兩點要素：
1.表單提交方式必須是post
2.enctype要指定為multipart/form-data。

enctype的預設值是application/x-www-form-urlencoded，
這種編碼格式只支援鍵值對的資料，且只能是字串，無法傳輸檔案。

需要的jar包：

實現：

import java.io.BufferedInputStream;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.PrintWriter;
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.FileItem;
import org.apache.commons.fileupload.FileUploadException;
import
 
 org.apache.commons.fileupload.disk.DiskFileItemFactory;
import org.apache.commons.fileupload.servlet.ServletFileUpload;

public class UploadServlet extends HttpServlet {

    //資料夾路徑常量
    private static String TEMP_PATH = "E:/Learning/temp/170926";

    private static final long serialVersionUID = 3364746345428715093
 
L;

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

    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();

        // 判斷內容是否包含檔案
        boolean isMultipart = ServletFileUpload.isMultipartContent(request);

        // 判斷是否是檔案上傳請求
        if (!isMultipart) {
            out.println("Error : 表單必須包含 enctype=multipart/form-data");
            return;
        }

        // 用於建立解析檔案上傳的工廠類
        DiskFileItemFactory factory = new DiskFileItemFactory();

        // 用於從請求中解析出檔案
        ServletFileUpload upload = new ServletFileUpload(factory);

        try {

            // 獲取所有提交過來的檔案(包含表單內容) 這個FileItem叫做一個項，這個項有可能是檔案也有可能是表單引數 
            List<FileItem> items = upload.parseRequest(request);

            // 定義一個輸出流
            FileOutputStream fos = null;

            //緩衝區
            byte [] buf = new byte[8192];

            if (items != null && items.size() > 0) { // 不為空且長度大於0則遍歷之
                for (FileItem item : items) {
                    if (item.isFormField()) { // isFormField = true 文字框的值
                        out.println("使用者名稱: " + item.getString() + "<br />");
                    } else {
                        //獲取輸入流
                        BufferedInputStream bis = new BufferedInputStream(item.getInputStream());

                        //初始化本地輸出流
                        fos = new FileOutputStream(new File(TEMP_PATH + "/" + item.getName()));

                        int len = -1;
                        while ((len = bis.read(buf)) != -1) {
                            fos.write(buf , 0 , len);
                            fos.flush();
                        }
                        out.println("檔案: " + item.getName() + "已上傳 <br />");
                    }
                }
            }
        } catch (FileUploadException e) {
            out.println("Error: 檔案解析異常");
        }

    }

}