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342 Latches
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A latching variable or condition is one that eventually receives a value from which it never again changes A binary latching variable or condition (normally just called a latch, also known as a oneshot) can change value only once, from its initial state to its final state Concurrency control techniques surrounding latches can be encapsulated using a simple Latch class that again obeys the usual acquire-release interface, but with the semantics that a single release permits all previous and future acquire operations to proceed Latches help structure solutions to initialization problems (see 241) where you do not want a set of activities to proceed until all objects and threads have been completely constructed For example, a more ambitious game-playing application than shown in 324 might need to ensure that all players wait until the game officially begins This could be arranged using code such as:
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class Player implements Runnable { // protected final Latch startSignal; Player(Latch l) { startSignal = l; }
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public void run() { try { startSignalacquire(); play(); } catch(InterruptedException ie) { return; } } // } class Game { // void begin(int nplayers) { Latch startSignal = new Latch(); for (int i = 0; i < nplayers; ++i) new Thread(new Player(startSignal))start(); startSignalrelease(); } }
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Extended forms of latches include countdowns, which allow acquire to proceed when a fixed number of releases occur, not just one Latches, countdowns, and other simple utilities built on top of them can be used to coordinate responses to conditions involving:
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Completion indicators For example, to force a set of threads to wait until some other activity completes Timing thresholds For example, to trigger a set of threads at a certain date Event indications For example, to trigger processing that cannot occur until a certain packet is received or button is clicked Error indications For example, to trigger a set of threads to proceed with global shut-down tasks 3421 Latching variables and predicates While utility classes are convenient for most one-shot triggering applications, latching fields (also known as permanent variables) and predicates can improve reliability, simplify usage, and improve efficiency in other contexts as well Among their other properties, latching predicates (including the common special case of threshold indicators) are among the very few conditions for which unsynchronized busy-wait loops (see 326) may be a possible (although rarely taken) implementation option for guarded methods If a predicate is known to latch, then there is no risk that it will slip (see 3241) Its value cannot change between the check to see if it is true and a subsequent action that requires it to remain true For example:
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class LatchingThermometer { // Seldom useful private volatile boolean ready; // latching private volatile float temperature; public double getReading() { while (!ready) Threadyield(); return temperature; } void sense(float t) { // called from sensor temperature = t; ready = true; } }
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Note that this kind of construction is confined to classes in which all relevant variables are either declared as volatile or are read and written only under synchronization (see 227)
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343 Exchangers
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An exchanger acts as a synchronous channel (see 3414) except that instead of supporting two methods, put and take, it supports only one method, rendezvous (sometimes just called exchange) that combines their effects (see 234) This operation takes an argument representing an Object offered by one thread to another, and returns the Object offered by the other thread Exchangers can be generalized to more than two parties, and can be further generalized to apply arbitrary functions on arguments rather than simply exchanging them These capabilities are supported
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by the Rendezvous class in utilconcurrent But the majority of applications are restricted to the exchange of resource objects among two threads (as arranged below by using only the default two-party constructor for Rendezvous) Exchange-based protocols extend those described in 234 to serve as alternatives to resource pools (see 3412) They can be used when two or more tasks running in different threads at all times each maintain one resource When one thread is finished with one resource and needs another, it exchanges with another thread The most common application of this protocol is buffer exchange Here, one thread fills up a buffer (for example by reading in data) When the buffer is full, it exchanges it with a thread that processes the buffer, thereby emptying it In this way, only two buffers are ever used, no copying is needed, and a resource management pool becomes unnecessary The following FillAndEmpty class gives a glimpse of the additional exception-handling obligations required with exchangers Because the protocol is symmetric, cancellation or time-out of one party in the midst of an attempted exchange must lead to an exception (here, BrokenBarrierException) in the other party In the example below, this is handled simply by returning from the run method A more realistic version would entail further cleanup, including additional adjustments to deal with incompletely filled or emptied buffers upon termination, as well as to deal with IO exceptions and end-of-file conditions surrounding the readByte method
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class FillAndEmpty { // Incomplete static final int SIZE = 1024; // buffer size, for demo protected Rendezvous exchanger = new Rendezvous(2); protected byte readByte() { /* */; } protected void useByte(byte b) { /* */ } public void start() { new Thread(new FillingLoop())start(); new Thread(new EmptyingLoop())start(); } class FillingLoop implements Runnable { // inner class public void run() { byte[] buffer = new byte[SIZE]; int position = 0; try { for (;;) { if (position == SIZE) { buffer = (byte[])(exchangerrendezvous(buffer)); position = 0; } buffer[position++] = readByte(); } } catch (BrokenBarrierException ex) {} // die catch (InterruptedException ie) {} // die }
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} class EmptyingLoop implements Runnable { // inner class public void run() { byte[] buffer = new byte[SIZE]; int position = SIZE; // force exchange first time through try { for (;;) { if (position == SIZE) { buffer = (byte[])(exchangerrendezvous(buffer)); position = 0; } useByte(buffer[position++]); } } catch (BrokenBarrierException ex) {} // die catch (InterruptedException ex) {} // die } } }
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The use of exchangers here illustrates one of the design advantages of utility classes that replace concerns surrounding the fields of objects with those surrounding the passing of messages This can be much easier to deal with as coordination schemes scale up (see 4)
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