老师你好，可以帮忙看下这个怎么解决吗，用restlet测试的时候500 internal server error，显示could not get jdbc connection，但我不知道该如何解决。。。谢谢老师！

HTTP Status 500 - Request processing failed; nested exception is org.mybatis.spring.MyBatisSystemException: nested exception is org.apache.ibatis.exceptions.PersistenceException:

type Exception report

message Request processing failed; nested exception is org.mybatis.spring.MyBatisSystemException: nested exception is org.apache.ibatis.exceptions.PersistenceException:

description The server encountered an internal error that prevented it from fulfilling this request.

exception

org.springframework.web.util.NestedServletException: Request processing failed; nested exception is org.mybatis.spring.MyBatisSystemException: 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

Last packet sent to the server was 0 ms ago.)

The error may exist in file [/Library/apache-tomcat-7.0.73/webapps/ROOT/WEB-INF/classes/mappers/UserMapper.xml]

The error may involve com.mmall.dao.UserMapper.checkUsername

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

Last packet sent to the server was 0 ms ago.)
org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:943)
org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:833)
javax.servlet.http.HttpServlet.service(HttpServlet.java:650)
org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:807)
javax.servlet.http.HttpServlet.service(HttpServlet.java:731)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:88)
org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:108)

root cause

org.mybatis.spring.MyBatisSystemException: 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