Servlet – CRUD 操作示例

CRUD 表示创建、读取、更新和删除。这些是在数据库和应用程序中执行的基本重要操作。我们将使用 Servlet、MYSQL 和 JDBC 构建一个简单的用户注册应用程序进行演示。在这个例子中，我们将能够创建用户、读取用户、更新用户和删除用户。

技术工具：

• MySQL（工作台）数据库
• IDE（智能）
• Apache Tomcat（我使用的是 Tomcat 9 版本）。

首先，我们从下载 Apache Tomcat 中下载 Apache Tomcat，解压到您的工作目录中。

然后我们创建一个新项目，将其命名为 app user。通常，有两种方法，第一种是从数据库创建项目，另一种是先创建应用程序，然后设置数据库。在本文中，我们将从数据库开始我们的项目。所以，让我们建立我们的数据库，在这个例子中使用 MYSQL，查看下图以了解如何建立数据库。

之后，我们设置了我们的项目，对于这个例子，我使用 Intellij IDE，

• 通过选择文件创建一个新项目（它将显示各种选项）
• 选择新建，然后项目
• 单击Java Enterprise 并选择 Web 应用程序作为您的项目模板

设置应用服务器

• 选择应用程序并选择新的
• 选择 Tomcat 基目录（保存在工作目录中的那个）

这是项目结构的样子：

编码方面。首先，我们创建一个名为 User 的模型类，它是一个简单的 POJO 类；

用户。Java

Java

public class User {
    private int id;
    private String username;
    private String password;
  
    public int getId() { return id; }
  
    public void setId(int id) { this.id = id; }
  
    public String getUsername() { return username; }
  
    public void setUsername(String username)
    {
        this.username = username;
    }
  
    public String getPassword() { return password; }
  
    public void setPassword(String password)
    {
        this.password = password;
    }
}

然后我们创建一个类来帮助对我们的数据库执行 CRUD 操作，

用户道处理程序。Java

Java

import java.sql.*;
import java.util.ArrayList;
import java.util.List;
  
public class UserDaoHandler {
    public static Connection connectDB()
    {
        // connection object
        Connection connection = null;
        try {
            // returns the class object
            Class.forName("com.mysql.jdbc.Driver");
            // it creates a connection to the database using
            // the url
            connection = DriverManager.getConnection(
                "jdbc:mysql://localhost:3306/appuserdb",
                "adminuser", "user123");
        }
  
        catch (Exception message) {
            System.out.println(message);
        }
        return connection;
    }
  
    public static int addUser(User user) throws SQLException
    {
        int result = 0;
        Connection connect = UserDaoHandler.connectDB();
        
        // SQL statement is precompiled and stored in a
        // PreparedStatement object.
        PreparedStatement preparedStatement
            = connect.prepareStatement(
                "insert into user(username,password) values (?,?)");
        
        // set the parameter to the given Java String value
        preparedStatement.setString(1, user.getUsername());
        preparedStatement.setString(2, user.getPassword());
        
        // execute SQl statement insert values to the
        // database
        result = preparedStatement.executeUpdate();
        
        // close the database connection
        connect.close();
        return result;
    }
    public static int updateUser(User user)
        throws SQLException
    {
        int result = 0;
        
        // create connection at the call of this method
        Connection connect = UserDaoHandler.connectDB();
        
        // SQL statement is precompiled and stored in a
        // PreparedStatement object
        PreparedStatement preparedStatement
            = connect.prepareStatement(
                "update user set username=?,password=? where id=?");
        
        // set the parameter to the given Java String value
        preparedStatement.setString(1, user.getUsername());
        preparedStatement.setString(2, user.getPassword());
        
        // execute SQl statement, insert values to the
        // database
        result = preparedStatement.executeUpdate();
        
        // close the database connection
        connect.close();
        return result;
    }
    public static int deleteUser(int id) throws SQLException
    {
        int result = 0;
        
        // create connection at the call of this method
        Connection connect = UserDaoHandler.connectDB();
        
        // SQL statement is precompiled and stored in a
        // PreparedStatement object
        PreparedStatement preparedStatement
            = connect.prepareStatement(
                "delete from USER where id =?");
        // set the integer value to the  user id,
        preparedStatement.setInt(1, id);
        
        // execute SQl statement, insert values to the
        // database
        result = preparedStatement.executeUpdate();
        
        // close the database connection
        connect.close();
        
        return result;
    }
    public static User getUserById(int id)
        throws SQLException
    {
        // create a user object
        User user = new User();
        
        // create connection at the call of the method
        Connection connect = UserDaoHandler.connectDB();
        
        // SQL statement is precompiled and stored in a
        // PreparedStatement object
        PreparedStatement preparedStatement
            = connect.prepareStatement(
                "select * from USER where id=?");
        
        // set the integer value to the  user id,
        preparedStatement.setInt(1, id);
        
        // A table of data representing a database result
        // set,generated after the query
        ResultSet resultSet
            = preparedStatement.executeQuery();
        
        // checking for saved fields,if more than one
        if (resultSet.next()) {
            // value of the column is assigned to the set
            // method
            user.setId(resultSet.getInt(1));
            user.setUsername(resultSet.getString(2));
            user.setPassword(resultSet.getString(3));
        }
        
        // close the database connection
        connect.close();
        return user;
    }
    public static List getAllUsers(int start,
                                         int total)
        throws SQLException
    {
        // creating an empty arraylist of type User.
        List list = new ArrayList();
        
        // create connection at the call of the method
        Connection connect = UserDaoHandler.connectDB();
        
        // SQL statement and telling it to select from the
        // first index
        PreparedStatement preparedStatement
            = connect.prepareStatement(
                "select * from user limit " + (start - 1)
                + "," + total);
        ResultSet resultSet
            = preparedStatement.executeQuery();
        
        // this keep iterating the list of user
        // setting the values to the corresponding integer
        while (resultSet.next()) {
            User user = new User();
            user.setId(resultSet.getInt(1));
            user.setUsername(resultSet.getString(2));
            user.setPassword(resultSet.getString(3));
            // store the values into the list
            list.add(user);
        }
        
        // close the database connection
        connect.close();
        
        return list;
    }
}

之后，我们创建我们的 Servlet 类，每个 servlet 类用于从数据库中创建用户、更新用户、读取用户和删除用户。这是课程；

添加用户 Servlet：

该方法是一个post方法，它收集用户信息并将其添加到数据库中，在这个例子中，我们使用“username”、“password”和“userId”作为要保存到数据库中的字段。

Java

import java.io.IOException;
import java.io.PrintWriter;
import java.sql.SQLException;
import javax.servlet.*;
import javax.servlet.annotation.*;
import javax.servlet.http.*;
  
@WebServlet(name = "addUser", value = "/addUser")
public class AddUser extends HttpServlet {
    protected void
    processRequest(HttpServletRequest request,
                   HttpServletResponse response)
        throws ServletException, IOException
    {
        response.setContentType("text/html;charset=UTF-8");
    }
    @Override
    protected void doGet(HttpServletRequest request,
                         HttpServletResponse response)
        throws ServletException, IOException
    {
    }
    
    // override the supertype method post
    @Override
    protected void doPost(HttpServletRequest request,
                          HttpServletResponse response)
        throws ServletException, IOException
    {
        processRequest(request, response);
        
        // print object for string formatting
        PrintWriter out = response.getWriter();
        
        // Httpservletrequest get parameters from user
        String username = request.getParameter("username");
        String password = request.getParameter("password");
        
        // Instances of User class
        User user = new User();
        
        // set the parameters gotten to the 'Username' field
        // of User class
        user.setUsername(username);
        user.setPassword(password);
  
        int status = 0;
        try {
            // static method add the values stored in the
            // user object to the database
            status = UserDaoHandler.addUser(user);
        }
        catch (SQLException e) {
            e.printStackTrace();
        }
        
        // check if the values correspond to the one
        // specified
        if (status > 0) {
            out.print("
Record saved successfully!
");
            request.getRequestDispatcher("index.html")
                .include(request, response);
        }
        else {
            out.println("Sorry! unable to save record");
        }
  
        // close database connection
        out.close();
    }
}

添加用户。 Java用于将用户添加到数据库。

更新用户 Servlet：

此方法获取用户 ID 并将其与数据库中的用户 ID 匹配，然后对其进行更新。

更新用户。Java

Java

import java.io.IOException;
import java.io.PrintWriter;
import java.sql.SQLException;
import javax.servlet.*;
import javax.servlet.annotation.*;
import javax.servlet.http.*;
  
@WebServlet(name = "updateUser", value = "/updateUser")
public class UpdateUser extends HttpServlet {
    // override the supertype method post
    @Override
    protected void doGet(HttpServletRequest request,
                         HttpServletResponse response)
        throws ServletException, IOException
    {
        PrintWriter out = response.getWriter();
        
        // collect user id parameter
        String userId = request.getParameter("id");
        
        // the string value is parse as integer to id
        int id = Integer.parseInt(userId);
        try {
            
            // this statement get user by id
            User user = UserDaoHandler.getUserById(id);
            
            // this print the jsp and render web page
            out.println("
Edit User Account
");
            out.print(
                "
");
            out.print("");
            out.print(
                "");
            out.print(
                "");
            out.print(
                "");
            out.print("
Name:
");
            out.print("
");
            
            // close database connection
            out.close();
        }
        catch (SQLException e) {
            e.printStackTrace();
        }
    }
  
    @Override
    protected void doPost(HttpServletRequest request,
                          HttpServletResponse response)
        throws ServletException, IOException
    {
    }
}

补丁方法：

该方法简单地将数据库信息与从网页获取的输入参数进行交换，并将其保存到数据库中。

补丁用户。Java

Java

import static java.lang.System.out;
  
import java.io.IOException;
import java.io.PrintWriter;
import java.sql.SQLException;
import javax.servlet.*;
import javax.servlet.annotation.*;
import javax.servlet.http.*;
  
@WebServlet(name = "patchUser", value = "/patchUser")
public class PatchUser extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request,
                         HttpServletResponse response)
        throws ServletException, IOException
    {
    }
  
    @Override
    protected void doPost(HttpServletRequest request,
                          HttpServletResponse response)
        throws ServletException, IOException
    {
        PrintWriter out = response.getWriter();
        String userId = request.getParameter("id");
        int id = Integer.parseInt(userId);
        String username = request.getParameter("name");
        String password = request.getParameter("password");
        User user = new User();
        user.setId(id);
        user.setUsername(username);
        user.setPassword(password);
        try {
            int result = UserDaoHandler.updateUser(user);
            if (result > 0) {
                response.sendRedirect("viewUser? page =1");
            }
            else {
                out.print("unable to connect");
            }
        }
        catch (SQLException e) {
            e.printStackTrace();
        }
        out.close();
    }
}

查看用户 Servlet：

此方法从数据库中获取所有用户并以简单的表格格式显示。

查看用户。Java

Java

import java.io.IOException;
import java.io.PrintWriter;
import java.sql.SQLException;
import java.util.List;
import javax.servlet.*;
import javax.servlet.annotation.*;
import javax.servlet.http.*;
  
@WebServlet(name = "viewUser", value = "/viewUser")
public class ViewUser extends HttpServlet {
    protected void
    processRequest(HttpServletRequest request,
                   HttpServletResponse response)
        throws ServletException, IOException
    {
        response.setContentType("text/html;charset=UTF-8");
    }
    
    // override the supertype method get
    @Override
    protected void doGet(HttpServletRequest request,
                         HttpServletResponse response)
        throws ServletException, IOException
    {
        processRequest(request, response);
        PrintWriter out = response.getWriter();
        
        // assigning integer values to web pages
        String pageId = request.getParameter("page");
        int total = 3;
        int pagesId = Integer.parseInt(pageId);
        if (pagesId == 1) {
        }
        else {
            pagesId = pagesId - 1;
            pagesId = pagesId * total + 1;
        }
        
        // initializing list of users
        List list = null;
        out.println(
            "Add user");
  
        out.print("
 User Table: 
");
        out.print(
            "");
        out.print("");
        try {
            // getting all users and assigning to the page
            // numbers
            list = UserDaoHandler.getAllUsers(pagesId,
                                              total);
        }
        catch (SQLException e) {
            e.printStackTrace();
        }
        
        // ensuring list is not null
        if (list != null) {
            // iterating through the list of Users
            // And getting username and id of users.
            for (User user : list) {
                out.print("");
            }
            
            // printing out in a jsp web format.
            out.print("
Id
username
" + user.getId()
                          + "
" + user.getUsername()
                          + "
");
            out.print("1 ");
            out.print("2 ");
            out.print("3 ");
        }
    }
  
    @Override
    protected void doPost(HttpServletRequest request,
                          HttpServletResponse response)
        throws ServletException, IOException
    {
    }
}

删除小服务程序：

下面的方法从数据库中删除用户并发送用户视图响应，显示数据库中的当前用户。

删除用户。Java

Java

import java.io.IOException;
import java.sql.SQLException;
import javax.servlet.*;
import javax.servlet.annotation.*;
import javax.servlet.http.*;
  
@WebServlet(name = "deleteUser", value = "/deleteUser")
public class DeleteUser extends HttpServlet {
    protected void
    processRequest(HttpServletRequest request,
                   HttpServletResponse response)
        throws ServletException, IOException
    {
        response.setContentType("text/html;charset=UTF-8");
    }
    
    // overriding the supertype method get
    @Override
    protected void doGet(HttpServletRequest request,
                         HttpServletResponse response)
        throws ServletException, IOException
    {
        processRequest(request, response);
        
        // get user by id
        String userId = request.getParameter("id");
        
        // the int value of the parameter
          // is parse to the id
        int id = Integer.parseInt(userId);
        try {
            // the delete method is 
              // invoked on user with the
            // specified id
            UserDaoHandler.deleteUser(id);
        }
        catch (SQLException e) {
            e.printStackTrace();
        }
        // it sent the current
         // user view as response
        response.sendRedirect("viewUser?page=1");
    }
  
    @Override
    protected void doPost(HttpServletRequest request,
                          HttpServletResponse response)
        throws ServletException, IOException
    {
    }
}

输出：

每当用户调用时， Java servlet 都会呈现Java服务器页面。