android 4.0 HttpURLConnection. getInputStream() 为空的有关问题[转载]

android 4.0 HttpURLConnection. getInputStream() 为空的问题[转载]
Java代码 

public String downTxt(String urlStr) {          StringBuffer sBuffer = new StringBuffer();          String line = null;          BufferedReader bReader = null;          try {              URL url = new URL(urlStr);              HttpURLConnection conn = (HttpURLConnection) url.openConnection();              bReader = new BufferedReader(new InputStreamReader(                      conn.getInputStream()));              while ((line = bReader.readLine()) != null) {                  sBuffer.append(line);              }            } catch (MalformedURLException e) {              e.printStackTrace();          } catch (IOException e) {              e.printStackTrace();          } finally {              try {                  bReader.close();              } catch (IOException e) {                  e.printStackTrace();              }          }            return sBuffer.toString();      }  

走到
Java代码 

bReader = new BufferedReader(new InputStreamReader(                      conn.getInputStream())); 

 
就会报空指针错误。为什么呢 ？
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<uses-sdk android:minSdkVersion="14"/> 改为 7 就能正常运行了。。。