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The problem with this code is that in the absence of synchronization, there is no guarantee as to when, if ever, the stoppable thread will see a change in the the value of stopRequested that was made by another thread As a result, the requestStop method might be completely ineffective Unless you are running on a multiprocessor, you are unlikely to observe the problematic behavior in practice, but there are no guarantees The straightforward way to fix the problem is simply to synchronize all access to the stopRequested field:
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// Properly synchronized cooperative thread termination public class StoppableThread extends Thread { private boolean stopRequested = false; public void run() { boolean done = false; while (!stopRequested() && !done) { // do what needs to be done } } public synchronized void requestStop() { stopRequested = true; } private synchronized boolean stopRequested() { return stopRequested; } }
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Note that the actions of each of the synchronized methods are atomic: The synchronization is being used solely for its communication effects, not for mutual exclusion It is clear that the revised code works, and the cost of synchronizing on each iteration of the loop is unlikely to be noticeable That said, there is a correct alternative that is slightly less verbose and whose performance may be slightly better The synchronization may be omitted if stopRequested is declared volatile The volatile modifier guarantees that any thread that reads a field will see the most recently written value The penalty for failing to synchronize access to stopRequested in the previous example is comparatively minor; the effect of the requestStop method may be delayed indefinitely The penalty for failing to synchronize access to mutable shared data can be much more severe Consider the double-check idiom for lazy initialization:
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// The double-check idiom for lazy initialization - broken! private static Foo foo = null; public static Foo getFoo() { if (foo == null) { synchronized (Fooclass) { if (foo == null) foo = new Foo(); } } return foo; }
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The idea behind this idiom is that you can avoid the cost of synchronization in the common case of accessing the field (foo) after it has been initialized Synchronization is used only to prevent multiple threads from initializing the field The idiom does guarantee that the field will be initialized at most once and that all threads invoking getFoo will get the correct value for the object reference Unfortunately, the object reference is not guaranteed to work properly If a thread reads the reference without synchronization and then invokes a method on the referenced object, the method may observe the object in a partially initialized state and fail catastrophically That a thread can observe the lazily constructed object in a partially initialized state is wildly counterintuitive The object is fully constructed before the reference is published in the field from which it is read by other threads (foo) But in the absence of synchronization, reading a published object reference does not guarantee that a thread will see all of the data that were stored in memory prior to the publication of the object reference In particular, reading a published object reference does not guarantee that the reading thread will see the most recent values of the data that constitute the internals of the referenced object In general, the doublecheck idiom does not work, although it does work if the shared variable contains a primitive value rather than an object reference [Pugh01b] There are several ways to fix the problem The easiest way is to dispense with lazy initialization entirely:
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// Normal static initialization (not lazy) private static final Foo foo = new Foo(); public static Foo getFoo() { return foo; }
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This clearly works, and the getFoo method is as fast as it could possibly be It does no synchronization and no computation either As discussed in Item 37, you should write simple, clear, correct programs, leaving optimization till last, and you should optimize only if measurement shows that it is necessary Therefore dispensing with lazy initialization is generally the best approach If you dispense with lazy initialization, measure the cost, and find that it is prohibitive, the next best thing is to use a properly synchronized method to perform lazy initialization:
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// Properly synchronized lazy initialization private static Foo foo = null; public static synchronized Foo getFoo() { if (foo == null) foo = new Foo(); return foo; }
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This method is guaranteed to work, but it incurs the cost of synchronization on every invocation On modern JVM implementations, this cost is relatively small However, if you've determined by measuring the performance of your system that you can afford neither the cost of normal initialization nor the cost of synchronizing every access, there is another option The initialize-on-demand holder class idiom is appropriate for use when a static field is
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