初识MVC

最新推荐文章于 2023-04-22 16:26:27 发布

原创最新推荐文章于 2023-04-22 16:26:27 发布 · 760 阅读

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工程结构：

Product.java：

Product实例是一个封装了产品信息的JavaBean。Product类包含3个属性：productName、description和price。它实现了java.io.Serializable接口，其实例可以安全地将数据保存到HttpSession中。根据Serializable要求，Product实现了一个serialVersionUID属性。

package app02c.domain;
import java.io.Serializable;

public class Product implements Serializable {
    private static final long serialVersionUID = 748392348L;
	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：

这是一个表单类，它与Html表单相映射，是Product在服务端的代表。和Product类相比，它不需要实现Serializable接口，因为表单对象很少保存在HttpSession中。其中price属性类型为String而非float，这样它就可以直接从表单取数据。

它看上去同Product类相似，这就引出一个问题：ProductForm类是否有存在的必要？

实际上，表单对象会传递ServletRequest给其他组件，类似Validator。而ServletRequest是一个Servlet层的对象，不应当暴露给应用的其他层。另一原因是，当数据校验失败时，表单对象将用于保存和展示用户在原始表单上的输入。校验不通过，则重定向回输入表单。

package app02c.form;

public class ProductForm {
    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;
    }
}

DispatcherServlet.java：

主要用作职责调度工作。此类检查每个URI，创建相应的controller，并调用其handleRequest方法。

Dispatcher servlet必须能够做如下事情：

(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)转向到一个视图(JSP页面)

package app02c.servlet;
import java.io.IOException;

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

import app02c.controller.InputProductController;
import app02c.controller.SaveProductController;

public class DispatcherServlet extends HttpServlet {
    
    private static final long serialVersionUID = 98279L;

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

    @Override
    public void doPost(HttpServletRequest request, 
            HttpServletResponse response)
            throws IOException, ServletException {
        process(request, response);
    }

    private void process(HttpServletRequest request,
            HttpServletResponse response) 
            throws IOException, ServletException {

        String uri = request.getRequestURI();
        /*
         * uri is in this form: /contextName/resourceName, 
         * for example: /app10a/product_input. 
         * However, in the case of a default context, the 
         * context name is empty, and uri has this form
         * /resourceName, e.g.: /product_input
         */
        int lastIndex = uri.lastIndexOf("/");
        String action = uri.substring(lastIndex + 1); 
        String dispatchUrl = null;

        if (action.equals("product_input.action")) {
        	InputProductController controller = new InputProductController();
        	dispatchUrl = controller.handleRequest(request, response);
        } else if (action.equals("product_save.action")) {
        	SaveProductController controller = new SaveProductController();
        	dispatchUrl = controller.handleRequest(request, response);
        }

        // forward to a view
        if (dispatchUrl != null) {
            RequestDispatcher rd = 
                    request.getRequestDispatcher(dispatchUrl);
            rd.forward(request, response);
        }
    }
}

process方法步骤如下：

a.创建并根据请求参数构建一个表单对象。product_save操作涉及3个属性：name、description和price。然后创建一个领域对象，并通过表单对象设置相应属性。

b.执行针对领域对象的业务逻辑，包括将其持久化到数据库中。

c.转发请求到视图(JSP页面)

Controller.java：

我们应该将后端业务逻辑提取到独立的被称为controller的类中，这是一个接口，它只有handleRequest一个方法。Controller接口的实现类通过该方法访问到当前请求的HttpServletRequest和HttpServletResponse对象。

package app02c.controller;

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

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

InputProductController.java：

一个controller，直接返回了ProductForm.jsp的路径。

package app02c.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：

一个controller，读取请求参数来构造一个ProductForm对象，如果validate成功，就用ProductForm对象来构造一个Product对象，并返回ProductDetail.jsp路径。要注意：它不仅返回ProductDetails.jsp(或ProductForm.jsp)路径，还完成了在ServletRequest上设置attribute：product(或productForm、errors)对象，此处设置的attribute用于在JSP页面展示。

package app02c.controller;

import java.util.List;

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

import app02c.domain.Product;
import app02c.form.ProductForm;
import app02c.validator.ProductValidator;

public class SaveProductController implements Controller {

    @Override
    public String handleRequest(HttpServletRequest request,
            HttpServletResponse response) {
        ProductForm productForm = new ProductForm();
        // populate action properties
        productForm.setName(request.getParameter("name"));
        productForm.setDescription(request.getParameter("description"));
        productForm.setPrice(request.getParameter("price"));

        // validate ProductForm
        ProductValidator productValidator = new ProductValidator();
        List<String> errors = productValidator.validate(productForm);
        if (errors.isEmpty()) {
            // create Product from ProductForm
            Product product = new Product();
            product.setName(productForm.getName());
            product.setDescription(productForm.getDescription());
            product.setPrice(Float.parseFloat(productForm.getPrice()));

            // no validation error, execute action method
            // insert code to save product to the database

            // store product in a scope variable for the view
            request.setAttribute("product", product);
            return "/WEB-INF/jsp/ProductDetails.jsp";
        } else {
            // store errors and form in a scope variable for the view
            request.setAttribute("errors", errors);
            request.setAttribute("form", productForm);
            return "/WEB-INF/jsp/ProductForm.jsp";
        }
    }

}

ProductValidator.java：

在Web应用执行action时，很重要的一个步骤就是进行输入校验。在本例中唯一需要用到产品校验的地方就是保存产品时，所以我们为SaveProductController类引入ProductValidator类。

package app02c.validator;

import java.util.ArrayList;
import java.util.List;

import app02c.form.ProductForm;

public class ProductValidator {
	
	public List<String> validate(ProductForm productForm) {
		List<String> errors = new ArrayList<String>();
		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;
	}
}

web.xml：

部署描述文件，用一个路径配置servlet的映射。

<?xml version="1.0" encoding="ISO-8859-1"?>
<web-app xmlns="http://java.sun.com/xml/ns/javaee"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
    version="3.0"
> 

	<servlet>
	    <servlet-name>DispatcherServlet</servlet-name>
	    <servlet-class>app02c.servlet.DispatcherServlet</servlet-class>
	</servlet>
	<servlet-mapping>
	    <servlet-name>DispatcherServlet</servlet-name>
	    <url-pattern>*.action</url-pattern>
	</servlet-mapping>  

</web-app>

注：若基于Servlet3.0规范，则可以采用注解的方式，而无需在部署描述符中进行映射：

@WebService(name="ControllerServlet", urlPatterns={"/product_input","/product_save"})
public class ControllerServlet extends HttpServlet{
...
}

ProductForm.jsp：

<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<!DOCTYPE HTML>
<html>
<head>
<title>Add Product Form</title>
</head>
<body>

<div id="global">
<c:if test="${requestScope.errors != null}">
        <p id="errors">
        Error(s)!
        <ul>
        <c:forEach var="error" items="${requestScope.errors}">
            <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>

ProductDetail.jsp：

<!DOCTYPE HTML>
<html>
<head>
<title>Save Product</title>
<style type="text/css">@import url(css/main.css);</style>
</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>

注：页面中用到“表达式语言”，在lib中包含jstl相关的jar包(见工程目录截图)。

==========================================================

测试：

1.在浏览器输入网址：http://localhost:8080/MVCTest/product_input.action