Condition Variables in Java

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344 Condition Variables
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Monitor operations in the Java programming language maintain a single wait set for each object Some other languages and thread libraries (in particular POSIX pthreads) include support for multiple wait sets associated with multiple condition variables managed under a common object or lock While any design requiring multiple wait sets can be implemented using other constructions such as semaphores, it is possible to create utilities that mimic the condition variables found in other systems In fact, support for pthreads-style condvars leads to usage patterns that are almost identical to those in concurrent C and C++ programs A CondVar class can be used to represent a condition variable that is managed in conjunction with a given Mutex, where this Mutex is also (unenforceably) used for all exclusion locking in the associated class(es) Thus, classes using CondVar must also rely on the "manual" locking techniques discussed in 251 More than one CondVar can use the same Mutex[6]
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The converse that more than one Mutex serve the same condition variable is logically possible but usually reflects a programming error and is not supported by this class
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The class supports analogs of the standard waiting and notification methods, here given names based on those in pthreads:
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class CondVar { // Implementation omitted protected final Sync mutex; public CondVar(Sync lock) { mutex = lock; } public void await() throws InterruptedException; public boolean timedwait(long ms) throws InterruptedException; public void signal(); // analog of notify public void broadcast(); // analog of notifyAll }
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(In the utilconcurrent version, the nuances of these operations also mirror those in pthreads For example, unlike notify, signal does not require the lock to be held) The main applications of such a class lie not in original design efforts, but in adapting code originally written using other languages and systems In other respects, a CondVar may be employed in the same design patterns, encountering the same design issues, as discussed in 33 For example, here is another bounded buffer class Except for the structured exception handling, this version almost looks as if it came out of a pthreads programming book (see Further Readings in 125)
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class PThreadsStyleBuffer { private final Mutex mutex = new Mutex(); private final CondVar notFull = new CondVar(mutex); private final CondVar notEmpty = new CondVar(mutex); private int count = 0; private int takePtr = 0; private int putPtr = 0; private final Object[] array; public PThreadsStyleBuffer(int capacity) { array = new Object[capacity]; } public void put(Object x) throws InterruptedException { mutexacquire(); try { while (count == arraylength) notFullawait(); array[putPtr] = x; putPtr = (putPtr + 1) % arraylength; ++count; notEmptysignal(); } finally { mutexrelease(); } }
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public Object take() throws InterruptedException { Object x = null; mutexacquire(); try { while (count == 0) notEmptyawait(); x = array[takePtr]; array[takePtr] = null; takePtr = (takePtr + 1) % arraylength; --count; notFullsignal(); } finally { mutexrelease(); } return x; } }
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345 Further Readings
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Additional discussions and examples of semaphores and condition variables can be found in almost any book on concurrent programming (see 125) Resource pools can be extended into more general Object Manager classes See: Sommerlad, Peter "Manager", in Robert Martin, Dirk Riehle, and Frank Buschmann (eds), Pattern Languages of Program Design, Volume 3, Addison-Wesley, 1998 Exchangers are described in more detail in: Sane, Aamod, and Roy Campbell "Resource Exchanger", in John Vlissides, James Coplien, and Norman Kerth (eds), Pattern Languages of Program Design, Volume 2, Addison-Wesley, 1996 The approximate fairness of some commonly used scheduling policies is discussed in: Epema, Dick H J "Decay-Usage Scheduling in Multiprocessors", ACM Transactions on Computer Systems, Vol 16, 367-415, 1998
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35 Joint Actions
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So far, this chapter has confined itself mainly to discussions of guarded actions that rely on the state of a single object Joint action frameworks provide a more general setting to attack more general design problems From a high-level design perspective, joint actions are atomic guarded methods that involve conditions and actions among multiple, otherwise independent participant objects They can be described abstractly as atomic methods involving two or more objects:
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void jointAction(A a, B b) {
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