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 {
IQue
d6f4
stionDao 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