使用commons-fileupload上传文件

import java.io.*;
import java.util.*;
import javax.servlet.ServletException;
import javax.servlet.http.*
import org.apache.commons.fileupload.FileItem;
import org.apache.commons.fileupload.FileUploadException;
import org.apache.commons.fileupload.FileUploadBase.SizeLimitExceededException;
import org.apache.commons.fileupload.disk.DiskFileItemFactory;
import org.apache.commons.fileupload.servlet.ServletFileUpload;

public class FileUploadServlet extends HttpServlet {
  public void doGet(HttpServletRequest request, HttpServletResponse response)
      throws ServletException, IOException {
    doPost(request, response);
  }

  public void doPost(HttpServletRequest request, HttpServletResponse response)
      throws ServletException, IOException {
    final long MAX_SIZE = 3 * 1024 * 1024;// 设置上传文件最大为 3M
    // 允许上传的文件格式的列表
    final String[] allowedExt = new String[] { "jpg", "jpeg", "gif", "txt",
        "doc", "docx", "mp3", "wma", "m4a" };
    response.setContentType("text/html");
    // 设置字符编码为UTF-8, 这样支持汉字显示
    response.setCharacterEncoding("UTF-8");
    
    
    // 实例化一个硬盘文件工厂,用来配置上传组件ServletFileUpload
    DiskFileItemFactory dfif = new DiskFileItemFactory();
    // 设置上传文件时用于临时存放文件的内存大小,这里是4K.多于的部分将临时存在硬盘
    dfif.setSizeThreshold(4096);
    // 设置存放临时文件的目录,web根目录下的ImagesUploadTemp目录
    dfif.setRepository(new File(request.getSession().getServletContext()
      .getRealPath("/") + "ImagesUploadTemp"));
    
    // 用以上工厂实例化上传组件
    ServletFileUpload sfu = new ServletFileUpload(dfif);
    // 设置最大上传尺寸
    sfu.setSizeMax(MAX_SIZE);
    PrintWriter out = response.getWriter();
    // 从request得到 所有 上传域的列表
    List fileList = null;
    try {
      fileList = sfu.parseRequest(request);
    } catch (FileUploadException e) {// 处理文件尺寸过大异常
      if (e instanceof SizeLimitExceededException) {
        out.println("文件尺寸超过规定大小:" + MAX_SIZE + "字节<p />");
        out.println("<a href=\"upload.html\" target=\"_top\">返回</a>");
        return;
      }
      e.printStackTrace();
    }
    // 没有文件上传
    if (fileList == null || fileList.size() == 0) {
      out.println("请选择上传文件<p />");
      out.println("<a href=\"upload.html\" target=\"_top\">返回</a>");
      return;
    }
    
    // 得到所有上传的文件
    Iterator it = fileList.iterator();
    // 循环处理所有文件
    while (it.hasNext()) {
      FileItem fileItem = null;
      String path = null;
      long size = 0;
      // 得到当前文件
      fileItem = (FileItem) it.next();
      // 忽略简单form字段而不是上传域的文件域(<input type="text" />等)
      if (fileItem == null || fileItem.isFormField()) {
        continue;
      }
      // 得到文件的完整路径
      path = fileItem.getName();
      // 得到文件的大小
      size = fileItem.getSize();
      if ("".equals(path) || size == 0) {
        out.println("请选择上传文件<p />");
        out.println("<a href=\"upload.html\" target=\"_top\">返回</a>");
        return;
      }
      // 得到去除路径的文件名
      String t_name = path.substring(path.lastIndexOf("\\") + 1);
      // 得到文件的扩展名(无扩展名时将得到全名)
      String t_ext = t_name.substring(t_name.lastIndexOf(".") + 1);
      // 拒绝接受规定文件格式之外的文件类型
      int allowFlag = 0;
      int allowedExtCount = allowedExt.length;
      for (; allowFlag < allowedExtCount; allowFlag++) {
        if (allowedExt[allowFlag].equals(t_ext))
          break;
      }
      if (allowFlag == allowedExtCount) {
        out.println("请上传以下类型的文件<p />");
        for (allowFlag = 0; allowFlag < allowedExtCount; allowFlag++)
          out.println("*." + allowedExt[allowFlag] + "&nbsp;&nbsp;&nbsp;");
        out.println("<p /><a href=\"upload.html\" target=\"_top\">返回</a>");
        return;
      }
      
      long now = System.currentTimeMillis();
      // 根据系统时间生成上传后保存的文件名
      String prefix = String.valueOf(now);
      // 保存的最终文件完整路径,保存在web根目录下的ImagesUploaded目录下
      String u_name = request.getSession().getServletContext()
        .getRealPath("/") + "ImagesUploaded/" + prefix + "." + t_ext;
      try {
        // 保存文件
        fileItem.write(new File(u_name));
        out.println("文件上传成功. 已保存为: " + prefix + "." + t_ext
            + " &nbsp;&nbsp;文件大小: " + size + "字节<p />");
        out.println("<a href=\"upload.html\" target=\"_top\">继续上传</a>");
      } catch (Exception e) {
        e.printStackTrace();
      }
    }
  }
}

页面

<html>
  <head>
  <meta http-equiv="Content-Type" content="text/html;charset=utf-8">
    <title>Untitled</title>
  </head>

  <body>
    <form action="FileUploadServlet" method="post"
      enctype="multipart/form-data">
      <input type="text" value="xxx"/> <br />
      <input type="file" size="30" name="test" /><br />
      <input name="up" type="submit" value="上传" />
    </form>
  </body>
</html>

web.xml文件中的配置

  <servlet>
  <servlet-name>FileUploadServlet</servlet-name>
  <servlet-class>FileUploadServlet</servlet-class>
  </servlet>

  <servlet-mapping>
  <servlet-name>FileUploadServlet</servlet-name>
  <url-pattern>/FileUploadServlet</url-pattern>
  </servlet-mapping>