问题：
java.sql.SQLNonTransientConnectionException: Data source rejected establishment of connection, message from server: "Too many connections"
at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:110)
at com.mysql.cj.jdbc.exceptions.SQLExceptionsMapping.translateException(SQLExceptionsMapping.java:122)
at com.mysql.cj.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:828)
at com.mysql.cj.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:448)
at com.mysql.cj.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:241)
at com.mysql.cj.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:198)
at com.alibaba.druid.filter.FilterChainImpl.connection_connect(FilterChainImpl.java:156)
at com.alibaba.druid.filter.FilterAdapter.connection_connect(FilterAdapter.java:787)
at com.alibaba.druid.filter.FilterEventAdapter.connection_connect(FilterEventAdapter.java:38)
at com.alibaba.druid.filter.FilterChainImpl.connection_connect(FilterChainImpl.java:150)
at com.alibaba.druid.filter.stat.StatFilter.connection_connect(StatFilter.java:227)
at com.alibaba.druid.filter.FilterChainImpl.connection_connect(FilterChainImpl.java:150)
at com.alibaba.druid.pool.DruidAbstractDataSource.createPhysicalConnection(DruidAbstractDataSource.java:1654)
解决方案：
show variables like 'max_connections';
set global max_connections=1000;原因：当前数据库的连接数量已经达到饱和需要，设大数据库的最大链接数