背景

项目使用了阿里druid jdbc连接池。某天环境出现网络波动，持续20分钟后，网络恢复，单服务一直无法连接数据库。重启之后正常运行

druid 版本：1.1.21

项目配置

spring.datasource.druid.game.driver-class-name = com.mysql.cj.jdbc.Driver
spring.datasource.druid.game.initial-size = 5
spring.datasource.druid.game.min-idle = 5
spring.datasource.druid.game.max-active = 100
spring.datasource.druid.game.max-wait = 30000
spring.datasource.druid.game.time-between-eviction-runs-millis = 60000
spring.datasource.druid.game.min-evictable-idle-time-millis = 300000
spring.datasource.druid.game.validationQuery = SELECT 'x'
spring.datasource.druid.game.test-while-idle = true
spring.datasource.druid.game.test-on-borrow = false
spring.datasource.druid.game.test-on-return = false
spring.datasource.druid.game.poolPreparedStatements = true
spring.datasource.druid.game.maxPoolPreparedStatementPerConnectionSize = 20
spring.datasource.druid.game.maxOpenPreparedStatements = 20

Druid的Bug现场

翻看当时日志，发现大量CannotGetJdbcConnectionException异常日：

org.mybatis.spring.MyBatisSystemException: nested exception is org.apache.ibatis.exceptions.PersistenceException:

### Error querying database. Cause: org.springframework.jdbc.CannotGetJdbcConnectionException: Failed to obtain JDBC Connection; nested exception is com.alibaba.druid.pool.GetConnectionTimeoutException: wait millis 60000, active 0, maxActive 100, creating 1, createElapseMillis 686180, createErrorCount 3

Caused by: org.apache.ibatis.exceptions.PersistenceException: 
### Error querying database.  Cause: org.springframework.jdbc.CannotGetJdbcConnectionException: Failed to obtain JDBC Connection; nested exception is com.alibaba.druid.pool.GetConnectionTimeoutException: wait millis 60000, active 0, maxActive 100, creating 1, createElapseMillis 686180, createErrorCount 3
### The error may exist in class path resource [mapper/SysDictMapper.xml]
### The error may involve com.stnts.suileyoo.game.api.dao.SysDictDao.select
### The error occurred while executing a query
### Cause: org.springframework.jdbc.CannotGetJdbcConnectionException: Failed to obtain JDBC Connection; nested exception is com.alibaba.druid.pool.GetConnectionTimeoutException: wait millis 60000, active 0, maxActive 100, creating 1, createElapseMillis 686180, createErrorCount 3
    at org.apache.ibatis.exceptions.ExceptionFactory.wrapException(ExceptionFactory.java:30)
    at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:149)
    at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:140)
    at org.apache.ibatis.session.defaults.DefaultSqlSession.selectOne(DefaultSqlSession.java:76)
    at sun.reflect.GeneratedMethodAccessor135.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.mybatis.spring.SqlSessionTemplate$SqlSessionInterceptor.invoke(SqlSessionTemplate.java:426)
    ... 95 common frames omitted
Caused by: org.springframework.jdbc.CannotGetJdbcConnectionException: Failed to obtain JDBC Connection; nested exception is com.alibaba.druid.pool.GetConnectionTimeoutException: wait millis 60000, active 0, maxActive 100, creating 1, createElapseMillis 686180, createErrorCount 3
    at org.springframework.jdbc.datasource.DataSourceUtils.getConnection(DataSourceUtils.java:82)
    at org.mybatis.spring.transaction.SpringManagedTransaction.openConnection(SpringManagedTransaction.java:80)
    at org.mybatis.spring.transaction.SpringManagedTransaction.getConnection(SpringManagedTransaction.java:67)
    at org.apache.ibatis.executor.BaseExecutor.getConnection(BaseExecutor.java:336)
    at org.apache.ibatis.executor.SimpleExecutor.prepareStatement(SimpleExecutor.java:86)
    at org.apache.ibatis.executor.SimpleExecutor.doQuery(SimpleExecutor.java:62)
    at org.apache.ibatis.executor.BaseExecutor.queryFromDatabase(BaseExecutor.java:324)
    at org.apache.ibatis.executor.BaseExecutor.query(BaseExecutor.java:156)
    at org.apache.ibatis.executor.CachingExecutor.query(CachingExecutor.java:109)
    at org.apache.ibatis.executor.CachingExecutor.query(CachingExecutor.java:83)
    at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:147)
    ... 101 common frames omitted
Caused by: com.alibaba.druid.pool.GetConnectionTimeoutException: wait millis 60000, active 0, maxActive 100, creating 1, createElapseMillis 686180, createErrorCount 3
    at com.alibaba.druid.pool.DruidDataSource.getConnectionInternal(DruidDataSource.java:1722)
    at com.alibaba.druid.pool.DruidDataSource.getConnectionDirect(DruidDataSource.java:1402)
    at com.alibaba.druid.pool.DruidDataSource.getConnection(DruidDataSource.java:1382)
    at com.alibaba.druid.pool.DruidDataSource.getConnection(DruidDataSource.java:1372)
    at com.alibaba.druid.pool.DruidDataSource.getConnection(DruidDataSource.java:109)
    at org.springframework.jdbc.datasource.DataSourceUtils.fetchConnection(DataSourceUtils.java:158)
    at org.springframework.jdbc.datasource.DataSourceUtils.doGetConnection(DataSourceUtils.java:116)
    at org.springframework.jdbc.datasource.DataSourceUtils.getConnection(DataSourceUtils.java:79)
    ... 111 common frames omitted

Bug定位

系统是通过Druid连接

获取连接超时(GetConnectionTimeoutException)此错误的出现，只有两种可能:

1.业务系统本身Druid获取连接失败。    
2.网络有问题或者数据库不可用。         

在这个Bug里面很明显是Druid创建连接失败，原因如下:

1.之前网络的确有问题，单已恢复，另外还有另外一个服务也是访问这个数据库的。但是那个访问正常。    
2.重启服务之后一切正常。  

如果说还是网络连接异常，那么并不能解释重启后正常这一现象。

Druid获取连接的过程

在分析这个问题之前，先得看下Druid是如何创建连接的,下面是本人在上网查找到的，示意图:

（备注：来源 https://my.oschina.net/alchemystar/blog/899987）
可见druid创建连接都是通过一个专门的线程来进行的，此图省略了大量的源码细节。

然后对比DruidDataSource源码，验证了的确是上图所示的流程。

定位源码

根据日志“

Failed to obtain JDBC Connection; nested exception is com.alibaba.druid.pool.GetConnectionTimeoutException: wait millis 60000, active 0, maxActive 100, creating 1, createElapseMillis 686180, createErrorCount 3

 ”，发现日志打印位置，如下图：

holder == null 是关键。然后查找holder是如果被赋值的，经查找源码，发现其如下图：

 

 因为已经配置过maxWait属性，故定位到pollLast方法中，再结合druid连接流程，找到CreateConnectionTask的runInternal方法。仔细阅读发现如下逻辑

 经过比对日志输出，最终发现创建连接时，没有创建就直接返回了。真相了，原来是未设置keepAlive属性导致的。。。。。泪奔

起伏

已经定位到了问题，但是在查找资料的过程中，发现有人说在1.1.21版本中配置了keepAlive也是出现类似的情况

详见https://github.com/alibaba/druid/issues/3889

后查找druid发版日志，找到如下日志：

 随将druid版本到1.1.24版本，但是发现日志中会频繁出现“discard long time none received connection”的错误日志。然后进一步查找相关问题，找到下面的一篇文章https://gitee.com/y_project/RuoYi-Vue/issues/I1XWZE?_from=gitee_search

 此错误不影响链接数据库，去除需要设置System.setProperty("druid.mysql.usePingMethod", "false");  因为采用的spring-boot,随将这行代码加入到Application的main方法中，经测试之后，已无此错误日志频繁出现的问题。

解决 

最终，升级druid版本到1.1.24版本，另外在配置中增加了keepAlive属性，并在Application的main方法中加入System.setProperty("druid.mysql.usePingMethod", "false");

题外

不配置maxWait可能带了的BUG https://zhuanlan.zhihu.com/p/144446516