handling the consequences of time-out or interruption in Java

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handling the consequences of time-out or interruption
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3732 FIFO semaphores Specific notifications can be used to implement the kinds of First-In-First-Out semaphore classes discussed in 3415 FIFO semaphores can in turn be used to build other utilities that rely on FIFO properties The following FIFOSemaphore class (a streamlined version of one in utilconcurrent) is defined as a subclass of the generic Semaphore class from 371 The FIFOSemaphore class maintains a linked WaitQueue holding WaitNodes, each serving as a monitor An acquire operation that cannot immediately obtain a permit enqueues a new monitor object that enters a wait The release operation dequeues the oldest waiting node and notifies it A released field in each WaitNode helps manage interactions between notifications and interruptions During a release, any monitor that has aborted due to interruption is skipped over Conversely, an interrupted wait first checks to see if it has been notified in addition to being
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interrupted If so, it must roll forward, ignoring the exception but resetting interrupt status (see 312) to preserve cancellation status (An unshown doTimedWait method can be implemented similarly, by setting released status upon time-out) The potential for interruptions at inconvenient times accounts for the retry loop in release The interactions among FIFOSemaphore, WaitQueue, and WaitNode ensure the necessary atomicity while avoiding nested monitor problems They also demonstrate some of the arbitrariness of decisions surrounding support of FIFO policies We can promise only that the semaphore is FIFO with respect to an arbitrary start point and end point The start point commences with the synchronized(this) in acquire The end point normally occurs upon release from a wait due to notify Two threads entering acquire might obtain the lock in different orders from their arrivals, for example if the first one is scheduled out by the JVM before it hits the synchronized(this) statement Similarly, a thread released before another might finally return to its caller after the other Especially on multiprocessors, the class provides as strong a guarantee as users of the class should expect The scheduling rules can be changed by substituting a different kind of queue here; for example one based on ThreadgetPriority However, it is trickier to adapt this class to handle semantic restrictions based on execution or logical state Most semantic restrictions require notified or interrupted threads to acquire additional locks This would introduce complications to the scheme here that exploits the fact that awakened threads need not access the main lock These would need to be resolved in an application-specific manner
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class FIFOSemaphore extends Semaphore { protected final WaitQueue queue = new WaitQueue(); public FIFOSemaphore(long initialPermits) { super(initialPermits); } public void acquire() throws InterruptedException { if (Threadinterrupted()) throw new InterruptedException(); WaitNode node = null; synchronized(this) { if (permits > 0) { // no need to queue --permits; return; } else { node = new WaitNode(); queueenq(node); } } // must release lock before node wait nodedoWait();
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} public synchronized void release() { for (;;) { // retry until success WaitNode node = queuedeq(); if (node == null) { // queue is empty ++permits; return; } else if (nodedoNotify()) return; // else node was already released due to // interruption or time-out, so must retry } } // Queue node class Each node serves as a monitor protected static class WaitNode { boolean released = false; WaitNode next = null; // to arrange in linked list synchronized void doWait() throws InterruptedException { try { while (!released) wait(); } catch (InterruptedException ie) { if (!released) { // interrupted before notified // Suppress future notifications: released = true; throw ie; } else { // interrupted after notified // ignore exception but propagate status: ThreadcurrentThread()interrupt(); } } } synchronized boolean doNotify() { // return true if notified if (released) return false; else { // was interrupted or timed out
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released = true; notify(); return true; } } synchronized boolean doTimedWait(long msecs) throws InterruptedException { // similar } } // Standard linked queue class // Used only when holding Semaphore lock protected static class WaitQueue { protected WaitNode head = null; protected WaitNode last = null; protected void enq(WaitNode node) { if (last == null) head = last = node; else { lastnext = node; last = node; } } protected WaitNode deq() { WaitNode node = head; if (node != null) { head = nodenext; if (head == null) last = null; nodenext = null; } return node; } } }
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