What does happens-before mean in the Java Memory Model?

Explains visibility and ordering through synchronization actions instead of misleading main-memory and thread-cache folklore.

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If action A happens-before action B, the effects of A are visible to B and ordered before it according to the Java Memory Model. Examples include releasing a monitor before another thread later acquires the same monitor, writing a volatile field before another thread reads that write, and actions before starting a thread becoming visible in that thread. Program order also establishes happens-before within one thread. Without a valid relationship, concurrent reads may observe stale or surprising state.

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Java Memory Model reasoning, safe communication, synchronization order, and publication guarantees.

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Happens-before is the guarantee that one action's effects are visible and ordered before another action.
If action A happens-before action B, the effects of A are visible to B and ordered before it according to the Java Memory Model. Examples include releasing a monitor before another thread later acquires the same monitor, writing a volatile field before another thread reads that write, and actions before starting a thread becoming visible in that thread. Program order also establishes happens-before within one thread. Without a valid relationship, concurrent reads may observe stale or surprising state.
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Happens-before is not wall-clock language; two operations can occur at different times without the required memory guarantee. A common publication pattern writes ordinary fields, then writes a volatile ready flag; a reader that observes the volatile write also sees the preceding writes. Locks provide both mutual exclusion and visibility when all participants use the same lock. Thread start, thread join, concurrent queues, futures, and other library contracts also establish useful ordering.

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Treating happens-before as chronological time.
Using sleep as a visibility mechanism.
Synchronizing readers and writers on different locks.

What does happens-before mean in the Java Memory Model?

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