制作的系统点击登录提示遇到了未知的错误，并且页面显示大段红色代码

Question

nested exception is org.apache.ibatis.exceptions.PersistenceException: ### Error querying database. Cause: org.springframework.jdbc.CannotGetJdbcConnectionException: Could not get JDBC Connection; nested exception is org.apache.commons.dbcp.SQLNestedException: Cannot create PoolableConnectionFactory (Communications link failure The last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.) ### The error may exist in file [D:\work_idea\HospitalizationManSys\out\artifacts\HospitalizationManSys_war_exploded\WEB-INF\classes\com\qut\mapper\UserMapper.xml] ### The error may involve com.qut.mapper.UserMapper.findUserById ### The error occurred while executing a query ### Cause: org.springframework.jdbc.CannotGetJdbcConnectionException: Could not get JDBC Connection; nested exception is org.apache.commons.dbcp.SQLNestedException: Cannot create PoolableConnectionFactory (Communications link failure The last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.)

Answer 1

先看看数据库的配置对么，连接的数据库是否是目标库，账号密码是否正确，IP地址端口号有没有问题，如果数据库连接都没有问题，

看一下UserMapper.xml的xml文件SQL有没有写错，

com.qut.mapper.UserMapper.findUserById这个方法有没有问题。