文件上传在Web应用中非常普遍,要在Java Web环境中实现文件上传功能非常容易,因为网上已经有许多用Java开发的组件用于文件上传,本文以使用最普遍的commons-fileupload组件为例,演示如何为Java Web应用添加文件上传功能。
commons-fileupload组件是Apache的一个开源项目之一,可以从http://commons.apache.org/fileupload/下载。该组件简单易用,可实现一次上传一个或多个文件,并可限制文件大小。
下载后解压zip包,将commons-fileupload-1.x.jar复制到tomcat的webapps/你的webapp/WEB-INF/lib/下,如果目录不存在请自建目录。
新建一个UploadServlet.java用于文件上传:
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package com.liaoxuefeng.web; public class FileUploadServlet extends HttpServlet { private String uploadDir = "C:\\temp" ; @Override protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { // TODO: } } |
当servlet收到浏览器发出的Post请求后,在doPost()方法中实现文件上传,我们需要遍历FileItemIterator,获得每一个FileItemStream:
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@Override protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { try { ServletFileUpload upload = new ServletFileUpload(); // set max file size to 1 MB: upload.setFileSizeMax( 1024 * 1024 ); FileItemIterator it = upload.getItemIterator(req); // handle with each file: while (it.hasNext()) { FileItemStream item = it.next(); if (! item.isFormField()) { // it is a file upload: handleFileItem(item); } } req.getRequestDispatcher( "success.jsp" ).forward(req, resp); } catch (FileUploadException e) { throw new ServletException( "Cannot upload file." , e); } } |
在handleFileItem()方法中读取上传文件的输入流,然后写入到uploadDir中,文件名通过UUID随机生成:
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void handleFileItem(FileItemStream item) throws IOException { System.out.println( "upload file: " + item.getName()); File newUploadFile = new File(uploadDir + "/" + UUID.randomUUID().toString()); byte [] buffer = new byte [ 4096 ]; InputStream input = null ; OutputStream output = null ; try { input = item.openStream(); output = new BufferedOutputStream( new FileOutputStream(newUploadFile)); for (;;) { int n = input.read(buffer); if (n==(- 1 )) break ; output.write(buffer, 0 , n); } } finally { if (input!= null ) { try { input.close(); } catch (IOException e) {} } if (output!= null ) { try { output.close(); } catch (IOException e) {} } } } |
如果要在web.xml配置文件中读取指定的上传文件夹,可以在init()方法中初始化:
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@Override public void init(ServletConfig config) throws ServletException { super .init(config); this .uploaddir = config.getInitParameter( "dir" ); } |
最后在web.xml中配置Servlet:
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<? xml version = "1.0" encoding = "UTF-8" ?> <!DOCTYPE web-app PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN" "http://java.sun.com/dtd/web-app_2_3.dtd"> < web-app > < servlet > < servlet-name >UploadServlet</ servlet-name > < servlet-class >com.liaoxuefeng.web.FileUploadServlet</ servlet-class > </ servlet > < servlet-mapping > < servlet-name >UploadServlet</ servlet-name > < url-pattern >/upload</ url-pattern > </ servlet-mapping > </ web-app > |
配置好Servlet后,启动Tomcat或Resin,写一个简单的index.htm测试:
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< html > < body > < p >FileUploadServlet Demo</ p > < form name = "form1" action = "upload" method = "post" enctype = "multipart/form-data" > < input type = "file" name = "file" /> < input type = "submit" name = "button" value = "Submit" /> </ form > </ body > </ html > |
注意action="upload"指定了处理上传文件的FileUploadServlet的映射URL。
当上传成功后,显示success.jsp,否则,抛出异常。如果上传的文件大小超过了我们设定的1MB,就会得到一个FileSizeLimitExceededException。
以上就是本文的全部内容,希望对大家的学习有所帮助,也希望大家多多支持服务器之家。