jsp 附件下载的实现
一、实现功能：页面上显示附件链接，点击链接，弹出“文件下载”框，打开或保存文件。
（ps:最简单常见的功能，却有很多地方需要注意）
二、过程：采用 servlet1 提供“附件列表”数据，在页面显示所有附件链接，点击链接经过servlet2处理，弹出“文件下载”框，打开或保存文件。
1、在index页面，通过点击链接向GotoAttachServlet发出请求。
2、GotoAttachServlet 提供附件列表，转向download.jsp 页面。
3、在download.jsp 点击某个附件链接，通过DownloadServlet处理，弹出“文件下载”框，打开或保存该文件。
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注:为了方便，数据没有从数据库中取。整个过程是完整的，其实只要关注DownloadServlet.java中相关代码即可。
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三、实现。
1、index中的相关代码。
<a href="GotoAttachServlet">附件页面</a>
?2、GotoAttachServlet中相关代码。
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {List<Attachment>attachList = new ArrayList<Attachment>();Attachment attach1=new Attachment(1,"flower.png","d://flower.jpg");attachList.add(attach1);Attachment attach2=new Attachment(2,"tree.txt","d://tree.txt");attachList.add(attach2);request.setAttribute("attachList", attachList);request.getRequestDispatcher("download.jsp").forward(request, response);}?3、download.jsp 代码。
<%String path = request.getContextPath();String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";%><%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%><%@taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%><%@taglib uri="http://java.sun.com/jsp/jstl/fmt" prefix="fmt" %><!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"><html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><title>下载</title><script src="<%=basePath%>js/jquery-1.4.4.js"></script><script type="text/javascript">function download(attachId){var url="/MyWeb/DownloadServlet?attachId="+attachId;$('#attachmentForm').attr('action',url).submit();}</script></head><body><div>附件下载</div><form id="attachmentForm" method="post"><table><tr><td>附件：</td><td><ul><c:forEach items="${attachList}" var="attach"><li><a href="#" onclick="download('${attach.id}')">${attach.name}</a></li></c:forEach></ul></td></tr></table></form></body></html>??4、DownLoadServlet.java中相关代码。
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protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {String idStr=request.getParameter("attachId");Attachment attachment=null;if(idStr.equals("1")){attachment=new Attachment(1,"flower.png","d://flower.jpg");}else{attachment=new Attachment(2,"tree.txt","d://tree.txt");}String downFilePath=attachment.getPath();String fileName = attachment.getName();fileName = URLEncoder.encode(fileName, "UTF-8");// 设置响应信息response.reset();response.setContentType("application/octet-stream");response.addHeader("Content-Disposition", "attachment;filename="+ fileName);response.setCharacterEncoding("utf-8");// 得到流InputStream inputStream = new FileInputStream(downFilePath);ServletOutputStream servletOutputStream=response.getOutputStream();final int size = 1024;byte[] buffer = new byte[size];int length;while ((length = inputStream.read(buffer)) > 0) {servletOutputStream.write(buffer, 0, length);}servletOutputStream.flush();servletOutputStream.close();} }?5、你可能看不明白的地方：
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1、response.setContentType("application/octet-stream");??? 参看：http://wuyechun.iteye.com/blog/1132423?
2、response.addHeader("Content-Disposition", "attachment;filename="?+ fileName);
在使用来自 Web 服务器的文档时，您可能希望立即提示用户将文件直接保存在用户磁盘上，而不要在浏览器中打开该文件。不过，对于已知的 MIME（多用途 Internet 邮件扩展）类型，如 Microsoft Word ("application/ms-word")，默认行为是在 Internet Explorer 中打开文档。
可以使用 Content-disposition 头来覆盖此默认行为。其格式是：
Content-disposition: attachment; filename=fname.ext
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