单例的一种实现方式
private static final Map<String, DownBigFileUtils> mMap = new HashMap<String, DownBigFileUtils>();

private File mStorageDirectory;

/**
* 根据不同的path，会创建不同的实例
* @param path 下载保存的路径，如childtongyao/shijingmp3
* @return
*/
public static DownBigFileUtils getDefaultInstance(String path) {
DownBigFileUtils downBigFileUtils = mMap.get(path);
if(downBigFileUtils == null) {
downBigFileUtils = new DownBigFileUtils(path);
mMap.put(path, downBigFileUtils);
}
return downBigFileUtils;
}