servlet层调用biz业务层出现浏览器 500错误，解决方法 dao数据访问层 数据库Util工具类都可能出错 通过新建一个测试类复制代码逐步测试查找出最终出错原因

package com.swift.jztk.servlet;

import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import com.swift.jztk.biz.IQuestionBiz;
import com.swift.jztk.biz.QuestionBizImpl;

@WebServlet("/getQuestions")
public class GetQuestions extends HttpServlet {
    private static final long serialVersionUID = 1L;
    IQuestionBiz biz=new QuestionBizImpl();//生成业务层对象
    public GetQuestions() {
        super();
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.getWriter().append("Served at: ").append(request.getContextPath());
        String testType=request.getParameter("testType");
        String json=biz.getQuestions(testType);//业务层对象调用方法，方法中使用dao数据访问层访问数据库，代码问题执行失败
        response.getWriter().println(json);
    }
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }

}

biz.getQuestions(testType);//biz业务层调用的方法.getQuestions(testType)如下：

package com.swift.jztk.biz;

import java.util.Collections;
import java.util.Comparator;
import java.util.HashSet;
import java.util.Iterator;
import java.util.List;
import java.util.Random;

import com.google.gson.Gson;
import com.swift.jztk.bean.Result;
import com.swift.jztk.bean.Root;
import com.swift.jztk.dao.IQuestionDao;
import com.swift.jztk.dao.QuestionDaoImpl;

public class QuestionBizImpl implements IQuestionBiz {
    IQuestionDao dao = new QuestionDaoImpl();

    @SuppressWarnings("null")
    @Override
    public String getQuestions(String testType) {
        List<Result> list = null;
        if (testType.equals("rand")) {
            HashSet<Integer> set = new HashSet<Integer>();//生成一个哈希集合，用于存放随机数
            Random ran = new Random();
            for (;;) {//无限循环
                int n = ran.nextInt(200) + 1;// 1~200间的随机整数，不加1是0到199
                set.add(n);// 随机数放到 整数类型的哈希集合中，保证没有相同的整数
                if (set.size() == 100) {
                    break;
                }
            }
            Iterator<Integer> it = set.iterator();//迭代器，用于获取集合中各条内容
            while (it.hasNext()) {
                int id = it.next();
                Result result = dao.getResultById(id);
                list.add(result);//加入对象列表集合
                Collections.sort(list, new Comparator<Result>() {// 比较器 匿名内部类

                    @Override
                    public int compare(Result o1, Result o2) {
                        int id1 = o1.getId();
                        int id2 = o2.getId();
                        return id1 - id2;// 按照id大小从小到大排序
                    }
                });
            }

        } else if (testType.equals("order")) {
            list = dao.getAll();//MVC架构，数据访问层用接口进行连接，得到数据库中全部数据
        }
        String json=listToJson(list);
        return json;
    }
    //把得到的List<Result>对象列表集合转换成字符串
    public String listToJson(List<Result> list) {
        Root root = new Root();
        root.setResult(list);
        root.setStatusCode("000000");
        root.setDesc("请求成功");//json实体类对象赋值
        Gson gson = new Gson();
        String json = gson.toJson(root);//json实体类对象用Gson解析成字符串
        return json;
    }

}

Result result = dao.getResultById(id);//biz层调用数据访问层的方法.getResultById();

list = dao.getAll();//biz层调用数据访问层的方法.getAll();

数据访问层代码如下：

package com.swift.jztk.dao;

import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;

import com.swift.jztk.bean.Result;
import com.swift.jztk.util.DBUtil;

public class QuestionDaoImpl implements IQuestionDao {

    //@Override
    public List<Result> getAll() {
        Connection conn = DBUtil.getConn();
        PreparedStatement ps = null;
        ResultSet rs = null;
        List<Result> list = null;
        try {
            ps = conn.prepareStatement("select * from sw_question");
            rs = ps.executeQuery();
            list = new ArrayList<Result>();
            while (rs.next()) {
                int id = rs.getInt("id");
                String question = rs.getString("question");
                String answer = rs.getString("answer");
                String item1 = rs.getString("item1");
                String item2 = rs.getString("item2");
                String item3 = rs.getString("item3");
                String item4 = rs.getString("item4");
                String expalins = rs.getString("expalins");
                String url = rs.getString("url");
                Result result = new Result(id, question, answer, item1, item2, item3, item4, expalins, url);
                list.add(result);
            }
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            DBUtil.closeAll(conn, ps, rs);
        }
        return list;
    }

    @Override
    public Result getResultById(int id) {
        Connection conn = DBUtil.getConn();
        PreparedStatement ps = null;
        ResultSet rs = null;
        Result result = null;
        try {
            ps = conn.prepareStatement("select * from sw_question where id=?");
            ps.setInt(1, id);
            rs = ps.executeQuery();
            if(rs.next()) {
            String question = rs.getString("question");
            String answer = rs.getString("answer");
            String item1 = rs.getString("item1");
            String item2 = rs.getString("item2");
            String item3 = rs.getString("item3");
            String item4 = rs.getString("item4");
            String expalins = rs.getString("expalins");
            String url = rs.getString("url");
            result = new Result(id, question, answer, item1, item2, item3, item4, expalins, url);
            }
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            DBUtil.closeAll(conn, ps, rs);
        }
        return result;
    }

}

提示com.mysql.jdbc.Driver找不到 NullClassException异常

导入的这些包都是正常的，没有红线，可是浏览器出现500错误：

import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;

import com.swift.jztk.bean.Result;
import com.swift.jztk.util.DBUtil;

Connection conn = DBUtil.getConn();

都已导入，没有问题
检查数据库工具类：

package com.swift.jztk.util;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;

public class DBUtil {
    // 连接MySQL数据库工具
    public static Connection getConn() {
        Connection conn = null;
        try {
            Class.forName("com.mysql.jdbc.Driver");
            try {
                conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/sw_database?user=root&password=root");
            } catch (SQLException e) {
                e.printStackTrace();
            }
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
        return conn;
    }

    // 关闭数据库连接、sql连接、结果集
    public static void closeAll(Connection conn, PreparedStatement ps, ResultSet rs) {
        if (conn != null)
            try {
                conn.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        if (ps != null) {
            try {
                ps.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
        if (rs != null) {
            try {
                rs.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }
}

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
这些包都导入了

com.mysql.jdbc.Driver的驱动却找不到，原因有两个：
一个是数据库突然崩溃，MySQL数据库软件发生问题，重新安装解决
一个是光导包不出红线是没用的，还需要把mysql-connector-java-5.1.7-bin.jar这个jar包没有复制到lib