https://qooah.com/2015/05/29/android-%e6%96%b0%e5%8a%9f%e8%83%bd-%e6%89%be%e5%87%ba%e5%81%b7%e8%b5%b0%e4%bd%a0-ram-%e7%9a%84%e7%a8%8b%e5%bc%8f/