// slot counts // counts of waiting threads in Java

Creating QR in Java // slot counts // counts of waiting threads
// slot counts // counts of waiting threads
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private final Object putMonitor = new Object(); private final Object takeMonitor = new Object(); public BoundedBufferWithMonitorObjects(int capacity) throws IllegalArgumentException { if (capacity <= 0) throw new IllegalArgumentException(); array = new Object[capacity]; emptySlots = capacity; } public void put(Object x) throws InterruptedException { synchronized(putMonitor) { while (emptySlots <= 0) { ++waitingPuts; try { putMonitorwait(); } catch(InterruptedException ie) { putMonitornotify(); throw ie; } finally { --waitingPuts; } } --emptySlots; array[putPtr] = x; putPtr = (putPtr + 1) % arraylength; } synchronized(takeMonitor) { // directly notify ++usedSlots; if (waitingTakes > 0) takeMonitornotify(); } } public Object take() throws InterruptedException { Object old = null; synchronized(takeMonitor) { while (usedSlots <= 0) { ++waitingTakes; try { takeMonitorwait(); } catch(InterruptedException ie) { takeMonitornotify();
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throw ie; } finally { --waitingTakes; } } --usedSlots; old = array[takePtr]; array[takePtr] = null; takePtr = (takePtr + 1) % arraylength; } synchronized(putMonitor) { ++emptySlots; if (waitingPuts > 0) putMonitornotify(); } return old; } }
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373 Specific Notifications
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Instead of treating the little helper Objects in classes such as BoundedBufferWithMonitorObjects as the culmination of design efforts, you can treat them as tools for implementing any design problem amenable to solution via split monitors The Specific Notification pattern devised by Tom Cargill takes precisely this tactic The basic idea is to put tasks to sleep via waits in monitors ordinary Objects (or more typically, instances of simple classes that help with bookkeeping) used solely for their wait sets One monitor is used for each task or set of tasks that must be individually notified In most cases, this requires one monitor per thread; in others, a group of threads that should all be awakened at once can use the same monitor These monitors serve similar purposes to the condition queues that are natively supported in some monitor-based concurrent programming languages (see 344) The main difference is that, without native support, these helper monitors must be dealt with more carefully to avoid nesting problems Specific notifications may be useful whenever you need threads to wait and the notification policy does not dynamically depend on the properties of the threads Once a thread is put in its wait set, it is impossible to access it in any way other than to wake it up Among the common applications to which these constraints apply are:
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Supporting specific scheduling policies through the use of an explicit queue (for example FIFO, LIFO, priority) Dividing incoming tasks into different queues depending on the method they are waiting to perform This can be used to extend techniques based on conflict sets (see 332)
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However, while it may be tempting to combine support for scheduling constraints such as FIFO with constraints based on logical state or execution state, interactions between these two applications usually lead to both conceptual and logistical problems For example, you need to consider cases where thread A should be enabled before thread B because it arrived earlier, but thread B is logically
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able to proceed while thread A is not This may necessitate elaborate apparatus to requeue threads, manage locking orders, and arbitrarily handle corner cases 3731 Design steps The main design steps are specializations of those described in 3721 Create or modify a class, say Host, as follows:
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For each thread or set of threads that needs specific notification, create an object serving as a monitor These monitors may be arranged in arrays or other collections, or dynamically created during execution Set up bookkeeping in the classes serving as monitors to manage waiting and notification operations and their interactions with time-out and interruption policies As shown in the WaitNode class in 3732, this usually entails maintaining a released field to remember if a waiting thread has been released due to notification, interruption, or time-out These classes may then support methods, say doWait, doTimedWait, doNotify, and doNotifyAll, that perform reliable waiting and notification and deal with interrupts and time-outs in the desired fashion If you cannot add bookkeeping to the classes serving as monitors, then these matters need to be addressed in the Host class methods In each Host method in which tasks are to be suspended, use monitordoWait() with the appropriate monitor object This code must avoid nested monitor problems by ensuring that the wait is performed within code regions that are not synchronized on the host object The simplest and most desirable form is:
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boolean needToWait; // to remember value after synch exit synchronized (this) { needToWait = ; if (needToWait) enqueue(monitor); // or any similar bookkeeping } if (needToWait) monitordoWait(); In each method in which tasks are to be resumed, use monitordoNotify(), also
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