The Java memory model question 14 of 180 mid

Which happens-before edges do you get for free?

Correct: Thread.start() and join(), monitor unlock to lock, volatile write to read, and final fields
Only synchronized
Any method call on a shared object
Anything inside a try block

The list worth memorising, because interviewers ask for it directly:

Program order inside a single thread.
Monitor: unlocking a monitor happens-before every later lock of the same monitor.
Volatile: a write happens-before every later read of that field.
Thread start: everything before t.start() happens-before everything in t.
Thread join: everything in t happens-before t.join() returning.
Final fields: correctly constructed final fields are visible without synchronisation.
Transitivity: if A happens-before B and B happens-before C, then A happens-before C.

Everything in java.util.concurrent builds on these and documents its own: putting into a BlockingQueue happens-before taking that element out, submitting a task happens-before it running, and a CountDownLatch.countDown happens-before a returning await.

memory-model happens-before

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