Safe Publication & Memory Model

Safe publication ensures that other threads see a fully constructed object, not a partially initialized one with null fields, by using final fields, volatile references, or synchronized blocks.

Happens-before is the JMM's visibility guarantee: if action A happens-before action B, everything A wrote is visible to B, and without this relationship a thread can read stale values forever.

The JMM evolved from a broken pre-Java 5 model (unsafe DCL, unreliable volatile) to JSR-133 in Java 5 (full happens-before), Java 9 (VarHandle with fine-grained access modes), and Java 21 (virtual threads, biased locking removed).

The compiler, JIT, and CPU can reorder instructions for performance, and for 'new Object()' this means another thread may see a non-null reference to an object whose constructor has not finished, which is why DCL requires volatile.

Memory barriers stop the CPU and compiler from reordering memory operations and control cross-core visibility; release publishes all prior writes before a flag write, acquire sees all prior writes after reading the flag, and Java exposes them via VarHandle fences.