[컴][안드로이드][자바] synchronize primitive 사용

synchronized primitive 를 제대로 사용하는 방법

References

1. http://snowbora.tistory.com/471
2. LCK01-J. Do not synchronize on objects that may be reused
3. LCK00-J. Use private final lock objects to synchronize classes that may interact with untrusted code