使用Servlet连接MySQL时遇java.lang.ClassNotFoundException错误求助

问题详情 #

我是编程初学者，正在使用Eclipse IDE学习Servlet。已将mysql-connector jar文件添加到项目库中，普通Java程序可正常连接数据库并获取数据，但Servlet执行时抛出java.lang.ClassNotFoundException: com.mysql.cj.jdbc.Driver错误。我的目标是将Servlet获取的表单数据存储到MySQL数据库中。

正常运行的普通Java代码 #

package learn.http_servlets.form.jdbc_connection;

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

public class JDBCConnection {
    public static void main(String[] args) {
        try {
            Class.forName("com.mysql.cj.jdbc.Driver");
            
            Connection con = DriverManager.getConnection("jdbc:mysql://localhost:3306/student", "root", "root");
            
            String query = "show tables";
            
            PreparedStatement pstmnt = con.prepareStatement(query);
            
            ResultSet rs = pstmnt.executeQuery();
            
            while(rs.next()) {
                System.out.println("list of tables :"+rs.getString(1));
            }
        }
        catch(Exception e) {
            e.printStackTrace();
        }
    }
}

抛出错误的Servlet代码 #

package learn.http_servlets.form;

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

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

@SuppressWarnings("serial")
public class Register extends HttpServlet {
    
    public void procesRequest(HttpServletRequest request, HttpServletResponse response) 
            throws ServletException, IOException {
        response.setContentType("text/html");
        PrintWriter out = response.getWriter();
        
        String name = request.getParameter("user_name");
        String password = request.getParameter("user_password");
        String email = request.getParameter("user_email");
        
        Connection connection = null;
        
        try {
            Class.forName("com.mysql.cj.jdbc.Driver"); 
            
            connection = DriverManager
                    .getConnection("jdbc:mysql://localhost:3306/register", "root", "root");
            
            out.println("connection established sucessfully");
            
//          String query = "insert into user(name, email, password) values(?,?,?)";
//          
//          PreparedStatement pstmnt = connection.prepareStatement(query);
//          pstmnt.setString(1, name);
//          pstmnt.setString(2, email);
//          pstmnt.setString(3, password);
//          
//          pstmnt.executeUpdate();
//          
//          pstmnt.close();
        }
        catch(Exception e ) {
            e.printStackTrace();
            out.println("<h1>error occured!!</h1>");
        }
        finally {
            if(connection != null) {
                try {
                    connection.close();
                } catch (SQLException e) {
                    e.printStackTrace();
                }
            }
        }
        out.println("<h3>"+name+" "+email+" "+password+"</h3>");
    }
    
    public void doGet(HttpServletRequest request, HttpServletResponse response) 
            throws ServletException, IOException {
        procesRequest(request, response);
    }
    
    public void doPost(HttpServletRequest request, HttpServletResponse response) 
            throws ServletException, IOException {
        procesRequest(request, response);
    }
}

项目结构中mysql-connector已添加到引用库，但仍报错。

解决方法 #

问题根源：Web容器（如Tomcat）的类加载机制与普通Java程序不同。添加到项目库的jar仅在编译阶段生效，Web应用运行时，容器只会加载WEB-INF/lib目录下的jar文件，导致驱动类无法被找到。

操作步骤：

1. 在Web项目中找到WEB-INF目录，若不存在lib文件夹，右键新建Folder并命名为lib。
2. 将mysql-connector jar文件复制到WEB-INF/lib文件夹内。
3. 右键项目选择Refresh，让Eclipse识别该jar文件。
4. 重新部署Web应用到Tomcat，再运行Servlet测试。

额外提示：JDBC 4.0及以上版本无需手动调用Class.forName("com.mysql.cj.jdbc.Driver")，驱动jar内的META-INF/services/java.sql.Driver文件会自动完成驱动注册，保留该代码也不会产生影响。

内容的提问来源于stack exchange，提问作者Manche Umesh