连接 mysql 报这个错误：nested exception is org.apache.ibatis.exceptions.PersistenceException: ### Error querying database. Cause: org.springframework.jdbc.CannotGetJdbcConnectionException: Could not get JDBC Connection; nested exception is java.sql.SQLException: Connections could not be acquired from the underlying database! ### The error may exist in file [D:\solutions.java\springDemo\target\springDemo\WEB-INF\classes\mapper\UserDaoMapper.xml] ### The error may involve dao.UserDao.selectByUsername ### The error occurred while executing a query ### Cause: org.springframework.jdbc.CannotGetJdbcConnectionException: Could not get JDBC Connection; nested exception is java.sql.SQLException: Connections could not be acquired from the underlying database!
怎么回事啊？