http://meilanie95.blogspot.co.id/2015/05/concurrency.html
http://www.cs.miami.edu/home/visser/csc329-files/Concurrency.pdf
http://stackoverflow.com/questions/7335950/semaphore-vs-monitors-whats-the-difference
https://en.wikipedia.org/wiki/Monitor_%28synchronization%29
http://www.answers.com/Q/What_is_basic_difference_between_semaphore_and_monitor_in_operating_system
http://www.programcreek.com/2011/12/monitors-java-synchronization-mechanism/