compile 'com.github.bumptech.glide:glide:3.7.0'

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

public class SDFileHelper {

  private Context context;

  public SDFileHelper() {
  }

  public SDFileHelper(Context context) {
    super();
    this.context = context;
  }

  //Glide保存圖片
  public void savePicture(final String fileName, String url){
    Glide.with(context).load(url).asBitmap().toBytes().into(new SimpleTarget<byte[]>() {
      @Override
      public void onResourceReady(byte[] bytes, GlideAnimation<? super byte[]> glideAnimation) {
        try {
          savaFileToSD(fileName,bytes);
        } catch (Exception e) {
          e.printStackTrace();
        }
      }
    });
  }
  //往SD卡寫入文件的方法
  public void savaFileToSD(String filename, byte[] bytes) throws Exception {
    //如果手機已插入sd卡,且app具有讀寫sd卡的權限
    if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
      String filePath = Environment.getExternalStorageDirectory().getCanonicalPath()+"/budejie";
      File dir1 = new File(filePath);
      if (!dir1.exists()){
        dir1.mkdirs();
      }
      filename = filePath+ "/" + filename;
      //這里就不要用openFileOutput了,那個是往手機內存中寫數據的
      FileOutputStream output = new FileOutputStream(filename);
      output.write(bytes);
      //將bytes寫入到輸出流中
      output.close();
      //關閉輸出流
      Toast.makeText(context, "圖片已成功保存到"+filePath, Toast.LENGTH_SHORT).show();
    } else Toast.makeText(context, "SD卡不存在或者不可讀寫", Toast.LENGTH_SHORT).show();
  }

}

 SDFileHelper helper = new SDFileHelper(MainActivity.this);
 helper.savePicture("bg.jpg",url);