Web文件上傳採用POST的方式，與POST提交表單不同的是，上傳文件需要設置FORM的enctype屬性為multipart/form-data.由於上傳的文件會比較大，因此需要設置該參數指定瀏覽器使用二進制上傳。如果不設置，enctype屬性默認為application/x-www-form-urlencoded，使用瀏覽器將使用ASCII向服務器發送數據，導致發送文件失敗。
上傳文件要使用文件域（<input type='file'/>，並把FORM的Enctype設置為multipart/form-data.
客戶端上傳頁面如圖所示：
代碼如下：
upload.html
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html><head><title>上傳文件</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>上傳文件</legend><br/> <div class='line'> <div align='left'>上傳文件一</div> <div align='left'> <input type="file" name="file1"> </div> </div> <div class='line'> <div align='left'>上傳文件二</div> <div align='left'> <input type="file" name="file2"> </div> </div> <div class='line'> <div align='left'>上傳文件說明一</div> <div align='left'><input type="text" name="description1"></div> </div> <div class='line'> <div align='left'>上傳文件說明二</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=" 上傳文件"> </div> </div> </fieldset></div></form></body></html>
客戶端運行的代碼很簡單，服務器要復雜一點。要想獲取其中的內容，必鬚根據HTTP協議所規定的格式來解析瀏覽器提交的Request.
解析二進制流比較麻煩。已經有許多類庫已經完成了這項工作，例如SmartUpload與Apache Commons Fileupload.SmartUpload是一個商業類庫，解析Request過程中數據存放在內存裡，因此速度較快，但上傳較大文件時會發生內存溢出。 Apache Commons Fileupload是一個免費的開源的類庫。一些框架比如Struts裡集成了Apache Common Fileupload類庫來實現文件上傳。
如圖所示：
代碼如下：
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("請以POST 方式上傳文件"); } @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>上傳文件</legend><br/>"); out.println(" <div class='line'>"); out.println(" <div align='left' class='leftDiv'>上傳日誌：</div>"); out.println(" <div align='left' class='rightDiv'>"); // 使用DiskFileUpload 對象解析request DiskFileUpload diskFileUpload = new DiskFileUpload(); try { // 將解析的結果放置在List 中List<FileItem> list = diskFileUpload.parseRequest(request); out.println("遍歷所有的FileItem ... <br/>"); // 遍歷list 中所有的FileItem for(FileItem fileItem : list){ if(fileItem.isFormField()){ // 如果是文本域if("description1".equals(fileItem.getFieldName())){ // 如果該FileItem 名稱為description1 out.println("遍歷到description1 ... <br/>"); description1 = new String(fileItem.getString().getBytes(), "UTF-8"); } if("description2".equals(fileItem.getFieldName())){ // 如果該FileItem 名稱為description2 out.println("遍歷到description2 ... <br/>"); description2 = new String(fileItem.getString().getBytes(), "UTF-8"); } } else{ // 否則，為文件域if("file1".equals(fileItem.getFieldName())){ // 客戶端文件路徑構建的File 對象File remoteFile = new File(new String(fileItem.getName().getBytes(), "UTF-8")); out.println("遍歷到file1 ... <br/>"); out.println("客戶端文件位置: " + remoteFile.getAbsolutePath() + "<br/>"); // 服務器端文件，放在upload 文件夾下file1 = new File(this.getServletContext().getRealPath("attachment"), remoteFile.getName()); file1.getParentFile().mkdirs(); file1.createNewFile(); // 寫文件，將FileItem 的文件內容寫到文件中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())){ // 客戶端文件路徑構建的File 對象File remoteFile = new File(new String(fileItem.getName().getBytes(), "UTF-8")); out.println("遍歷到file2 ... <br/>"); out.println("客戶端文件位置: " + remoteFile.getAbsolutePath() + "<br/>"); // 服務器端文件，放在upload 文件夾下file2 = new File(this.getServletContext().getRealPath("attachment"), remoteFile.getName()); file2.getParentFile().mkdirs(); file2.createNewFile(); // 寫文件，將FileItem 的文件內容寫到文件中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 解析完畢"); } 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(); }}程序運行效果如圖所示：
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