java实现文件的上传和下载

【文件上传和下载】是很多系统必备功能，比如PM\OA\ERP等；系统中常见的开发模式有B/S和C/S，而前者主要是通过浏览器来访问web服务器，一般采用七层协议中的【应用层http】进行数据传输，后者主要通过编程语言开发的app作为客户端来访问服务端，一般采用七层协议中的【传输层tcp】进行数据传输。
文章主要完成简单java web涉及的文件上传和下载功能。

正文

1. java原生servlet实现：

pom.xml配置：

<dependency>
	  <groupId>javax</groupId>
	  <artifactId>javaee-api</artifactId>
	  <version>8.0</version>
	  <scope>provided</scope>
</dependency>
<dependency>
	  <groupId>javax</groupId>
	  <artifactId>javaee-web-api</artifactId>
	  <version>8.0</version>
	  <scope>provided</scope>
</dependency>
<dependency>
	  <groupId>javax.servlet</groupId>
	  <artifactId>jstl</artifactId>
	  <version>1.</version>
	  <scope>runtime</scope>
</dependency>

web.xml配置：

<welcome-file-list>  
	<welcome-file>/WEB-INF/view/jsp/upload/upload.jsp</welcome-file>
</welcome-file-list>
<!-- 声明Servlet对象 -->
<servlet>
	<!-- 指定Servlet对象的名称 -->
	<servlet-name>UploadServlet</servlet-name>
	<!-- 指定Servlet对象的完整位置，包含包名和类名 -->
	<servlet-class>pers.chaffee.servlet.UploadServlet</servlet-class>
</servlet>
<!-- 映射Servlet -->
<servlet-mapping>
	<!--<servlet-name>与上面<Servlet>标签的<servlet-name>元素相对应，不可以随便起名  -->
	<servlet-name>UploadServlet</servlet-name>
	<!-- 用于映射访问URL -->
	<url-pattern>/UploadFileServlet</url-pattern>
</servlet-mapping>

servlet实现：

@WebServlet("/UploadServlet")
public class UploadServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) {

        //根据文件扩展名设置文件MIME类型
        resp.setContentType(getServletContext().getMimeType("hello.txt"));

        //设置下载消息响应,提示文件保存attachment
        resp.setHeader("Content-Disposition", "attachment;filename=" + "hello");

        /*
         * 设置缓冲区
         * is.read(b)当文件读完时返回-1
         */
        try {
            //输入流为项目文件，输出流指向浏览器
            InputStream is = new FileInputStream("E:\\hello.txt");

            // 提供了将二进制数据写入响应的流
            ServletOutputStream os = resp.getOutputStream();

            int len = -1;
            byte[] b = new byte[1024];
            while ((len = is.read(b)) != -1) {
                os.write(b, 0, len);
            }
            //关闭流
            is.close();
            os.close();
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) {
        // 文件上传到本地磁盘工厂类
        DiskFileItemFactory factory = new DiskFileItemFactory();
        //设置内存缓冲区的大小
        factory.setSizeThreshold(20480);
        //设置临时文件目录,供上传文件过大时临时存储
        factory.setRepository(new File("F:\\f"));

        //文件上传操作核心类
        ServletFileUpload upload = new ServletFileUpload(factory);
        //设置限制单个上传文件的大小
        upload.setFileSizeMax(50480);
        //设置限制总上传文件大小
        upload.setSizeMax(80480);
        // 这个路径相对当前应用的目录
        try {
            List<FileItem> formItems = upload.parseRequest(req);
            if (formItems != null && formItems.size() > 0) {
                // 迭代表单数据
                for (FileItem item : formItems) {
                    // 处理不在表单中的字段
                    if (!item.isFormField()) {
                        File storeFile = new File("F:\\fromweb_" + item.getName());
                        // 保存文件到硬盘
                        item.write(storeFile);
                    }
                }
            }
        } catch (FileUploadException e) {
            e.printStackTrace();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

upload.jsp:

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<h1>文件上传实例</h1>
<!--请求编码类型必须为"multipart/form-data"-->
<form method="post" action="/UploadFileServlet" enctype="multipart/form-data">
    选择一个文件:
    <input type="file" name="uploadFile"/>
    <input type="file" name="uploadFile2"/>
    <br/><br/>
    <input type="submit" value="上传"/>
</form>
<a href="/UploadFileServlet" rel="nofollow">下载</a>
</body>
</html>

2. java web主流框架【ssm】实现：

pom.xml配置

<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-core</artifactId>
    <version>${spring.version}</version>
</dependency>
<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-webmvc</artifactId>
    <version>${spring.version}</version>
</dependency>
<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-web</artifactId>
    <version>${spring.version}</version>
</dependency>
<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-context</artifactId>
    <version>${spring.version}</version>
</dependency>
<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-context-support</artifactId>
    <version>${spring.version}</version>
</dependency>
<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-aop</artifactId>
    <version>${spring.version}</version>
</dependency>
<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-aspects</artifactId>
    <version>${spring.version}</version>
</dependency>

web.xml配置

<context-param>
    <param-name>contextConfigLocation</param-name>
    <!--<param-value>classpath*:applicationContext-*.xml</param-value>-->
    <param-value>classpath*:*.xml</param-value>
</context-param>
<listener>
    <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>
<listener>
    <listener-class>org.springframework.web.util.IntrospectorCleanupListener</listener-class>
</listener>
<servlet>
    <servlet-name>dispatcherServlet</servlet-name>
    <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
    <init-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>classpath:spring-mvc.xml</param-value>
    </init-param>
    <load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
    <servlet-name>dispatcherServlet</servlet-name>
    <url-pattern>/service/*</url-pattern>
</servlet-mapping>

spring-mvc.xml:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:p="http://www.springframework.org/schema/p"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:context="http://www.springframework.org/schema/context"
       xmlns:mvc="http://www.springframework.org/schema/mvc"
       xsi:schemaLocation="
        http://www.springframework.org/schema/beans  
        http://www.springframework.org/schema/beans/spring-beans-3.2.xsd  
        http://www.springframework.org/schema/context  
        http://www.springframework.org/schema/context/spring-context-3.2.xsd  
        http://www.springframework.org/schema/mvc  
        http://www.springframework.org/schema/mvc/spring-mvc-3.2.xsd">

    <!-- 启动SpringMVC的注解功能 -->
    <mvc:annotation-driven/>

    <!-- 扫描controller（controller层注入） -->
    <context:component-scan base-package="pers.chaffee.controller"/>

    <!-- 对模型视图添加前后缀 -->
    <bean id="viewResolver"
          class="org.springframework.web.servlet.view.InternalResourceViewResolver"
          p:prefix="/WEB-INF/view/jsp/" p:suffix=".jsp"/>

    <!-- 静态资源不走controller -->
    <mvc:resources mapping="/resources/**" location="/resources/"/>
</beans>

servlet实现:

	@RequestMapping(value = "/upload2", method = RequestMethod.POST, consumes = {"multipart/form-data"})
	public ModelAndView postUpload(HttpServletRequest request, HttpServletResponse response) {
		
		//文件上传核心类
		CommonsMultipartResolver multipartResolver = new CommonsMultipartResolver(request.getSession().getServletContext());
		
		//判断request是否有文件上传
		if (multipartResolver.isMultipart(request)) {
		    //通过MultipartHttpServletRequest解析上传请求中的文件
		    MultipartHttpServletRequest multiRequest = (MultipartHttpServletRequest) request;
		    //获取上传请求中的所有文件
		    Iterator<String> iter = multiRequest.getFileNames();
		    while (iter.hasNext()) {
		        //转换成spring支持的文件类型MultipartFile
		        MultipartFile file = multiRequest.getFile(iter.next());
		        if (file != null) {
		            File localFile = new File("F:\\f\\" + file.getOriginalFilename());
		            try {
		                //将上传文件写到指定位置,此处是本地文件夹
		                file.transferTo(localFile);
		            } catch (IllegalStateException e) {
		                e.printStackTrace();
		            } catch (IOException e) {
		                e.printStackTrace();
		            }
		        }
		    }
		}
		
		ModelAndView modelAndView = new ModelAndView();
		modelAndView.setViewName("upload/upload2");
		return modelAndView;
		}
	
	@RequestMapping(value = "/download", method = RequestMethod.GET)
	public ResponseEntity<byte[]> downLoad() throws IOException {
		byte[] responseBody;
		//获取文件
		File file = new File("E:\\hello.txt");
		InputStream is = new FileInputStream(file);
		responseBody = new byte[is.available()];
		is.read(responseBody);
		HttpHeaders headers = new HttpHeaders();
		
		//设置文件类型
		headers.add("Content-Disposition", "attachment;filename=" + file.getName());
		
		//设置Http状态码
		HttpStatus stateCode = HttpStatus.OK;
		
		//返回数据
		ResponseEntity<byte[]> entity = new ResponseEntity<>(responseBody, headers, stateCode);
		return entity;
	}

小结

截至目前，只初步完成B/S方式的上传和下载，且文件存储方式仅限本地磁盘。而C/S方式的文件传输可以考虑thrift框架，thrift是一种跨平台跨语言的RPC框架，使用过程中发现在数据量不是很大的情况下thrift应用是一种比较合适的C/S解决方案；另外，文件存储方式可以根据具体情况选用文件服务器、数据库等。

功能介绍

在页面中选择好相应的上传目录，点击粘贴上传按钮，数据即可快速开始上传

说明: http://bbsres2.ncmem.com/731fda07.png