What is happens-before

The rules for happens-before are:


Program order rule. Each action in a thread happens-before every action in that thread that comes later in the program order.


Monitor lock rule. An unlock on a monitor lock happens-before every subsequent lock on that same monitor lock.


Volatile variable rule. A write to a volatile field happens-before every subsequent read of that same field.


Thread start rule. A call to Thread.start on a thread happens-before every action in the started thread.


Thread termination rule. Any action in a thread happens-before any other thread detects that thread has terminated, either by successfully return from Thread.join or by Thread.isAlive returning false.


Interruption rule. A thread calling interrupt on another thread happens-before the interrupted thread detects the interrupt (either by having InterruptedException tHRown, or invoking isInterrupted or interrupted).


Finalizer rule. The end of a constructor for an object happens-before the start of the finalizer for that object.


Transitivity. If A happens-before B, and B happens-before C, then A happens-before C.