Web文件上传采用POST的方式，与POST提交表单不同的是，上传文件需要设置FORM的enctype属性为multipart/form-data.由于上传的文件会比较大，因此需要设置该参数指定浏览器使用二进制上传。如果不设置，enctype属性默认为application/x-www-form-urlencoded，使用浏览器将使用ASCII向服务器发送数据，导致发送文件失败。
上传文件要使用文件域（<input type='file'/>，并把FORM的Enctype设置为multipart/form-data.
客户端上传页面如图所示：
代码如下：
upload.html
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html><head><title>上传文件</title><meta http-equiv="keywords" content="keyword1,keyword2,keyword3"><meta http-equiv="description" content="this is my page"><meta http-equiv="content-type" content="text/html; charset=UTF-8"><link rel="stylesheet" type="text/css" href="css/style.css"></head><body><form action="servlet/UploadServlet" method="post" enctype="multipart/form-data"><div align="center"><br/> <fieldset> <legend>上传文件</legend><br/> <div class='line'> <div align='left'>上传文件一</div> <div align='left'> <input type="file" name="file1"> </div> </div> <div class='line'> <div align='left'>上传文件二</div> <div align='left'> <input type="file" name="file2"> </div> </div> <div class='line'> <div align='left'>上传文件说明一</div> <div align='left'><input type="text" name="description1"></div> </div> <div class='line'> <div align='left'>上传文件说明二</div> <div align='left'><input type="text" name="description2"></div> </div> <div class='line'> <div align='left'></div> <div align='left'><br/> <input type="submit" value=" 上传文件 "> </div> </div> </fieldset></div></form></body></html>
客户端运行的代码很简单，服务器要复杂一点。要想获取其中的内容，必须根据HTTP协议所规定的格式来解析浏览器提交的Request.
解析二进制流比较麻烦。已经有许多类库已经完成了这项工作，例如SmartUpload与Apache Commons Fileupload.SmartUpload是一个商业类库，解析Request过程中数据存放在内存里，因此速度较快，但上传较大文件时会发生内存溢出。Apache Commons Fileupload是一个免费的开源的类库。一些框架比如Struts里集成了Apache Common Fileupload类库来实现文件上传。
如图所示：
代码如下：
package com.helloweenvsfei.servlet;import java.io.File;import java.io.FileOutputStream;import java.io.IOException;import java.io.InputStream;import java.io.OutputStream;import java.io.PrintWriter;import java.net.URLEncoder;import java.util.List;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import org.apache.commons.fileupload.DiskFileUpload;import org.apache.commons.fileupload.FileItem;import org.apache.commons.fileupload.FileUploadException;public class UploadServlet extends HttpServlet { private static final long serialVersionUID = 7523024737218332088L; public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setCharacterEncoding("UTF-8"); response.getWriter().println("请以 POST 方式上传文件"); } @SuppressWarnings("unchecked") public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { File file1 = null, file2 = null; String description1 = null, description2 = null; response.setCharacterEncoding("UTF-8"); response.setContentType("text/html"); PrintWriter out = response.getWriter(); out.println("<!DOCTYPE HTML PUBLIC /"-//W3C//DTD HTML 4.01 Transitional//EN/">"); out.println("<HTML>"); out.println(" <HEAD><TITLE>A Servlet</TITLE></HEAD>"); out.println(" <link rel='stylesheet' type='text/css' href='../css/style.css'>"); out.println(" <BODY>"); out.println("<div align=center><br/>"); out.println("<fieldset style='width:90%'><legend>上传文件</legend><br/>"); out.println(" <div class='line'>"); out.println(" <div align='left' class='leftDiv'>上传日志：</div>"); out.println(" <div align='left' class='rightDiv'>"); // 使用 DiskFileUpload 对象解析 request DiskFileUpload diskFileUpload = new DiskFileUpload(); try { // 将 解析的结果 放置在 List 中 List<FileItem> list = diskFileUpload.parseRequest(request); out.println("遍历所有的 FileItem ... <br/>"); // 遍历 list 中所有的 FileItem for(FileItem fileItem : list){ if(fileItem.isFormField()){ // 如果是 文本域 if("description1".equals(fileItem.getFieldName())){ // 如果该 FileItem 名称为 description1 out.println("遍历到 description1 ... <br/>"); description1 = new String(fileItem.getString().getBytes(), "UTF-8"); } if("description2".equals(fileItem.getFieldName())){ // 如果该 FileItem 名称为 description2 out.println("遍历到 description2 ... <br/>"); description2 = new String(fileItem.getString().getBytes(), "UTF-8"); } } else{ // 否则，为文件域 if("file1".equals(fileItem.getFieldName())){ // 客户端文件路径构建的 File 对象 File remoteFile = new File(new String(fileItem.getName().getBytes(), "UTF-8")); out.println("遍历到 file1 ... <br/>"); out.println("客户端文件位置: " + remoteFile.getAbsolutePath() + "<br/>"); // 服务器端文件，放在 upload 文件夹下 file1 = new File(this.getServletContext().getRealPath("attachment"), remoteFile.getName()); file1.getParentFile().mkdirs(); file1.createNewFile(); // 写文件，将 FileItem 的文件内容写到文件中 InputStream ins = fileItem.getInputStream(); OutputStream ous = new FileOutputStream(file1); try{ byte[] buffer = new byte[1024]; int len = 0; while((len=ins.read(buffer)) > -1) ous.write(buffer, 0, len); out.println("已保存文件" + file1.getAbsolutePath() + "<br/>"); }finally{ ous.close(); ins.close(); } } if("file2".equals(fileItem.getFieldName())){ // 客户端文件路径构建的 File 对象 File remoteFile = new File(new String(fileItem.getName().getBytes(), "UTF-8")); out.println("遍历到 file2 ... <br/>"); out.println("客户端文件位置: " + remoteFile.getAbsolutePath() + "<br/>"); // 服务器端文件，放在 upload 文件夹下 file2 = new File(this.getServletContext().getRealPath("attachment"), remoteFile.getName()); file2.getParentFile().mkdirs(); file2.createNewFile(); // 写文件，将 FileItem 的文件内容写到文件中 InputStream ins = fileItem.getInputStream(); OutputStream ous = new FileOutputStream(file2); try{ byte[] buffer = new byte[1024]; int len = 0; while((len=ins.read(buffer)) > -1) ous.write(buffer, 0, len); out.println("已保存文件" + file2.getAbsolutePath() + "<br/>"); }finally{ ous.close(); ins.close(); } } } } out.println("Request 解析完毕"); } catch (FileUploadException e) { // TODO Auto-generated catch block e.printStackTrace(); } out.println(" </div>"); out.println(" </div>"); if(file1 != null){ out.println(" <div class='line'>"); out.println(" <div align='left' class='leftDiv'>file1：</div>"); out.println(" <div align='left' class='rightDiv'>"); out.println(" <a href='" + request.getContextPath() + "/attachment/" + file1.getName() + "' target=_blank>" + file1.getName() + "</a>" ); out.println(" </div>"); out.println(" </div>"); } if(file2 != null){ out.println(" <div class='line'>"); out.println(" <div align='left' class='leftDiv'>file2：</div>"); out.println(" <div align='left' class='rightDiv'>"); out.println(" <a href='" + request.getContextPath() + "/attachment/" + URLEncoder.encode(file2.getName(), "UTF-8") + "' target=_blank>" + file2.getName() + "</a>" ); out.println(" </div>"); out.println(" </div>"); } out.println(" <div class='line'>"); out.println(" <div align='left' class='leftDiv'>description1：</div>"); out.println(" <div align='left' class='rightDiv'>"); out.println(description1); out.println(" </div>"); out.println(" </div>"); out.println(" <div class='line'>"); out.println(" <div align='left' class='leftDiv'>description2：</div>"); out.println(" <div align='left' class='rightDiv'>"); out.println(description2); out.println(" </div>"); out.println(" </div>"); out.println("</fieldset></div>"); out.println(" </BODY>"); out.println("</HTML>"); out.flush(); out.close(); }}程序运行效果如图所示：
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