happens-before relation on memory operations such as reads and writes of shared variables. The results of a write by one thread

are guaranteed to be visible to a read by another thread only if the write operation happens-before the read operation.

　　The synchronized and volatile constructs, as well as the Thread.start() and Thread.join()methods, can form happens-before 

relationships. In particular:

• Each action in a thread happens-before every action in that thread that comes later in the program’s order.

• An unlock (synchronized block or method exit) of a monitor happens-before every subsequent lock (synchronized block or method

entry) of that same monitor. And because the happens-before relation is transitive, all actions of a thread prior to unlocking

happen-before all actions subsequent to any thread locking that monitor.

• A write to a volatile field happens-before every subsequent read of that same field. Writes and reads of volatile fields have similar

memory consistency effects as entering and exiting monitors, but do not entail mutual exclusion locking.

• A call to start on a thread happens-before any action in the started thread.

• All actions in a thread happen-before any other thread successfully returns from a join on that thread.

　　The methods of all classes in java.util.concurrent and its subpackages extend these guarantees to higher-level synchronization.

In particular:

• Actions in a thread prior to placing an object into any concurrent collection happen-before actions subsequent to the access or

removal of that element from the collection in another thread.

• Actions in a thread prior to the submission of a Runnable to an Executorhappen-before its execution begins. Similarly for Callables

submitted to an ExecutorService.

• Actions taken by the asynchronous computation represented by a Futurehappen-before actions subsequent to the retrieval of the

result via Future.get() in another thread.

• Actions prior to “releasing” synchronizer methods such as Lock.unlock, Semaphore.release, and CountDownLatch.countDown happen-before

actions subsequent to a successful “acquiring” method such as Lock.lock, Semaphore.acquire, Condition.await, and CountDownLatch.await on

the same synchronizer object in another thread.

• For each pair of threads that successfully exchange objects via an Exchanger, actions prior to the exchange() in each thread 

happen-before those subsequent to the corresponding exchange() in another thread.

• Actions prior to calling CyclicBarrier.awaithappen-before actions performed by the barrier action, and actions performed by the barrier

action happen-before actions subsequent to a successful return from the corresponding await in other threads.