mysql.cj.protocol.a.MultiPacketReader.readHeader(MultiPacketReader.java:44) ~[mysql-connector-j-8.0.33.jar:8.0.33]at com.mysql.cj.protocol.a.NativeProtocol.readMessage(NativeProtocol.java:576) ~[mysql-connector-j-8.0.33.jar:8.0.33]... 88 common frames omitted

这个问题在github也有类似的情况，但是不一定是笔者遇到的问题：https://github.com/alibaba/druid/issues/5963

对于连接泄露可以配置泄露连接强制回收，但是这个仅在排查问题使用

removeAbandoned: true
removeAbandonedTimeoutMillis: 30000 #(以毫秒数为单位)
logAbandoned: true

貌似有用，但是实际情况是占用数据库连接的线程并不一定是问题的代码逻辑，我这里仅仅写了一个controller，刚好对应上

所以排查还需要另外的思路，下面演示解决A的锅B来背的案例。

解决方法

开启连接池连接的状态日志，这里以druid为例

com.alibaba.druid.filter.logging.LogFilter

既然查看连接情况，那么这个最关键

spring.datasource.druid.filters=stat,wall,slf4j
logging.level.com.alibaba.druid=TRACE
#最关键的日志
logging.level.druid.sql.Connection=TRACE

当然笔者使用的MySQL版本8.0，这里是客户端缓存，MySQL8.0已经删除了查询缓存，而且这里是更新操作，毕竟查询缓存命中效率比较低，还是吞吐量比较重要，还增加了查询重写插件

以另一个不是事务的请求为例，用于验证问题

update的连接调用

等待1分钟，并发多次请求查询连接后就会报错

错误日志分析

错误的id为10002，从日志往上追溯

这里就是查询业务背了开始更新业务的锅，毕竟查询没有事务。事务代码BUG导致连接没被连接池回收，但被查询业务使用了连接，而且有时候可能和mysql Server的一些超时断掉连接的特性结合，那么问题就复杂了。

找到问题就好处理有问题的代码了，毕竟解决问题很简单，找到问题很难。

总结

我们在写代码时，尽量还是使用框架封装的逻辑，比如连接池，比如Spring声明式事务，可以避免编程式事务在写代码时的逻辑漏洞，因为在没触发的时候很难出现，测试很多时候不充分。实际上一起写jdbc的时候还很少出现这个问题，因为那个时候会非常注意连接的回收，事务的提交逻辑，但是Spring自动管理后，这个就很少注意了，因为注解声明式事务太方便了，只有特殊时候才会自己管理事务。

另外这个问题具备隐蔽性，因为报错是MySQL驱动连接MySQL Server报超时未使用关闭连接，但是此处示例的根源却是编程式事务导致连接池泄露，单个问题实际上可以头痛医头，但是当一些mysqlServer的特性加持下，就变成了头痛医脚的感觉了，解决问题很容易，发现问题很难。