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
