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BufferArray helper class (In fact, as illustrated in 434, a completely different underlying data
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clients receive permits and could otherwise insert or extract elements concurrently
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class BufferArray { protected final Object[] array; // the elements protected int putPtr = 0; // circular indices protected int takePtr = 0; BufferArray(int n) { array = new Object[n]; } synchronized void insert(Object x) { // put mechanics array[putPtr] = x; putPtr = (putPtr + 1) % arraylength; } synchronized Object extract() { // take mechanics Object x = array[takePtr]; array[takePtr] = null; takePtr = (takePtr + 1) % arraylength; return x; } }
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The corresponding BoundedBufferWithSemaphores class surrounds buffer operations with semaphore operations to implement put and take Even though each method starts with an acquire and ends with a release, they follow a different usage pattern than seen with locks in 25 The release is on a different semaphore from the acquire, and is performed only after the element is successfully inserted or extracted So, among other consequences, these releases are not placed in finally clauses: If there were any chance that buffer operations could fail, some recovery actions would be needed, but these trailing release statements are not among them
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class BoundedBufferWithSemaphores { protected final BufferArray buff; protected final Semaphore putPermits; protected final Semaphore takePermits; public BoundedBufferWithSemaphores(int capacity) { if (capacity <= 0) throw new IllegalArgumentException(); buff = new BufferArray(capacity); putPermits = new Semaphore(capacity); takePermits = new Semaphore(0); }
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public void put(Object x) throws InterruptedException { putPermitsacquire(); buffinsert(x); takePermitsrelease(); } public Object take() throws InterruptedException { takePermitsacquire(); Object x = buffextract(); putPermitsrelease(); return x; } public Object poll(long msecs) throws InterruptedException { if (!takePermitsattempt(msecs)) return null; Object x = buffextract(); putPermitsrelease(); return x; } public boolean offer(Object x, long msecs) throws InterruptedException { if (!putPermitsattempt(msecs)) return false; buffinsert(x); takePermitsrelease(); return true; } }
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This class also includes variants of put and take, called offer and poll, that support balking (when msecs is 0) or time-out policies These methods are implemented using Semaphoreattempt, which handles the messy time-based constructions described in 325 Methods offer and poll allow clients to choose the guarding policy most appropriate for their needs However, clients must still pick compatible policies For example, if a producer relied solely on offer(x, 0) and its only consumer used poll(0), items would rarely be transferred The BoundedBufferWithSemaphores class is likely to run more efficiently than the BoundedBufferWithStateTracking class in 331 when there are many threads all using the buffer BoundedBufferWithSemaphores relies on two different underlying wait sets The BoundedBufferWithStateTracking class gets by with only one, so any emptyto-partial or full-to-partial state transition causes all waiting threads to wake up, including those waiting for the other logical condition and those that will immediately rewait because some other thread took the only item or filled the only available slot The BoundedBufferWithSemaphores class isolates the monitors for these two conditions This can be exploited by the underlying Semaphore implementation (see 371) to eliminate unnecessary context switching by using notify instead of notifyAll This reduces the worstcase number of wakeups from being a quadratic function of the number of invocations to being linear
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More generally, whenever you can isolate a condition using a semaphore, you can usually improve performance as compared to notifyAll-based solutions 3414 Synchronous channels As mentioned in 331, the interface for BoundedBuffer can be broadened to describe any kind of Channel that supports a put and a take operation:
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interface Channel { // Repeated void put(Object x) throws InterruptedException; Object take() throws InterruptedException; }
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(The utilconcurrent version of this interface also includes the offer and poll methods that support time-outs, and declares it to extend interfaces Puttable and Takable to allow type enforcement of one-sided usages) There are many possible semantics that you might attach to a Channel For example, the queue class in 242 has unbounded capacity (at least conceptually failing only when a system runs out of memory), while bounded buffers have finite predetermined capacity A limiting case is the idea of a synchronous channel that has no internal capacity With synchronous channels, every thread attempting a put must wait for a thread attempting a take, and vice versa This allows the precise control over thread interaction needed in several of the design frameworks and patterns discussed in 414 and 451 Semaphores can be used to implement synchronous channels Here, we can use the same approach as with bounded buffers, adding another semaphore that permits a put to continue only after its offered item has been taken However, this introduces a new problem So far, we have used only blocking constructions that can throw InterruptedExceptions as the first lines of methods, allowing simple clean exit upon interruption But here, we need to do a second acquire at the end of the put method Aborting at this point of no return would break the protocol While it is possible to define a version of this class that performs full rollback, the simplest solution here is to roll forward (see 3114), ignoring any interrupt until after the second acquire completes:
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class SynchronousChannel implements Channel { protected Object item = null;// to hold while in transit protected final Semaphore putPermit; protected final Semaphore takePermit; protected final Semaphore taken; public SynchronousChannel() { putPermit = new Semaphore(1); takePermit = new Semaphore(0); taken = new Semaphore(0); } public void put(Object x) throws InterruptedException {
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putPermitacquire(); item = x; takePermitrelease(); // Must wait until signaled by taker InterruptedException caught = null; for (;;) { try { takenacquire(); break; } catch(InterruptedException ie) { caught = ie; } } if (caught != null) throw caught; // can now rethrow } public Object take() throws InterruptedException { takePermitacquire(); Object x = item; item = null; putPermitrelease(); takenrelease(); return x; } }
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3415 Fairness and scheduling Built-in waiting and notification methods do not provide any fairness guarantees They make no promises about which of the threads in a wait set will be chosen in a notify operation, or which thread will grab the lock first and be able to proceed (thus excluding others) in a notifyAll operation This flexibility in JVM implementations permitted by the JLS makes it all but impossible to prove particular liveness properties of a system But this is not a practical concern in most contexts For example, in most buffer applications, it doesn't matter at all which of the several threads trying to take an item actually do so On the other hand, in a resource pool management class, it is prudent to ensure that threads waiting for needed resource items don't continually get pushed aside by others because of unfairness in how the underlying notify operations choose which threads to unblock Similar concerns arise in many applications of synchronous channels It is not possible to change the semantics of notify, but it is possible to implement Semaphore (sub)class acquire operations to provide stronger fairness properties A range of policies can be supported, varying in exactly how fairness is defined The best-known policy is First-In-First-Out (FIFO), in which the thread that has been waiting the longest is always selected This is intuitively desirable, but can be unnecessarily demanding and even somewhat arbitrary on multiprocessors where different threads on different processors start waiting at
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(approximately) the same time However, various weakenings of and approximations to FIFO are available that provide sufficient fairness for applications that need to avoid indefinite postponement There are, however, some intrinsic limitations to such guarantees: There is no way to ensure that an underlying system will ever actually execute a given runnable process or thread unless the system provides guarantees that go beyond the minimal requirements stated in the JLS However, this is unlikely to be a significant pragmatic issue Most if not all JVM implementations strive to provide sensible scheduling policies that extend well beyond minimal requirements They display some sort of weak, restricted, or probabilistic fairness properties with respect to executing runnable threads However, it is difficult for a language specification to state all the reasonable ways in which this may occur The matter is left as a quality-of-implementation issue in the JLS Utility classes such as semaphores are convenient vehicles for establishing different fairness policies, modulo these scheduling caveats For example, 373 describes implementation of a FIFOSemaphore class that maintains FIFO notification order Applications such as the Pool class can use this or other implementations of semaphores that provide any supported fairness properties, at the potential cost of additional overhead 3416 Priorities In addition to addressing fairness, semaphore implementation classes can pay attention to thread priorities The notify method is not guaranteed to do so, but it is of course allowed to, and does so on some JVM implementations Priority settings (see 1123) tend to be of value only when there may be many more runnable threads than CPUs, and the tasks running in these threads intrinsically have different urgencies or importances This occurs most commonly in embedded (soft) real-time systems where a single small processor must carry out many tasks that interact with its environment Reliance on priority settings can complicate notification policies Even if notifications unblock (and run) threads in highest-priority-first order, systems may still encounter priority inversions A priority inversion occurs when a high-priority thread becomes blocked waiting for a low-priority thread to complete and then release a lock or change a condition needed by the high-priority thread In a system using strict priority scheduling, this can cause the high-priority thread to starve if the low-priority thread does not get a chance to run One solution is to use special semaphore classes or lock classes constructed via such semaphores Here, the concurrency control objects themselves manipulate priorities When a high-priority thread becomes blocked, the concurrency control object can temporarily raise the priority of a low-priority thread that could unblock it This reflects the fact that proceeding to a release point is a high-priority action (see Further Readings in 125) For this to work, all relevant synchronization and locking must rely on such priority-adjusting utility classes Further, this tactic is guaranteed to maintain the intended properties only on particular JVM implementations that use strict priority scheduling In practice, any usable JVM implementation supporting strict priority scheduling is sure to apply priority adjustment for built-in lock and monitor operations Doing otherwise would defeat most of the rationale for adopting strict priority scheduling in the first place The main practical consequence is that programs that absolutely rely on strict priority scheduling sacrifice portability They need additional JVM implementation-specific guarantees that may be bolstered via construction and use of additional concurrency control utilities In other more portable
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programs, semaphore classes and related utilities that prefer higher-priority threads can still be used occasionally as devices for heuristically improving responsiveness
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