调试了一下午了，请高手指点一下吧！
contex.xml配置如下：
                  <Resource
 
           name="jdbc/mysqlds"
     auth="container"
     type="javax.sql.DataSource"
     maxActive="100"
     maxIdle="30"
     maxWait="10000"
     username="root"
     password="315423"
     dirverClassName="com.mysql.jdbc.Driver"
     url="jdbc:mysql://127.0.0.1/myblog"/>

myblog是数据库名
BlogServlet.java如下：
package com.oursky.blog;

import java.io.IOException;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.SQLException;

import javax.naming.Context;
import javax.naming.InitialContext;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.sql.DataSource;

public class BlogServlet extends HttpServlet {


public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
doPost(request,response);

}


public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
request.setCharacterEncoding("UTF-8");
String title=request.getParameter("title");
String content=request.getParameter("content");
String categoryId=request.getParameter("category");



DataSource ds=null;
try {
Context context=new InitialContext();
ds=(DataSource) context.lookup("java:/comp/env/jdbc/mysqlds");
} catch (Exception e) {
System.out.println("获取数据出错");
}


try {
Connection conn=ds.getConnection();
String sql="insert into blog(title,content,category_id,create_time) values (?,?,?,now())";
PreparedStatement pstmt=conn.prepareStatement(sql);
pstmt.setString(1, title);
pstmt.setString(2, content);
pstmt.setInt(3, Integer.parseInt(categoryId));
int result=pstmt.executeUpdate();
System.out.println(result);
} catch (SQLException e) {
e.printStackTrace();
}
}

}

运行结果是：
org.apache.tomcat.dbcp.dbcp.SQLNestedException: Cannot create JDBC driver of class '' for connect URL 'jdbc:mysql://127.0.0.1/myblog'
at org.apache.tomcat.dbcp.dbcp.BasicDataSource.createDataSource(BasicDataSource.java:1150)
at org.apache.tomcat.dbcp.dbcp.BasicDataSource.getConnection(BasicDataSource.java:880)
at com.oursky.blog.BlogServlet.doPost(BlogServlet.java:46)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:637)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:293)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:849)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:583)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:454)
at java.lang.Thread.run(Thread.java:619)
Caused by: java.sql.SQLException: No suitable driver
at java.sql.DriverManager.getDriver(DriverManager.java:264)
at org.apache.tomcat.dbcp.dbcp.BasicDataSource.createDataSource(BasicDataSource.java:1143)
... 16 more

出现以上问题从网上搜集的资料是把context.xml配置改成这样
                    <ResourceLink
           global="jdbc/mysqlds"
           name="jdbc/mysqlds"
     auth="container"
     type="javax.sql.DataSource"
     maxActive="100"
     maxIdle="30"
     maxWait="10000"
     username="root"
     password="315423"
     dirverClassName="com.mysql.jdbc.Driver"
     url="jdbc:mysql://127.0.0.1/myblog"/>

但仍然会出现错误，但错误和上一个不一样
严重: Servlet.service() for servlet BlogServlet threw exception
java.lang.NullPointerException
at com.oursky.blog.BlogServlet.doPost(BlogServlet.java:46)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:637)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:293)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:849)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:583)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:454)
at java.lang.Thread.run(Thread.java:619)