Sharing Objects - Java Concurrency in Practice - page 36

Listing 3.1. Sharing Variables without Synchronization. Don't Do this .
NoVisibility could loop forever because the value of ready might never become vis-
ible to the reader thread. Even more strangely, NoVisibility could print zero because the
write to ready might be made visible to the reader thread before the write to number , a
phenomenon known as reordering . There is no guarantee that operations in one thread will
be performed in the order given by the program, as long as the reordering is not detectable
from within that thread— even if the reordering is apparent to other threads . [1] When the
main thread writes first to number and then to done without synchronization, the reader
thread could see those writes happen in the opposite order—or not at all.
In the absence of synchronization, the compiler, processor, and runtime can do some down-
right weird things to the order in which operations appear to execute. Attempts to reason
about the order in which memory actions “must” happen in insufflciently synchronized mul-
tithreaded programs will almost certainly be incorrect.