1.文件上传的条件

 1）表单的提交方式必须是POST方式。（才有content-type属性）
 2）有文件上传表单，表单中有<input type="file"/>的选择文件的标签
 3）把表单设置为enctype="multipart/form-data"，提交的数据不再是key-value对，而是字节数据
# jsp中的代码
 <form action="${pageContext.request.contextPath }/UploadDemo1" method="post" enctype="multipart/form-data">
 请选择文件： <input type="file" name="img"/><br/>
 <input type="submit" value="上传" />
 </form>

2.自动书写文件上传的代码

/**
 * 手动处理上传文件的逻辑
 * @author APPle
 *
 */
public class UploadDemo1 extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        //得到实体内容数据
        InputStream in = request.getInputStream();
        BufferedReader br = new BufferedReader(new InputStreamReader(in));
        //读取文件的开始符
        String startTag = br.readLine();
        
        //读取文件名： Content-Disposition: form-data; name="img"; filename="news.txt" 
        String line = br.readLine();
        String fileName = line.substring(line.lastIndexOf("filename=\"")+10, line.lastIndexOf("\"") );
        System.out.println("文件名："+fileName);
        
        //跳过2行
        br.readLine();
        br.readLine();
        
        //读取文件的实际内容
        String str = null;
        BufferedWriter bw = new BufferedWriter(new FileWriter("E:/files/"+fileName));
        while((str=br.readLine())!=null){
            //读到文件结束符时退出循环
            if((startTag+"--").equals(str)){
                break;
            }
            
            //把内容写出文件中
            bw.write(str);
            bw.newLine();
            bw.flush();
        }
        //关闭
        bw.close();
        br.close();
    }
    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        doGet(request, response);
    }
}