What are happens-before relationships in JMM?

The Happens-Before principle is a formal guarantee that memory writes by one specific statement are visible to another specific statement.

Key Rules:

1. Program Order Rule: Actions in a single thread happen according to code order.
2. Monitor Lock Rule: An unlock on a monitor happens-before every subsequent lock on that same monitor.
3. Volatile Variable Rule: A write to a volatile field happens-before every subsequent read of that same field.
4. Thread Start Rule: A call to Thread.start() happens-before any action in the started thread.
5. Thread Termination Rule: Any action in a thread happens-before any other thread detects that thread has terminated.