The Java Language Specification, Third Edition

THREADS AND LOCKS Happens-before Order 17.4.5
DISCUSSION
It follows from the above definitions that:
• An unlock on a monitor happens-before every subsequent lock on that monitor.
• A write to a volatile field (§8.3.1.4) happens-before every subsequent read of that field.
• A call to start() on a thread happens-before any actions in the started thread.
• All actions in a thread happen-before any other thread successfully returns from a
join() on that thread.
• The default initialization of any object happens-before any other actions (other than
default-writes) of a program.
When a program contains two conflicting accesses (§17.4.1) that are not ordered
by a happens-before relationship, it is said to contain a data race.
The semantics of operations other than inter-thread actions, such as reads of
array lengths (§10.7), executions of checked casts (§5.5, §15.16), and invocations
of virtual methods (§15.12), are not directly affected by data races.
DISCUSSION
Therefore, a data race cannot cause incorrect behavior such as returning the wrong length
for an array.
DRAFT
A program is correctly synchronized if and only if all sequentially consistent executions
are free of data races.
DISCUSSION
A subtle example of incorrectly synchronized code can be seen below. The figures show
two different executions of the same program, both of which contain conflicting accesses to
shared variables X and Y. The two threads in the program lock and unlock a monitor M1.
In execution (a), there is a happens-before relationship between all pairs of conflicting
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