Doing Two or More Tasks At Once: Threads |
notifyAll
and wait
Methods
TheCubbyHole
stores its value in a private member variable calledcontents
.CubbyHole
has another private member variable,available
, that is aboolean
.available
istrue
when the value has just been put but not yet gotten and is false when the value has been gotten but not yet put. So, here's one possible implementation for theput
andget
methods:As implemented, these two methods won't work. Look at thepublic synchronized int get() { // won't work! if (available == true) { available = false; return contents; } } public synchronized int put(int value) { // won't work! if (available == false) { available = true; value = ; } }get
method. What happens if theProducer
hasn't put anything in theCubbyHole
andavailable
isn'ttrue
?get
does nothing. Similarly, if theProducer
callsput
before theConsumer
got the value,put
doesn't do anything.You really want the
Consumer
to wait until theProducer
puts something in theCubbyHole
and theProducer
must notify theConsumer
when it's done so. Similarly, theProducer
must wait until theConsumer
takes a value (and notifies theProducer
of its activities) before replacing it with a new value. The two threads must coordinate more fully and can useObject
'swait
andnotifyAll
methods to do so.Here are the new implementations of
get
andput
that wait on and notify each other of their activities:The code in thepublic synchronized int get() { while (available == false) { try { // wait for Producer to put value wait(); } catch (InterruptedException e) { } } available = false; // notify Producer that value has been retrieved notifyAll(); return contents; } public synchronized void put(int value) { while (available == true) { try { // wait for Consumer to get value wait(); } catch (InterruptedException e) { } } contents = value; available = true; // notify Consumer that value has been set notifyAll(); }get
method loops until theProducer
has produced a new value. Each time through the loop,get
calls thewait
method. Thewait
method relinquishes the lock held by theConsumer
on theCubbyHole
(thereby allowing theProducer
to get the lock and update theCubbyHole
) and then waits for notification from theProducer
. When theProducer
puts something in theCubbyHole
, it notifies theConsumer
by callingnotifyAll
. TheConsumer
then comes out of the wait state,available
is nowtrue
, the loop exits, and theget
method returns the value in theCubbyHole
.The
put
method works in a similar fashion, waiting for theConsumer
thread to consume the current value before allowing theProducer
to produce a new one.The
notifyAll
method wakes up all threads waiting on the object in question (in this case, theCubbyHole
). The awakened threads compete for the lock. One thread gets it, and the others go back to waiting. TheObject
class also defines the notify method, which arbitrarily wakes up one of the threads waiting on this object.The
Object
class contains not only the version ofwait
that is used in the producer/consumer example and which waits indefinitely for notification, but also two other versions of thewait
method:
- wait(long timeout)
- Waits for notification or until the
timeout
period has elapsed.timeout
is measured in milliseconds.- wait(long timeout, int nanos)
- Waits for notification or until
timeout
milliseconds plusnanos
nanoseconds have elapsed.
Note: Besides using these timedwait
methods to synchronize threads, you also can use them in place ofsleep
. Bothwait
andsleep
delay for the requested amount of time, but you can easily wake upwait
with a notify but a sleeping thread cannot be awoken prematurely. This doesn't matter too much for threads that don't sleep for long, but it could be important for threads that sleep for minutes at a time.
Doing Two or More Tasks At Once: Threads |