JMM - Java memory model

JMM
An archtecture's memory model tells programs what guarantees they can expect from the memory system, and specifies the special instructions required (called memory barriers) to get the additional memory coordination guarantees required when sharing data.

The JMM defines a partial ordering called happens-before on all actions within the program. To guarantee that the thread executing action B can see the results of action A, there must be a happens-before relationship between A and B. In the absence of a happens-before ordering between two operations, the JVM is free to reorder them as is pleases.

The rules for happen-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.

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.