nested exception is org.apache.ibatis.exceptions.PersistenceException: \r\n### Error querying database. Cause: org.springframework.jdbc.CannotGetJdbcConnectionException: Failed to obtain JDBC Connection; nested exception is com.mysql.cj.jdbc.exceptions.CommunicationsException: Communications link failure\n\nThe last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.\r\n### The error may exist in com/changgou/goods/dao/CategoryMapper.java (best guess)\r\n### The error may involve com.changgou.goods.dao.CategoryMapper.select\r\n### The error occurred while executing a query\r\n### Cause: org.springframework.jdbc.CannotGetJdbcConnectionException: Failed to obtain JDBC Connection; nested exception is com.mysql.cj.jdbc.exceptions.CommunicationsException: Communications link failure\n\nThe last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.--畅购商城学习日志
导航：
目录
- nested exception is org.apache.ibatis.exceptions.PersistenceException: \r\n### Error querying database. Cause: org.springframework.jdbc.CannotGetJdbcConnectionException: Failed to obtain JDBC Connection; nested exception is com.mysql.cj.jdbc.exceptions.CommunicationsException: Communications link failure\n\nThe last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.\r\n### The error may exist in com/changgou/goods/dao/CategoryMapper.java (best guess)\r\n### The error may involve com.changgou.goods.dao.CategoryMapper.select\r\n### The error occurred while executing a query\r\n### Cause: org.springframework.jdbc.CannotGetJdbcConnectionException: Failed to obtain JDBC Connection; nested exception is com.mysql.cj.jdbc.exceptions.CommunicationsException: Communications link failure\n\nThe last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.--畅购商城学习日志
- 导航：
- 1.问题描述：
- 2.思路经历：
- 3.产生原因：
- 4.解决方法：
1.问题描述：
- nested exception is org.apache.ibatis.exceptions.PersistenceException: \r\n### Error querying database. Cause: org.springframework.jdbc.CannotGetJdbcConnectionException: Failed to obtain JDBC Connection; nested exception is com.mysql.cj.jdbc.exceptions.CommunicationsException: Communications link failure\n\nThe last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.\r\n### The error may exist in com/changgou/goods/dao/CategoryMapper.java (best guess)\r\n### The error may involve com.changgou.goods.dao.CategoryMapper.select\r\n### The error occurred while executing a query\r\n### Cause: org.springframework.jdbc.CannotGetJdbcConnectionException: Failed to obtain JDBC Connection; nested exception is com.mysql.cj.jdbc.exceptions.CommunicationsException: Communications link failure\n\nThe last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.
2.思路经历：
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今天也是继续搞搞代码，突然Apifox说连接超时，冒了这一堆英文出来，大概是说连不上数据库啦，Navicat for MySQL提示说2003Unkown error。我寻思这代码我应该是没弄错的，而且显然是连接不上。所以得进入虚拟机看看啥毛病了，可是我发现我虚拟机的防火墙本来就是关着的，接着相互都ping通了，可真是奇了怪。
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那会不会是数据库没开？可是我再次启动MySQL服务，它说我已经开着。
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那会不会是端口被占用了？我Navicat那边端口用的是3306，是个默认端口，很有可能是idea帮我连接数据库的配置有问题了，我一看，哇端口是动态分配的
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所以，关闭idea，先连接Navicat，再开idea端口就能用了
3.产生原因：
- 这是一个持久化异常，可能是由于无法获取到JDBC连接导致的。同时，也提示了与MySQL通讯时出现了问题，可能是因为最后一个数据包从服务器发送成功的时间太久了，或者驱动程序没有从服务器接收到任何数据包。建议检查数据库连接配置是否正确，确保数据库服务正在运行，并检查网络连接是否正常。
4.解决方法：
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方法一
- 将idea数据库配置的动态端口改为固定值3307或者别的数字
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方法二
- 先启动Navicat，后启动idea，idea就会拿走一个还没占用的端口了