Java Reference
In-Depth Information
fer, ensuring that the balances are updated atomically and without violating invariants such
as “an account cannot have a negative balance”.
How can
transferMoney
deadlock? It may appear as if all the threads acquire their locks
in the same order, but in fact the lock order depends on the order of arguments passed to
transferMoney
, and these in turn might depend on external inputs. Deadlock can occur
if two threads call
transferMoney
at the same time, one transferring from
X
to
Y
, and the
other doing the opposite:
Listing 10.2. Dynamic Lock-ordering Deadlock.
Don't do this.
A: transferMoney(myAccount, yourAccount, 10);
B: transferMoney(yourAccount, myAccount, 20);
With unlucky timing,
A
will acquire the lock on
myAccount
and wait for the lock on
yourAccount
, while
B
is holding the lock on
yourAccount
and waiting for the lock on
myAccount
.
Deadlocks like this one can be spotted the same way as in
Listing 10.1
—look for nested lock
acquisitions. Since the order of arguments is out of our control, to fix the problem we must
induce
an ordering on the locks and acquire them according to the induced ordering consist-
ently throughout the application.
