• ThreadLocal

Each thread holds an implicit reference to its copy of a thread-local variable
as long as the thread is alive and the ThreadLocal instance is accessible;
after a thread goes away, all of its copies of thread-local instances are subject 
to garbage collection (unless other references to these copies exist).

• 초기값을 생성하는 lambda 를 지정해두면 최초에 값이 없을 때에 적절한 초기값을 제공해준다.
• remove() 를 호출해서 초기값이 없어진 상태에서 다시 get()을 하면 withInitialValue로 설정했다면 다시 초기값을 생성해서 반환해준다.

• java - ThreadLocal & Memory Leak