周末放假，突然收到短信提示“服务器故障”，顿时紧张起来，毕竟是正式环境。

回家，查看发现如下错误

com.mysql.jdbc.exceptions.jdbc4.MySQLNonTransientConnectionException: Data source rejected establishment of connection,  message from server: "Too many connections"

at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[na:1.8.0_144]

at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) ~[na:1.8.0_144]

at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[na:1.8.0_144]

at java.lang.reflect.Constructor.newInstance(Constructor.java:423) ~[na:1.8.0_144]

at com.mysql.jdbc.Util.handleNewInstance(Util.java:425) ~[mysql-connector-java-5.1.41.jar!/:5.1.41]

at com.mysql.jdbc.Util.getInstance(Util.java:408) ~[mysql-connector-java-5.1.41.jar!/:5.1.41]

at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:918) ~[mysql-connector-java-5.1.41.jar!/:5.1.41]

at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:897) ~[mysql-connector-java-5.1.41.jar!/:5.1.41]

at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:886) ~[mysql-connector-java-5.1.41.jar!/:5.1.41]

at com.mysql.jdbc.MysqlIO.doHandshake(MysqlIO.java:1040) ~[mysql-connector-java-5.1.41.jar!/:5.1.41]

at com.mysql.jdbc.ConnectionImpl.coreConnect(ConnectionImpl.java:2205) ~[mysql-connector-java-5.1.41.jar!/:5.1.41]

at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2236) ~[mysql-connector-java-5.1.41.jar!/:5.1.41]

at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2035) ~[mysql-connector-java-5.1.41.jar!/:5.1.41]

at com.mysql.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:790) ~[mysql-connector-java-5.1.41.jar!/:5.1.41]

at com.mysql.jdbc.JDBC4Connection.<init>(JDBC4Connection.java:47) ~[mysql-connector-java-5.1.41.jar!/:5.1.41]

at sun.reflect.GeneratedConstructorAccessor137.newInstance(Unknown Source) ~[na:na]

at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[na:1.8.0_144]

at java.lang.reflect.Constructor.newInstance(Constructor.java:423) ~[na:1.8.0_144]

at com.mysql.jdbc.Util.handleNewInstance(Util.java:425) ~[mysql-connector-java-5.1.41.jar!/:5.1.41]

at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:400) ~[mysql-connector-java-5.1.41.jar!/:5.1.41]

at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:330) ~[mysql-connector-java-5.1.41.jar!/:5.1.41]

at com.alibaba.druid.filter.FilterChainImpl.connection_connect(FilterChainImpl.java:156) ~[druid-1.1.20.jar!/:1.1.20]

at com.alibaba.druid.filter.FilterAdapter.connection_connect(FilterAdapter.java:787) ~[druid-1.1.20.jar!/:1.1.20]

at com.alibaba.druid.filter.FilterEventAdapter.connection_connect(FilterEventAdapter.java:38) ~[druid-1.1.20.jar!/:1.1.20]

at com.alibaba.druid.filter.FilterChainImpl.connection_connect(FilterChainImpl.java:150) ~[druid-1.1.20.jar!/:1.1.20]

at com.alibaba.druid.filter.FilterAdapter.connection_connect(FilterAdapter.java:787) ~[druid-1.1.20.jar!/:1.1.20]

at com.alibaba.druid.filter.FilterChainImpl.connection_connect(FilterChainImpl.java:150) ~[druid-1.1.20.jar!/:1.1.20]

at com.alibaba.druid.filter.stat.StatFilter.connection_connect(StatFilter.java:218) ~[druid-1.1.20.jar!/:1.1.20]

at com.alibaba.druid.filter.FilterChainImpl.connection_connect(FilterChainImpl.java:150) ~[druid-1.1.20.jar!/:1.1.20]

at com.alibaba.druid.pool.DruidAbstractDataSource.createPhysicalConnection(DruidAbstractDataSource.java:1645) ~[druid-1.1.20.jar!/:1.1.20]

at com.alibaba.druid.pool.DruidAbstractDataSource.createPhysicalConnection(DruidAbstractDataSource.java:1709) ~[druid-1.1.20.jar!/:1.1.20]

at com.alibaba.druid.pool.DruidDataSource$CreateConnectionThread.run(DruidDataSource.java:2715) ~[druid-1.1.20.jar!/:1.1.20]

一看日志就猜到是数据库连接数问题，虽然是正式环境，但也没多少用户的。
最直接的方法，重启项目，但发现还是不行。。。

后来通过下面查看mysql的最大连接数，只有151

SHOW VARIABLES LIKE '%max_connections%';  

然后就把最大连接数设置大点

 set GLOBAL max_connections=500;

果然就好了，回去查看哪里访问比较多，还有是否没有释放连接