在项目中发现同一个文件（约500M），在浏览器和别的下载工具上使用http协议下载速度能达5M+KB/s，而我们的app中只能到6、700+KB/s。一开始以为是没有使用多线程下载，或者某逻辑代码是耗时操作，经过多次对比实验和调试，发现将下载时写文件用的RandomAccessFile的构造函数中的参数mode由rwd改为rw即可。
RandomAccessFile的mode官方文档：
| Value | Meaning|
| ------------- |:-------------:| -----:|
| "r" | for reading only. Invoking any of the write methods of the resulting object will cause an IOException. |
|"rw"|Open for reading and writing. If the file does not already exist then an attempt will be made to create it.|
|"rws"|Open for reading and writing, as with "rw", and also require that every update to the file's content or metadata be written synchronously to the underlying storage device.|
|"rwd" |Open for reading and writing, as with "rw", and also require that every update to the file's content be written synchronously to the underlying storage device. |
