Java Reference
In-Depth Information
NumberRange
is not thread-safe; it does not preserve the invariant that constrains
lower
and
upper
. The
setLower
and
setUpper
methods
attempt
to respect this invariant, but
do so poorly. Both
setLower
and
setUpper
are check-then-act sequences, but they do
not use sufficient locking to make them atomic. If the number range holds (0, 10), and one
thread calls
setLower(5)
while another thread calls
setUpper(4)
, with some unlucky
timing both will pass the checks in the setters and both modifications will be applied. The res-
ult is that the range now holds (5, 4)—an invalid state. So while the underlying
AtomicIn-
teger
s are thread-safe, the composite class is not. Because the underlying state variables
lower
and
upper
are not independent,
NumberRange
cannot simply delegate thread
safety to its thread-safe state varaibles.
