MVC 模式和模型 2

MVC框架

一个实现 MVC 模式的应用包含模型、视图、控制器 3 个模块：

模型：封装了应用的数据和业务逻辑，负责管理系统业务数据

视图：负责应用的展示

控制器：负责与用户进行交互，接收用户输入、改变模型、调整视图的显示

基于MVC架构模式的 Java Web 开发

采用了 Servlet + JSP + JavaBean 的技术实现

基于MVC架构模式的 Java Web 开发步骤

1. 定义一系列的 Bean 来表示数据

2. 使用一个 Dispatcher Servlet 和控制类来处理用户请求

3. 在 Servlet 中填充 Bean

4. 在 Servlet 中，将 Bean 存储到 request、session、servletContext中

5. 将请求转发到 JSP 页面

6. 在 JSP 页面中，从 Bean 中提取数据。

其中 Dispatcher servlet 必须完成如下功能：

1. 根据 URI 调用相应的 action

2. 实例化正确的控制器类

3. 根据请求参数值来构造表单 bean

4. 调用控制器对象的相应方法

5. 转发到一个视图（JSP页面）

每个 HTTP 请求都发送给控制器，请求中的 URI 标识出对应的 action。action 代表了应用可以执行的一个操作。一个提供了 Action 的 Java 对象称为 action 对象。

控制器会解析 URI 并调用相应的 action，然后将模型对象放到视图可以访问的区域，以便服务器端数据可以展示在浏览器上。最后控制器利用 RequestDispatcher 跳转到视图（JSP页面），在JSP页面使用 EL 以及定制标签显示数据。

注意：调用 RequestDispatcher.forward 方法并不会停止执行剩余的代码。因此，若 forward 方法不是最后一行代码，则应显示的返回。

使用MVC模式的实例

 目录结构如下

代码如下

DispatcherServlet.java

package app16c.servlet;

import java.io.IOException;

import javax.servlet.RequestDispatcher;
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 app16c.controller.InputProductController;
import app16c.controller.SaveProductController;

@WebServlet(name = "DispatcherServlet", urlPatterns = {"/product_input.action", "/product_save.action"})
public class DispatcherServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;
       
    public DispatcherServlet() {
        super();
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        process(request, response);
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
    
    private void process(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String uri = request.getRequestURI();
        int lastIndex = uri.lastIndexOf("/");
        String action = uri.substring(lastIndex + 1);
        String dispatcherUrl = null;
        if (action.equals("product_input.action")) {
            InputProductController controller = new InputProductController();
            dispatcherUrl = controller.handleRequest(request, response);
        } else if (action.equals("product_save.action")) {
            SaveProductController controller = new SaveProductController();
            dispatcherUrl = controller.handleRequest(request, response);
        }
        if (dispatcherUrl != null) {
            RequestDispatcher rd = request.getRequestDispatcher(dispatcherUrl);
            rd.forward(request, response);
        }
    }
}

Controller.java

package app16c.controller;

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

public interface Controller {
    public abstract String handleRequest(HttpServletRequest request, HttpServletResponse response);
}

InputProductController.java

package app16c.controller;

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

public class InputProductController implements Controller {

    @Override
    public String handleRequest(HttpServletRequest request, HttpServletResponse response) {
        return "/WEB-INF/jsp/ProductForm.jsp";
    }
}

SaveProductController.java

package app16c.controller;

import java.util.List;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import app16c.domain.Product;
import app16c.form.ProductForm;
import app16c.validator.ProductValidator;

public class SaveProductController implements Controller {

    @Override
    public String handleRequest(HttpServletRequest request, HttpServletResponse response) {
        ProductForm productForm = new ProductForm();
        productForm.setName(request.getParameter("name"));
        productForm.setDescription(request.getParameter("description"));
        productForm.setPrice(request.getParameter("price"));
        ProductValidator productValidator = new ProductValidator();
        List<String> errors = productValidator.validate(productForm);
        System.out.println(errors);
        if (errors.isEmpty()) {
            Product product = new Product();
            product.setName(productForm.getName());
            product.setDescription(productForm.getDescription());
            product.setPrice(Float.parseFloat(productForm.getPrice()));
            // insert code to save product to the database
            request.setAttribute("product", product);
            return "/WEB-INF/jsp/ProductDetails.jsp";
        } else {
            request.setAttribute("errors", errors);
            request.setAttribute("form", productForm);
            return "/WEB-INF/jsp/ProductForm.jsp";
        }
    }
}

ProductValidator.java

package app16c.validator;

import java.util.ArrayList;
import java.util.List;
import app16c.form.ProductForm;

public class ProductValidator {
    public List<String> validate(ProductForm productForm) {
        List<String> errors = new ArrayList<>();
        String name = productForm.getName();
        if (name == null || name.trim().isEmpty()) {
            errors.add("Product must have a name");
        }
        String price = productForm.getPrice();
        if (price == null || price.trim().isEmpty()) {
            errors.add("Product must have a price");
        }else {
            try {
                Float.parseFloat(price);
            } catch (NumberFormatException e) {
                errors.add("Invalid price value.");
            }
        }
        return errors;
    }
}

Product.java

package app16c.domain;

import java.io.Serializable;

public class Product implements Serializable { // 一个JavaBean 实现该接口，其实例可以安全的将数据保存到 HttpSession 中。
    private static final long serialVersionUID = 1L;
    private String name;
    private String description;
    private float price;
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getDescription() {
        return description;
    }
    public void setDescription(String description) {
        this.description = description;
    }
    public float getPrice() {
        return price;
    }
    public void setPrice(float price) {
        this.price = price;
    }
}

ProductForm.java

package app16c.form;

// 表单类与 HTML表单相映射，是HTML表单在服务端的代表
public class ProductForm {  // 表单类不需要实现 Serializable 接口，因为表单对象很少保存在 HttpSession 中。
    private String name;
    private String description;
    private String price;
    
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getDescription() {
        return description;
    }
    public void setDescription(String description) {
        this.description = description;
    }
    public String getPrice() {
        return price;
    }
    public void setPrice(String price) {
        this.price = price;
    }
}

ProductForm.jsp

<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>  <!-- taglib指令用来引用标签库并设置标签库的前缀  -->

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>Add Product Form</title>
    <link rel="stylesheet" type="text/css" href="css/main.css" />  <!-- 外部样式表 -->
</head>
<body>
    <div id="global">
        <c:if test="${requestScope.errors != null }">  <!-- EL表达式 -->
            <p id="errors">
                Error(s)!
                <ul>
                    <c:forEach var="error" items="${requestScope.errors }">  <!-- jstl 遍历 -->
                        <li>${error }</li>
                    </c:forEach>
                </ul>
            </p>
        </c:if>
        <form action="product_save.action" method="post">
            <fieldset>
                <legend>Add a product</legend>
                <p>
                    <label for="name">Product Name: </label>
                    <input type="text" id="name" name="name" tabindex="1" />
                </p>
                <p>
                    <label for="description">Description: </label>
                    <input type="text" id="description" name="description" tabindex="2" />
                </p>
                <p>
                    <label for="price">Price: </label>
                    <input type="text" id="price" name="price" tabindex="3" />
                </p>
                <p id="buttons">
                    <input id="reset" type="reset" tabindex="4" />
                    <input id="submit" type="submit" tabindex="5" value="Add Product" />
                </p>
            </fieldset>
        </form>
    </div>
</body>
</html>

ProductDetails.jsp

<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>Save Product</title>
    <link rel="stylesheet" type="text/scc" href="css/main.css" />
</head>
<body>
    <div id="global">
        <h4>The product has been saved.</h4>
        <p>
            <h5>Details</h5>
            Product Name: ${product.name } <br />
            Description: ${product.description } <br />
            Price: $${product.price }
        </p>
    </div>
</body>
</html>

测试结果