Android用HTTP下载报错“android.os.StrictMode$AndroidBlockGuardPolicy.onNetwork”

此错误是因为Android不能在主线程中访问网络导致，可将访问网络代码另启线程运行：

Runnable r = new Runnable() {
	@Override
	public void run() {
		// TODO Auto-generated method stub
		try {
			URL url = new URL(strURL);
			HttpURLConnection conn = (HttpURLConnection)url.openConnection();
			
			String path = "file";
			String fileName = fileNa + "." + fileEx;
			OutputStream output = null;
			/*获取SD卡路径*/
			String SDCard = "/mnt/sdcard/HTTPget"; //Environment.getExternalStorageDirectory()+"";
			System.out.println(SDCard);
			String pathName = SDCard + "/" + path + "/" + fileName; //文件保存路径
			
			File file = new File(pathName);
			InputStream input = conn.getInputStream();
			if(file.exists())
			{
				System.out.println("file exits");
				return;
			}
			else
			{	
				String dir = SDCard + "/" + path;
				new File(dir).mkdirs(); //创建文件夹
				file.createNewFile();  //创建文件
				output = new FileOutputStream(file);
				
				byte[] buffer = new byte[4096];
				int len;
				while((len = input.read(buffer)) != -1)
				{
					output.write(buffer, 0, len);
				}
				output.flush();
				
			}
		} catch (MalformedURLException e) {
			// TODO: handle exception
			e.printStackTrace();
		} catch (IOException e) {
			// TODO: handle exception
				e.printStackTrace();
			} 

		}
	};
	new Thread(r).start();
}