下载文件到SD卡中

       URL url = new URL("http://192.168.1.51/res/ban/ad/12/mv/ad1.avi");
       HttpURLConnection connect=(HttpURLConnection)url.openConnection();
       InputStream input=connect.getInputStream();
       File file=new File(Environment.getExternalStorageDirectory()+"/MP4/1.mp4");
       OutputStream output=new FileOutputStream(file);
       byte buffer [] = new byte[1024]; 
       int i=0;
       while((i=input.read(buffer)) !=-1){ 
        output.write(buffer,0,i); 
       } 
       output.flush();

设置访问网络的权限：<uses-permission android:name="android.permission.INTERNET"/>  

设置SD卡读写权限：

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>