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protected synchronized void signalCond() { if (cond) { // simulate notifyAll for (int i = nWaiting; i > 0; --i) notify(); } }
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In open, extensible designs (see 134), the conditions under which notify apply are rather special and fragile The use of notify, and optimizations of guarded constructions more generally, are common sources of error As a general design tactic, it is a better idea to isolate uses of notify to concurrency control utility classes (see 34) that can be heavily optimized and carefully reviewed and tested We adopt this approach throughout the remainder of this chapter The conditions for using notify hold much more frequently in closed designs, where you are in full control of all participating threads For example, the following sketch of a closed-system two-player game uses waits for turn-taking A single notify suffices to wake the only thread that can possibly be awaiting its turn On the other hand, because there is only one thread waiting anyway, the performance differences between this version and one using notifyAll are probably too small to measure the main overhead associated with notifyAll is context switching, not the call to notifyAll itself Note that giveTurn is invoked as an open call (see 2413) in method GamePlayerreleaseTurn It is good practice to release as much synchronization as possible when performing notifications (see 372)
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class GamePlayer implements Runnable { protected GamePlayer other; protected boolean myturn = false;
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protected synchronized void setOther(GamePlayer p) { other = p; } synchronized void giveTurn() { // called by other player myturn = true; notify(); // unblock thread } void releaseTurn() { GamePlayer p; synchronized(this) { myturn = false;
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p = other; } pgiveTurn(); // open call } synchronized void awaitTurn() throws InterruptedException { while (!myturn) wait(); } void move() { /* perform one move */ } public void run() { try { for (;;) { awaitTurn(); move(); releaseTurn(); } } catch (InterruptedException ie) {} // die } public static void main(String[] args) { GamePlayer one = new GamePlayer(); GamePlayer two = new GamePlayer(); onesetOther(two); twosetOther(one); onegiveTurn(); new Thread(one)start(); new Thread(two)start(); } }
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Rather than waiting forever for a condition to become true in a guarded method, time-out designs place a bound on how long any given wait should remain suspended Responses to time-outs are of course situation-dependent When time-outs are used heuristically, the fact that a predicate does not hold may be only of informational value In other cases, time-outs force cancellation of attempted actions, in which case it is often appropriate to declare a TimeoutException as a subclass of InterruptedException Time-outs are typically more useful than other techniques that detect unanticipated liveness problems (such as deadlock[5]) because they make fewer assumptions about contexts any stall that causes unacceptably long blockages can be detected by a time-out that then triggers failure responses (see 311) Since most responses to delays of any kind are the same, they can all be triggered by time-out exceptions or related notification techniques
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Deadlock detection algorithms are discussed, for example, in the texts by Andrews and by Bernstein and Lewis listed in the Further Readings in 125 Implementation requires use of special lock classes However, some run-time systems and debuggers contain features allowing detection of deadlocks involving built-in synchronization
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The parameters controlling wait time and condition re-evaluation are sometimes completely arbitrary, and often require some trial and error Usually, it is not too hard to provide values that will catch true liveness problems without false-alarming on waits that just happen to be slow Since many such failures will at some point require human intervention, policies can be backed up via mechanisms that query users about remedial actions Time-outs are somewhat awkward to express using wait(msec) In the following TimeoutBoundedCounter class, the wait is placed in a loop in order to deal with the fact that unrelated notifications may occur This loop is slightly messy but has the identical rationale as versions without time-outs The condition being waited on is always checked first after waking up from the wait, before checking for time-out This helps avoid programming errors stemming from cases in which a wait is released by a time-out, but other contending threads execute before the timedout thread gets a chance to resume One of those other threads could have changed the condition, in which case it would not be necessary or appropriate to return a failure indication If the condition does not hold, the time-out value is checked and adjusted for use in the next iteration You could change this code to make the opposite decision about ordering the condition check and time check if time-outs are always considered to denote failure, even if the condition does hold upon resumption from the time-out
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class TimeoutException extends InterruptedException { } class TimeOutBoundedCounter { protected long count = 0; protected long TIMEOUT = 5000; // for illustration // synchronized void inc() throws InterruptedException { if (count >= MAX) { long start = SystemcurrentTimeMillis(); long waitTime = TIMEOUT; for (;;) { if (waitTime <= 0) throw new TimeoutException(); else { try { wait(waitTime); } catch (InterruptedException ie) { throw ie; // coded this way just for emphasis } if (count < MAX) break;
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else { long now = SystemcurrentTimeMillis(); waitTime = TIMEOUT - (now - start); } } } } ++count; notifyAll(); } synchronized void dec() throws InterruptedException { // similar } }
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