主线程中一定不能放耗时操作，必须要开子线程，比如下载文件，不然会不让你拿到输入流--报错显示android.os.NetworkOnMainThreadException

new Thread(new Runnable() {

@Override
public void run() {
// TODO Auto-generated method stub
try {
updateFile = Environment.getExternalStorageDirectory()
+ "/3530.jpg";
//downloadUpdateFile("http://image.anzimall.com/3530/3530_K21D_app_V2.3_2015050528.bin",  updateFile);
downloadUpdateFile("http://img1.cache.netease.com/catchpic/F/FC/FCC085159B92C5EE4FDDB9618166051E.jpg",  updateFile);
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();  //android.os.NetworkOnMainThreadException
System.out.println(e.toString()+"");
}
}
}).start();

public static long downloadUpdateFile(String down_url, String file)
throws Exception {

int downloadCount = 0;// 已经下载好的大小

InputStream inputStream;
OutputStream outputStream;
URL url = new URL(down_url);
HttpURLConnection httpurlconnection = (HttpURLConnection) url
.openConnection();
httpurlconnection.setConnectTimeout(5000);
httpurlconnection.setReadTimeout(5000);
// 获取下载文件的size
//totalSize = httpURLConnection.getContentLength();

if (httpurlconnection.getResponseCode() == 404) {
throw new Exception("fail!");
// 这个地方应该加一个下载失败的处理，但是，因为我们在外面加了一个try---catch，已经处理了Exception,
// 所以不用处理
}

inputStream = httpurlconnection.getInputStream();
File file_ok = new File(file);
outputStream = new FileOutputStream(file_ok);// 文件存在则覆盖掉

byte buffer[] = new byte[1024];
int readsize = 0;

while ((readsize = inputStream.read(buffer)) != -1) {
outputStream.write(buffer, 0, readsize);
downloadCount += readsize;// 时时获取下载到的大小
}
if (httpurlconnection != null) {
httpurlconnection.disconnect();
}
inputStream.close();
outputStream.close();

return downloadCount;
}