1.文件上傳的條件

 1）表單的提交方式必須是POST方式。（才有content-type屬性）
 2）有文件上傳表單，表單中有<input type="file"/>的選擇文件的標簽
 3）把表單設置為enctype="multipart/form-data"，提交的數據不再是key-value對，而是字節數據
# jsp中的代碼
 <form action="${pageContext.request.contextPath }/UploadDemo1" method="post" enctype="multipart/form-data">
 請選擇文件： <input type="file" name="img"/><br/>
 <input type="submit" value="上傳" />
 </form>

2.自動書寫文件上傳的代碼

/**
 * 手動處理上傳文件的邏輯
 * @author APPle
 *
 */
public class UploadDemo1 extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        //得到實體內容數據
        InputStream in = request.getInputStream();
        BufferedReader br = new BufferedReader(new InputStreamReader(in));
        //讀取文件的開始符
        String startTag = br.readLine();
        
        //讀取文件名： 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);
        
        //跳過2行
        br.readLine();
        br.readLine();
        
        //讀取文件的實際內容
        String str = null;
        BufferedWriter bw = new BufferedWriter(new FileWriter("E:/files/"+fileName));
        while((str=br.readLine())!=null){
            //讀到文件結束符時退出循環
            if((startTag+"--").equals(str)){
                break;
            }
            
            //把內容寫出文件中
            bw.write(str);
            bw.newLine();
            bw.flush();
        }
        //關閉
        bw.close();
        br.close();
    }
    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        doGet(request, response);
    }
}