Servlet – CRUD

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。我们可以使用 CRUD 操作来操作任何数据库的表。在本文中，让我们以 MYSQL 为例。 MySQL中的表创建

-- Let us keep the db name as geeksforgeeks
-- Table name geekusers
CREATE TABLE `geeksforgeeks`.`geekusers` (
  `geekUserId` INT NOT NULL AUTO_INCREMENT,
  `geekUserName` VARCHAR(25) NULL,
  `numberOfPosts` INT NULL,
  `technologiesPrefered` VARCHAR(45) NULL,
  PRIMARY KEY (`geekUserId`));

让我们有一个 index.html 文件，流程开始的地方。一个 HTML 文件可以用 CSS 来美化和 javascript 来验证。

HTML

    
  

    



  
Add New GeekUser
    
  


  
  
  
  
Name:
Number of Posts:
Technologies Preferred:



  


view GeekUsers
  

在运行 HTML 文件时，我们可以看到输出为

这里让我们给出如下输入

点击“ Save GeekUsers ”，我们可以看到下面的输出。这表明记录保存成功，并且还有通过“查看GeekUsers”的方式查看记录的规定

我们也可以看到一条记录被插入到 MySQL 中

让我们看看它的后端代码。首先，让我们看看“模型” Java文件。这里是 GeekUsers。Java它将包含一些类似于 MySQL 表中名为“geekusers”的列的属性。定义了其对应的 setter 和 getter 方法，这些方法在整个程序中都是必需的。

Java

// Model file equivalent 
// to MySQL table GeekUsers
public class GeekUsers {
    private int geekUserId;
    private int numberOfPosts;
    private String geekUserName, technologiesPrefered;
  
    public int getId() { return geekUserId; }
  
    public void setId(int id) { this.geekUserId = id; }
  
    public String getName() { return geekUserName; }
  
    public void setName(String name)
    {
        this.geekUserName = name;
    }
  
    public int getNumberOfPosts() { return numberOfPosts; }
  
    public void setNumberOfPosts(int numberOfPosts)
    {
        this.numberOfPosts = numberOfPosts;
    }
  
    public String gettechnologiesPrefered()
    {
        return technologiesPrefered;
    }
  
    public void
    setTechnologiesPrefered(String technologiesPrefered)
    {
        this.technologiesPrefered = technologiesPrefered;
    }
}

需要一个 DAO 类（数据访问对象），它可以帮助获取 MySQL 的 JDBC 连接，并且这里定义了执行 CRUD 操作所需的所有方法。下面是获取连接所需的代码片段。当我们使用 JDBC 时，我们应该有一个 URL 类型

jdbcURL = "jdbc:mysql://localhost:3306/";
// Let us use "geeksforgeeks" for it
jdbcUsername = "root"; // username generally "root"
jdbcPassword = "xxxxx";  // password that is used to connect to mysql

用于获取连接的Java代码片段

Java

import java.util.*;
import java.sql.*;  
public class GeekUsersDao { 
    
    // Here geeksforgeeks is the name of the database
    private static String jdbcURL = "jdbc:mysql://localhost:3306/geeksforgeeks";
    private static String jdbcUsername = "root";
    
    // provide your appropriate password  here
    private static String jdbcPassword = "*****"; 
  
    public static Connection getConnection(){
        Connection con=null;        
        try {
             // This is the way of connecting MySQL
            Class.forName("com.mysql.jdbc.Driver"); 
            con = DriverManager.getConnection(jdbcURL, jdbcUsername, jdbcPassword);
        } catch (SQLException e) {
            System.out.println("Message.. " + e.getMessage());
            e.printStackTrace();            
        } catch (ClassNotFoundException e) {
            System.out.println("Message.. " + e.getMessage());
            e.printStackTrace();
        }        
        return con;
    }
    // Rest of the methods like Save,
    // Update, Delete etc., should come here
}

现在让我们看看 servlet 代码。从 index.html 中，输入值被发送到“/SaveServlet”。 'SaveServlet' 是查找的 servlet 名称，'POST' 是被调用的方法

SaveGeekUserServlet。Java

Java

import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
@WebServlet("/SaveServlet")
public class SaveGeekUserServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request,
                          HttpServletResponse response)
        throws ServletException, IOException
    {
        response.setContentType("text/html");
        PrintWriter out = response.getWriter();
  
        // Getting all the request parameters from
        // index.html
        String name = request.getParameter("name");
        int numberOfPosts = Integer.parseInt(
            request.getParameter("numberOfPosts"));
        String technologiesPrefered
            = request.getParameter("technologiesPrefered");
  
        // GeekUsers object is created
        GeekUsers geekUser = new GeekUsers();
  
        // Collected parameters like name, numberOfPosts and
        // technologiesPrefered are set for the object so
        // that it can be retrieved in other places
        geekUser.setName(name);
        geekUser.setNumberOfPosts(numberOfPosts);
        geekUser.setTechnologiesPrefered(
            technologiesPrefered);
  
        // Calling save method in GeekUsersDao by passing
        // geekUser
        int status = GeekUsersDao.save(geekUser);
  
        // This is possible when the record is saved
        // successfully
        if (status > 0) {
            out.print("
Record saved successfully!
");
            request.getRequestDispatcher("index.html")
                .include(request, response);
        }
        else {
            // If there is an issue in saving the record, we
            // need to show this message
            out.println("Sorry! unable to save record");
        }
  
        out.close();
    }
}

下面给出了将记录保存到表中的Java方法

Using PreparedStatement, we can execute any SQL statement. In order to save the record, we need to insert the details in ‘geekusers’ table

Java

public static int save(GeekUsers e)
{
    int status = 0;
    try {
        Connection con = GeekUsersDao.getConnection();
        // Using PreparedStatement, we can execute any SQL
        // statement In order to save the record, we need to
        // insert the details in 'geekusers' table "e" is an
        // object of type "GeekUsers" which exactly resembles
        // geekusers table From html page, the values has
        // been passed to a servlet and from there, this
        // method is called
        PreparedStatement ps = con.prepareStatement(
            "insert into geekusers(geekUserName,numberOfPosts,technologiesPrefered) values (?,?,?)");
        ps.setString(1, e.getName());
        ps.setInt(2, e.getNumberOfPosts());
        ps.setString(3, e.gettechnologiesPrefered());
  
        status = ps.executeUpdate();
  
        con.close();
    }
    catch (Exception ex) {
        System.out.println("Message.." + ex.getMessage());
        ex.printStackTrace();
    }
  
    return status;
}

为了查看保存的记录，我们可以使用“查看 GeekUsers”链接

View Servlet的相关代码

Java

import java.io.IOException;
import java.io.PrintWriter;
import java.util.List;
  
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
@WebServlet("/ViewServlet")
public class ViewGeekUserServlet extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setContentType("text/html");
        PrintWriter printWriter=response.getWriter();
        printWriter.println("Add New GeekUser");
        printWriter.println("
GeekUsers List
");
          
        List list=GeekUsersDao.getAllGeekUses();
          
        printWriter.print("");
        for(GeekUsers e:list){
            // each row is identified by means of its id
            // hence on click of 'edit', it is sent as
            // edit
            // on click of 'delete', it is sent as
            // delete
            printWriter.print("");
        }
        printWriter.print("
Id
Name
Number Of Post
Preferred Technology
Edit
Delete
"+e.getId()+"
"+e.getName()+"
"+e.getNumberOfPosts()+"
"+e.gettechnologiesPrefered()+"
edit
delete
");
          
        printWriter.close();
    }
}

为了获取所有极客用户，使用“GeekUsersDao”中的“getAllGeekUses”方法

PreparedStatement ps=con.prepareStatement(“select * from geekusers”);

ResultSet rs=ps.executeQuery();

// and then the resultset is 

// iterated as shown in the below code

Java

public static List getAllGeekUses()
{
    List list = new ArrayList();
  
    try {
        Connection con = GeekUsersDao.getConnection();
        PreparedStatement ps = con.prepareStatement(
            "select * from geekusers");
        ResultSet rs = ps.executeQuery();
        while (rs.next()) {
            GeekUsers e = new GeekUsers();
            e.setId(rs.getInt(1));
            e.setName(rs.getString(2));
            e.setNumberOfPosts(rs.getInt(3));
            e.setTechnologiesPrefered(rs.getString(4));
            list.add(e);
        }
        con.close();
    }
    catch (Exception e) {
        e.printStackTrace();
    }
  
    return list;
}

我们可以选择编辑数据以及删除数据。让我们看看他们

“edit”上对应的Servlet代码是“EditGeekUserServlet.Java”

Java

import java.io.IOException;
import java.io.PrintWriter;
  
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
@WebServlet("/EditServlet")
public class EditGeekUserServlet extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setContentType("text/html");
        PrintWriter out=response.getWriter();
        out.println("
Update GeekUser
");
        String sid=request.getParameter("id");
        int id=Integer.parseInt(sid);
        
        // We need to get the id from view servlet
        // request and it has to be parsed with
        // Integer.parseInt in order to get as number
        // Then specific user is retrieved by means 
        // of GeekUsersDao.getGeekUsersById(id)
        GeekUsers e=GeekUsersDao.getGeekUsersById(id);
          
        out.print("
");
        out.print("");
        out.print("");
        out.print("");
        out.print("");
        out.print("");
          
        out.print("");
        out.print("
Name:
Number Of Posts:
Technologies Prefered:
");
        out.print("
");
          
        out.close();
    }
}

让我们看看 GeekUsersDao.getGeekUsersById()

Java

public static GeekUsers getGeekUsersById(int id)
{
    GeekUsers e = new GeekUsers();
  
    try {
        Connection con = GeekUsersDao.getConnection();
        // We are getting the details for a specific user
        // and hence the query has to be sent in the below
        // way
        PreparedStatement ps = con.prepareStatement(
            "select * from geekusers where geekUserId=?");
        ps.setInt(1, id);
        ResultSet rs = ps.executeQuery();
        if (rs.next()) {
            e.setId(rs.getInt(1));
            e.setName(rs.getString(2));
            e.setNumberOfPosts(rs.getInt(3));
            e.setTechnologiesPrefered(rs.getString(4));
        }
        con.close();
    }
    catch (Exception ex) {
        ex.printStackTrace();
    }
  
    return e;
}

现在，如果我们更新（更改）数据，相应的详细信息将更新并反映在屏幕以及 MySQL 中

geekUserId = 1 的数据已更新

让我们也查询 MySQL 部分

做上述操作的相关Servlet代码是“ EditGeekUserServlet2.Java“

Java

import java.io.IOException;
import java.io.PrintWriter;
  
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
@WebServlet("/EditServlet2")
public class EditGeekUserServlet2 extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setContentType("text/html");
        PrintWriter out=response.getWriter();
          
        String sid=request.getParameter("id");
        int id=Integer.parseInt(sid);
        String name=request.getParameter("name");
        int numberOfPosts=Integer.parseInt(request.getParameter("numberOfPosts"));
        String technologiesPrefered=request.getParameter("technologiesPrefered");    
          
        GeekUsers geekUser=new GeekUsers();
        geekUser.setId(id);
        geekUser.setName(name);
        geekUser.setNumberOfPosts(numberOfPosts);
        geekUser.setTechnologiesPrefered(technologiesPrefered);
          
        // GeekUsersDao.update method is called
        // along with the modified values for geekUser
        int status=GeekUsersDao.update(geekUser);
        if(status>0){
            response.sendRedirect("ViewServlet");
        }else{
            out.println("Sorry! unable to update record");
        }
          
        out.close();
    }
  
}

GeekUsersDao.update()

Java

public static int update(GeekUsers e)
{
    int status = 0;
    try {
        Connection con = GeekUsersDao.getConnection();
        // AS we are not sure about what fields need to be
        // updated, we are setting for all the firlds by
        // means of update query This will update the record
        // for the corresponding geekUserId
        PreparedStatement ps = con.prepareStatement(
            "update geekusers set geekUserName=?,numberOfPosts=?,technologiesPrefered=? where geekUserId=?");
        ps.setString(1, e.getName());
        ps.setInt(2, e.getNumberOfPosts());
        ps.setString(3, e.gettechnologiesPrefered());
        ps.setInt(4, e.getId());
  
        status = ps.executeUpdate();
  
        con.close();
    }
    catch (Exception ex) {
        ex.printStackTrace();
    }
  
    return status;
}

现在让我们看看删除部分。假设我们有 2 条记录，点击删除第 2 条记录，第 2 行被删除

让我们看看它的相关代码。

“删除GeekUserServlet”。Java

Java

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;
  
@WebServlet("/DeleteServlet")
public class DeleteGeekUserServlet extends HttpServlet {
    protected void doGet(HttpServletRequest request,
                         HttpServletResponse response)
        throws ServletException, IOException
    {
        String sid = request.getParameter("id");
        int id = Integer.parseInt(sid);
        // GeekUsersDao.delete methpd is called with the
        // corresponding id and then it is redirected to
        // viewservlet
        GeekUsersDao.delete(id);
        response.sendRedirect("ViewServlet");
    }
}

GeekUsersDao.delete

Java

public static int delete(int id)
{
    int status = 0;
    try {
        Connection con = GeekUsersDao.getConnection();
        // delete query is given to delete the record for
        // the given geekUserId
        PreparedStatement ps = con.prepareStatement(
            "delete from geekusers where geekUserId=?");
        ps.setInt(1, id);
        status = ps.executeUpdate();
        con.close();
    }
    catch (Exception e) {
        e.printStackTrace();
    }
  
    return status;
}

CRUD 流的可视化表示

结论

通过上述方式，我们可以使用 Servlet 技术对任何数据库进行“创建/读取/更新/删除”操作。一个模型类、DAO 类和相应的 servlet 类可以很好地执行简单的 CRUD 操作。