The Java Memory Model

Key points

• Compilers and CPUs reorder code freely. Your own thread always sees a sensible order. Other threads have no such promise.
• happens-before is that promise: if A happens-before B, then everything A wrote is visible to B, in order.
• You get an edge from unlocking then locking the same lock, writing then reading the same volatile, Thread.start(), and Thread.join().
• Library handoffs create edges too: putting into a queue, counting down a latch, completing a future, publishing into a concurrent map.
• final fields are special. As long as this does not escape the constructor, other threads always see them fully built.
• No edge means no guarantee, not “usually fine”. A data race is undefined behaviour, and it stays invisible until production load.

Every safe handoff between threads is a happens-before edge. Find yours.