MySQLNonTransientConnectionException: Too many connections
线上出了一个故障
2020-11-04.13:47:33.105 ERROR [Druid-ConnectionPool-Create-1979930572] DruidDataSource.run:2779 [run] create connection SQLException, url: jdbc:mysql://xxxxxxxxxxxxxx:xxxx/xxxxxx?useunicode=true&autoReconnect=true&zeroDateTimeBehavior=convertToNull&socketTimeout=10000, errorCode 1040, state 08004
com.mysql.jdbc.exceptions.jdbc4.MySQLNonTransientConnectionException: Too many connections
at sun.reflect.GeneratedConstructorAccessor644.newInstance(Unknown Source) ~[na:na]
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[na:1.8.0_91]
at java.lang.reflect.Constructor.newInstance(Constructor.java:423) ~[na:1.8.0_91]
at com.mysql.jdbc.Util.handleNewInstance(Util.java:404) ~[mysql-connector-java-5.1.38.jar:5.1.38]
at com.mysql.jdbc.Util.getInstance(Util.java:387) ~[mysql-connector-java-5.1.38.jar:5.1.38]
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:917) ~[mysql-connector-java-5.1.38.jar:5.1.38]
at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3878) ~[mysql-connector-java-5.1.38.jar:5.1.38]
at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3814) ~[mysql-connector-java-5.1.38.jar:5.1.38]
at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:871) ~[mysql-connector-java-5.1.38.jar:5.1.38]
at com.mysql.jdbc.MysqlIO.proceedHandshakeWithPluggableAuthentication(MysqlIO.java:1694) ~[mysql-connector-java-5.1.38.jar:5.1.38]
at com.mysql.jdbc.MysqlIO.doHandshake(MysqlIO.java:1215) ~[mysql-connector-java-5.1.38.jar:5.1.38]
at com.mysql.jdbc.ConnectionImpl.coreConnect(ConnectionImpl.java:2255) ~[mysql-connector-java-5.1.38.jar:5.1.38]
at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2286) ~[mysql-connector-java-5.1.38.jar:5.1.38]
at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2085) ~[mysql-connector-java-5.1.38.jar:5.1.38]
at com.mysql.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:795) ~[mysql-connector-java-5.1.38.jar:5.1.38]
at com.mysql.jdbc.JDBC4Connection.<init>(JDBC4Connection.java:44) ~[mysql-connector-java-5.1.38.jar:5.1.38]
at sun.reflect.GeneratedConstructorAccessor70.newInstance(Unknown Source) ~[na:na]
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[na:1.8.0_91]
at java.lang.reflect.Constructor.newInstance(Constructor.java:423) ~[na:1.8.0_91]
at com.mysql.jdbc.Util.handleNewInstance(Util.java:404) ~[mysql-connector-java-5.1.38.jar:5.1.38]
at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:400) ~[mysql-connector-java-5.1.38.jar:5.1.38]
at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:327) ~[mysql-connector-java-5.1.38.jar:5.1.38]
at com.alibaba.druid.filter.FilterChainImpl.connection_connect(FilterChainImpl.java:156) ~[druid-1.1.24.jar:1.1.24]
at com.alibaba.druid.filter.stat.StatFilter.connection_connect(StatFilter.java:218) ~[druid-1.1.24.jar:1.1.24]
at com.alibaba.druid.filter.FilterChainImpl.connection_connect(FilterChainImpl.java:150) ~[druid-1.1.24.jar:1.1.24]
at com.alibaba.druid.pool.DruidAbstractDataSource.createPhysicalConnection(DruidAbstractDataSource.java:1646) ~[druid-1.1.24.jar:1.1.24]
at com.alibaba.druid.pool.DruidAbstractDataSource.createPhysicalConnection(DruidAbstractDataSource.java:1710) ~[druid-1.1.24.jar:1.1.24]
at com.alibaba.druid.pool.DruidDataSource$CreateConnectionThread.run(DruidDataSource.java:2777) ~[druid-1.1.24.jar:1.1.24]
原因是同一个数据库集群的其他应用把数据库连接数打满了，导致项目申请不了数据连接，druid会维护好核心连接数和最大连接数，源码中有一个线程再不断验证连接数是否需要申请。
com.alibaba.druid.pool.DruidDataSource.CreateConnectionThread
截取部分代码
中间代码忽略，满足条件就会创建连接，下面截图就是捕获异常的地方。
这个异常不会影响现有已经创建好的连接，但是影响新申请的连接，如果遇到业务量上涨可能会因为申请不到连接导致整体时间上涨。但是还是要修复跑慢连接的应用问题，这才是重点。
