基于Spring MVC的Web应用开发(4) - FileUpload

牧场SZShepherd 2012-03-20

上一篇文章介绍了Spring MVC如何处理静态资源文件,本文讲解如何使用Spring MVC做文件上传,附带深入一下Spring MVC的ModelAndView。增加一个Controller,叫FileUploadController:

package org.springframework.samples.mvc.fileupload;

import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;

@Controller
@RequestMapping("/fileupload")
public class FileUploadController {

	@RequestMapping(method=RequestMethod.GET)
	public void fileUploadForm() {
	}

}

这个类和HelloWorld中的Controller类就有点差别了,首先类名上加入了@RequestMapping注解,这样在做HandlerMapping时,SpringMVC会将类名和方法名的@RequestMapping连接起来,而本例方法名前并没有具体路径(当然也可以写),因此最终映射的URL还是"/fileupload",另外一点就是在方法级@RequestMapping的注解增加了一个RequestMothod.GET,意思是只有以GET方式提交的"/fileupload"URL请求才会进入fileUploadForm()方法,其实根据HTTP协议,HTTP支持一系列提交方法(GET,POST,PUT,DELETE),同一个URL都可以使用这几种提交方式,事实上SpringMVC正是通过将同一个URL的不同提交方法对应到不同的方法上达到RESTful。

访问http://localhost:8080/web/fileupload,后台报错:

2012-03-20 19:12:44.557:WARN::/web/fileupload
javax.servlet.ServletException: Circular view path [fileupload]: would dispatch back to the current handler URL [/web/fileupload] again. Check your ViewResolver setup! (Hint: This may be the result of an unspecified view, due to default view name generation.)
	at org.springframework.web.servlet.view.InternalResourceView.prepareForRendering(InternalResourceView.java:292)
...

正如Hint中指出的,没有指定一个View,大家可能会有疑问,HelloWorld中不是可以将结果返回到浏览器么?注意HelloWorld中的@ResponseBody跟View没有任何关系,HelloWorld中其实也是返回了一个ModelAndView,但这个View为null,并且在返回前,已经将结果通过HttpServletResponse发送给浏览器了。那么怎么指定一个View呢?通常我们会想到一个默认的View,即如果没有写特殊的View,所有的结果都将转到这个默认的View上,最后将这个View推到浏览器展示。在servlet-context.xml中增加一个配置:

<!-- Resolves views selected for rendering by @Controllers to .jsp resources in the /WEB-INF/views directory -->
	<beans:bean class="org.springframework.web.servlet.view.InternalResourceViewResolver">
		<beans:property name="prefix" value="/WEB-INF/views/" />
		<beans:property name="suffix" value=".jsp" />
	</beans:bean>

配置了一个Bean,对应的类为InternalResourceViewResolver,并设置了prefix属性为"/WEB-INF/views",suffix属性为".jsp",这个类为视图解析器(view resolver),支持InternalResourceView(比如Servlet和JSP)及其子类如JstlView,默认的是InternalResourceView,如果有JSTL API存在的话,就是JstlView。

现在有一个默认的View了,这个View将view名字解析到/WEB-INF/views/目录下对应的jsp文件。哪里有view名字?当带有@RequestMapping的方法返回void时,@RequestMapping映射的URL路径即view名字。

所以当使用GET方式访问http://localhost:8080/web/fileupload时,最终会找WEB-INF/views/目录下有没有fileupload.jsp文件,有则将此jsp文件显示在浏览器上,没有则报如下错误:

ERROR: PWC6117: File &quot;/Users/stephansun/Documents/workspace/samples/samples-web/src/main/webapp/WEB-INF/views/fileupload.jsp&quot; not found

fileUploadForm.jsp文件为:

<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<html>
<head>
	<title>fileupload | mvc-showcase</title>
	<link href="<c:url value="/resources/form.css" />" rel="stylesheet"  type="text/css" />		
	<script type="text/javascript" src="<c:url value="/resources/jquery/1.6/jquery.js" />"></script>
	<script type="text/javascript" src="<c:url value="/resources/jqueryform/2.8/jquery.form.js" />"></script>	
</head>
<body>
	<div id="fileuploadContent">
		<h2>File Upload</h2>
		<p>
			See the <code>org.springframework.samples.mvc.fileupload</code> package for the @Controller code	
		</p>
		<form id="fileuploadForm" action="fileupload" method="POST" enctype="multipart/form-data" class="cleanform">
			<div class="header">
		  		<h2>Form</h2>
		  		<c:if test="${not empty message}">
					<div id="message" class="success">${message}</div>	  		
		  		</c:if>
			</div>
			<label for="file">File</label>
			<input id="file" type="file" name="file" />
			<p><button type="submit">Upload</button></p>		
		</form>	
	</div>
</body>
</html>

 上传页面正确显示后,我们需要一个方法处理上传请求,processUpload,现在FileUploadController看起来是这样的:

package org.springframework.samples.mvc.fileupload;

import java.io.IOException;

import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.multipart.MultipartFile;

@Controller
@RequestMapping("/fileupload")
public class FileUploadController {

	@RequestMapping(method=RequestMethod.GET)
	public void fileUploadForm() {
	}

	@RequestMapping(method=RequestMethod.POST)
	public void processUpload(@RequestParam MultipartFile file, Model model) throws IOException {
		model.addAttribute("message", "File '" + file.getOriginalFilename() + "' uploaded successfully");
	}
	
}

 这里出现了一个新注解@RequestParam,先看一下JSP页面的代码片段:

<input id="file" type="file" name="file" />

 input框提交的参数名就是file,@RequestParam注解自动将POST提交的参数中名为file的封装到一个MultipartFile类中,一切都在开发人员眼皮底下做好了,大大减少了开发人员的代码量。现在我们在上传页面选择一个文件上传,后台报错,看报错日志:

java.lang.IllegalArgumentException: Expected MultipartHttpServletRequest: is a MultipartResolver configured?
	at org.springframework.util.Assert.notNull(Assert.java:112)

说"是不是没配置MultipartResolver?",恩,还没有配,所有在servlet-context.xml中加上这么一段配置:

<!-- Only needed because we require fileupload in the org.springframework.samples.mvc.fileupload package -->
	<beans:bean id="multipartResolver" class="org.springframework.web.multipart.commons.CommonsMultipartResolver" />

这个Bean也是一个视图解析器(view resolver)。

完整的XML:

<?xml version="1.0" encoding="UTF-8"?>
<beans:beans xmlns="http://www.springframework.org/schema/mvc"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xmlns:beans="http://www.springframework.org/schema/beans"
	xmlns:context="http://www.springframework.org/schema/context"
	xmlns:mvc="http://www.springframework.org/schema/mvc"
	xsi:schemaLocation="
        http://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc-3.1.xsd
        http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.1.xsd
        http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.1.xsd">

	<!-- DispatcherServlet Context: defines this servlet's request-processing infrastructure -->
	
	<!-- Handles HTTP GET requests for /resources/** by efficiently serving up static resources in the ${webappRoot}/resources/ directory -->
	<resources mapping="/resources/**" location="/resources/" />
	
	<!-- Imports user-defined @Controller beans that process client requests -->
	<beans:import resource="controllers.xml" />
	
	<!-- You have to add this because you had a <resources/> declare -->
	<mvc:annotation-driven/>
	
	<!-- Resolves views selected for rendering by @Controllers to .jsp resources in the /WEB-INF/views directory -->
	<beans:bean class="org.springframework.web.servlet.view.InternalResourceViewResolver">
		<beans:property name="prefix" value="/WEB-INF/views/" />
		<beans:property name="suffix" value=".jsp" />
	</beans:bean>
	
	<!-- Only needed because we require fileupload in the org.springframework.samples.mvc.fileupload package -->
	<beans:bean id="multipartResolver" class="org.springframework.web.multipart.commons.CommonsMultipartResolver" />
	
</beans:beans>

然后再次上传文件,页面显示:

File 'a.js' uploaded successfully

上传成功,最后我们分析一下FileUploadController中的processUpload方法:

@RequestMapping(method=RequestMethod.POST)
	public void processUpload(@RequestParam MultipartFile file, Model model) throws IOException {
		model.addAttribute("message", "File '" + file.getOriginalFilename() + "' uploaded successfully");
	}

Model就是MVC模式中的M,Model层封装了要传递给View层显示的值。

最后更新一下pom.xml文件,添加两个依赖:

<!-- File Upload -->
	<dependency>
		<groupId>commons-fileupload</groupId>
		<artifactId>commons-fileupload</artifactId>
		<version>1.2.2</version>
	</dependency>	
	<dependency>
		<groupId>commons-io</groupId>
		<artifactId>commons-io</artifactId>
		<version>2.0.1</version>
	</dependency>

好像前面写的代码根本没有涉及到这两个jar包啊,为什么要引入?

 之前我写道

@RequestParam注解自动将POST提交的参数中名为file的封装到一个MultipartFile类中,一切都在开发人员眼皮底下做好了,大大减少了开发人员的代码量。

 底层真正由谁来做的呢?我们加入了multipartResolver这个Bean,它对应的类为

org.springframework.web.multipart.commons.CommonsMultipartResolver,

看看这个类的源代码,import中赫然写着:

import org.apache.commons.fileupload.FileItem;
import org.apache.commons.fileupload.FileItemFactory;
import org.apache.commons.fileupload.FileUpload;
import org.apache.commons.fileupload.FileUploadBase;
import org.apache.commons.fileupload.FileUploadException;
import org.apache.commons.fileupload.servlet.ServletFileUpload;

因此SpringMVC封装了commons-fileupload上传组件,真正起上传作用的还是commons-fileupload-1.2.2.jar这个jar包里面的类。

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