具体报错如下：

HTTP Status 500 - Servlet execution threw an exception

type Exception report

message Servlet execution threw an exception

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

exception

javax.servlet.ServletException: Servlet execution threw an exception
        org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)

root cause

java.lang.AbstractMethodError: com.mysql.jdbc.Connection.isValid(I)Z
        org.apache.tomcat.dbcp.dbcp2.DelegatingConnection.isValid(DelegatingConnection.java:913)
        org.apache.tomcat.dbcp.dbcp2.PoolableConnection.validate(PoolableConnection.java:226)
        org.apache.tomcat.dbcp.dbcp2.PoolableConnectionFactory.validateConnection(PoolableConnectionFactory.java:302)
        org.apache.tomcat.dbcp.dbcp2.BasicDataSource.validateConnectionFactory(BasicDataSource.java:2208)
        org.apache.tomcat.dbcp.dbcp2.BasicDataSource.createPoolableConnectionFactory(BasicDataSource.java:2191)
        org.apache.tomcat.dbcp.dbcp2.BasicDataSource.createDataSource(BasicDataSource.java:1945)
        org.apache.tomcat.dbcp.dbcp2.BasicDataSource.getConnection(BasicDataSource.java:1412)
        Dao.DBConnectionFromPool.(DBConnectionFromPool.java:16)
        Dao.DBConnectionFromPool.(DBConnectionFromPool.java:10)
        Dao.JdbcTools.getConnectionFromPool(JdbcTools.java:16)
        Dao.UserDao.(UserDao.java:16)
        Servlet.RegServlet.doPost(RegServlet.java:36)
        javax.servlet.http.HttpServlet.service(HttpServlet.java:648)
        javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
        org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)

note The full stack trace of the root cause is available in the Apache Tomcat/8.0.21 logs.

Apache Tomcat/8.0.21