浏览器报错如下：
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 java.sql.SQLException: Connections could not be acquired from the underlying database!
The error may exist in file [F:\develop\workplace.metadata.plugins\org.eclipse.wst.server.core\tmp0\wtpwebapps\o2o\WEB-INF\classes\mapper\ProductCategoryDao.xml]
The error may involve com.imooc.o2o.dao.ProductCategoryDao.queryProductCategoryList
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!
org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:982)
org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:861)
javax.servlet.http.HttpServlet.service(HttpServlet.java:622)
org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:846)
javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) 
打开tomcar日志后台报错如下：
后台报异常:
com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: 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.
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
at java.lang.reflect.Constructor.newInstance(Unknown Source)
at com.mysql.jdbc.Util.handleNewInstance(Util.java:404)
at com.mysql.jdbc.SQLError.createCommunicationsException(SQLError.java:983)
at com.mysql.jdbc.MysqlIO.(MysqlIO.java:339)
at com.mysql.jdbc.ConnectionImpl.coreConnect(ConnectionImpl.java:2252)
根本问题为:mysql连接失败导致 
解决办法:
重启mysql服务：
1、使用 service 启动：service mysqld restart
2、使用 mysqld 脚本启动：/etc/inint.d/mysqld restart 
查看mysql是否启动命令
ps -aux | grep mysqld
查看mysql是否在监 听端口命令
netstat -tl | grep mysql 