Effective Java: Programming Language Guide in Java

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One way to do this is to provide a wrapper class (Item 14) that implements an interface describing the class and performs appropriate synchronization before forwarding method invocations to the corresponding method of the wrapped object This is the approach that was taken by the Collections Framework Arguably, it should have been taken by javautilRandom as well A second approach, suitable for classes that are not designed to be extended or reimplemented, is to provide an unsynchronized class and a subclass consisting solely of synchronized methods that invoke their counterparts in the superclass One good reason to synchronize a class internally is because it is intended for heavily concurrent use and you can achieve significantly higher concurrency by performing internal fine-grained synchronization For example, it is possible to implement a nonresizable hash table that independently synchronizes access to each bucket This affords much greater concurrency than locking the entire table to access a single entry If a class or a static method relies on a mutable static field, it must be synchronized internally, even if it is typically used by a single thread Unlike a shared instance, it is not possible for the client to perform external synchronization because there can be no guarantee that other clients will do likewise The static method Mathrandom exemplifies this situation In summary, to avoid deadlock and data corruption, never call an alien method from within a synchronized region More generally, try to limit the amount of work that you do from within synchronized regions When you are designing a mutable class, think about whether it should do its own synchronization The cost savings that you can hope to achieve by dispensing with synchronization is no longer huge, but it is measurable Base your decision on whether the primary use of the abstraction will be multithreaded, and document your decision clearly
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Item 50: Never invoke wait outside a loop
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The Objectwait method is used to make a thread wait for some condition It must be invoked inside a synchronized region that locks the object on which it is invoked This is the standard idiom for using the wait method:
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synchronized (obj) { while (<condition does not hold>) objwait(); } // Perform action appropriate to condition
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Always use the wait loop idiom to invoke the wait method Never invoke it outside of a loop The loop serves to test the condition before and after waiting Testing the condition before waiting and skipping the wait if the condition already holds are necessary to ensure liveness If the condition already holds and notify (or notifyAll) method has already been invoked before a thread waits, there is no guarantee that the thread will ever waken from the wait Testing the condition after waiting and waiting again if the condition does not hold are necessary to ensure safety If the thread proceeds with the action when the condition does not
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Effective Java: Programming Language Guide
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hold, it can destroy the invariants protected by the lock There are several reasons a thread might wake up when the condition does not hold:
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Another thread could have obtained the lock and changed the protected state between the time a thread invoked notify and the time the waiting thread woke up Another thread could have invoked notify accidentally or maliciously when the condition did not hold Classes expose themselves to this sort of mischief by waiting on publicly accessible objects Any wait contained in a synchronized method of a publicly accessible object is susceptible to this problem The notifying thread could be overly generous in waking waiting threads For example, the notifying thread must invoke notifyAll even if only some of the waiting threads have their condition satisfied The waiting thread could wake up in the absence of a notify This is known as a spurious wakeup Although The Java Language Specification[JLS] does not mention this possibility, many JVM implementations use threading facilities in which spurious wakeups are known to occur, albeit rarely [Posix, 114361]
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A related issue is whether you should use notify or notifyAll to wake waiting threads (Recall that notify wakes a single waiting thread, assuming such a thread exists, and notifyAll wakes all waiting threads) It is often said that you should always use notifyAll This is reasonable, conservative advice, assuming that all wait invocations are inside while loops It will always yield correct results because it guarantees that you'll wake the threads that need to be awakened You may wake some other threads too, but this won't affect the correctness of your program These threads will check the condition for which they're waiting and, finding it false, will continue waiting As an optimization, you may choose to invoke notify instead of notifyAll if all threads that could be in the wait-set are waiting for the same condition and only one thread at a time can benefit from the condition becoming true Both of these conditions are trivially satisfied if only a single thread waits on a particular object (as in the WorkQueue example, Item 49) Even if these conditions appear true, there may be cause to use notifyAll in place of notify Just as placing the wait invocation in a loop protects against accidental or malicious notifications on a publicly accessible object, using notifyAll in place of notify protects against accidental or malicious waits by an unrelated thread Such waits could otherwise swallow a critical notification, leaving its intended recipient waiting indefinitely The reason that notifyAll was not used in the WorkQueue example is that the worker thread waits on a private object (queue) so there is no danger of accidental or malicious waits There is one caveat concerning the advice to use notifyAll in preference to notify While the use of notifyAll cannot harm correctness, it can harm performance In fact, it systematically degrades the performance of certain data structures from linear in the number of waiting threads to quadratic The class of data structures so affected are those for which only a certain number of threads are granted some special status at any given time and other threads must wait Examples include semaphores, bounded buffers, and read-write locks If you are implementing this sort of data structure and you wake up each thread as it becomes eligible for special status, you wake each thread once for a total of n wakeups If you wake all n threads when only one can obtain special status and the remaining n-1 threads go back to waiting, you will end up with n + (n 1) + (n 2) + 1 wakeups by the time all waiting
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