Save State at Program Close (Shutdown Hook)

[Daksh]

1. https://stackoverflow.com/questions/5747803/running-code-on-program-exit-in-java
2. https://docs.oracle.com/javase/7/docs/api/java/lang/Runtime.html#addShutdownHook(java.lang.Thread)
3. https://dzone.com/articles/know-jvm-series-2-shutdown