Planned lessonMID-004-L02

Visibility and happens-before

On This Lesson 4 topics

This lesson is planned. The outline below lists the topics it will cover. The full lesson has not been published yet.

In this lesson

explain why a thread may not see another's write.

CPU caches and compiler reordering mean a write may not be visible to other threads.
The Java Memory Model defines happens-before: if A happens-before B, B sees A's writes.
Practical edges: unlock then lock of the same monitor; write then read of a volatile; Thread.start() and join(); executor submit then task run, and task completion then Future.get().
Without such an edge, a loop on a plain flag may never stop.
Visibility and atomicity are different problems.

Draft outline details

java

boolean running = true;                  // not volatile
while (running) { work(); }              // may spin forever

Name two actions that create a happens-before edge.

Show answer

Monitor unlock/lock, volatile write/read, start/join.

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