Effective Java: Programming Language Guide in Java

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Because memory leaks typically do not manifest themselves as obvious failures, they may remain present in a system for years They are typically discovered only as a result of careful code inspection or with the aid of a debugging tool known as a heap profiler Therefore it is very desirable to learn to anticipate problems like this before they occur and prevent them from happening
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Item 6: Avoid finalizers
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Finalizers are unpredictable, often dangerous, and generally unnecessary Their use can cause erratic behavior, poor performance, and portability problems Finalizers have a few valid uses, which we'll cover later in this item, but as a rule of thumb, finalizers should be avoided C++ programmers are cautioned not to think of finalizers as the analog of C++ destructors In C++, destructors are the normal way to reclaim the resources associated with an object, a necessary counterpart to constructors In the Java programming language, the garbage collector reclaims the storage associated with an object when it becomes unreachable, requiring no special effort on the part of the programmer C++ destructors are also used to reclaim other nonmemory resources In the Java programming language, the try-finally block is generally used for this purpose There is no guarantee that finalizers will be executed promptly [JLS, 126] It can take arbitrarily long between the time that an object becomes unreachable and the time that its finalizer is executed This means that nothing time-critical should ever be done by a finalizer For example, it is a grave error to depend on a finalizer to close open files because open file descriptors are a limited resource If many files are left open because the JVM is tardy in executing finalizers, a program may fail because it can no longer open files The promptness with which finalizers are executed is primarily a function of the garbage collection algorithm, which varies widely from JVM implementation to JVM implementation The behavior of a program that depends on the promptness of finalizer execution may likewise vary It is entirely possible that such a program will run perfectly on the JVM on which you test it and then fail miserably on the JVM favored by your most important customer Tardy finalization is not just a theoretical problem Providing a finalizer for a class can, under rare conditions, arbitrarily delay reclamation of its instances A colleague recently debugged a long-running GUI application that was mysteriously dying with an OutOfMemoryError Analysis revealed that at the time of its death, the application had thousands of graphics objects on its finalizer queue just waiting to be finalized and reclaimed Unfortunately, the finalizer thread was running at a lower priority than another thread in the application, so objects weren't getting finalized at the rate they became eligible for finalization The JLS makes no guarantees as to which thread will execute finalizers, so there is no portable way to prevent this sort of problem other than to refrain from using finalizers Not only does the JLS provide no guarantee that finalizers will get executed promptly, it provides no guarantee that they'll get executed at all It is entirely possible, even likely, that a program terminates without executing finalizers on some objects that are no longer reachable As a consequence, you should never depend on a finalizer to update critical persistent state For example, depending on a finalizer to release a persistent lock on a shared resource such as a database is a good way to bring your entire distributed system to a grinding halt
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Don't be seduced by the methods Systemgc and SystemrunFinalization They may increase the odds of finalizers getting executed, but they don't guarantee it The only methods that claim to guarantee finalization are SystemrunFinalizersOnExit and its evil twin, RuntimerunFinalizersOnExit These methods are fatally flawed and have been deprecated In case you are not yet convinced that finalizers should be avoided, here's another tidbit worth considering: If an uncaught exception is thrown during finalization, the exception is ignored, and finalization of that object terminates [JLS, 126] Uncaught exceptions can leave objects in a corrupt state If another thread attempts to use such a corrupted object, arbitrary nondeterministic behavior may result Normally, an uncaught exception will terminate the thread and print a stack trace, but not if it occurs in a finalizer it won't even print a warning So what should you do instead of writing a finalizer for a class whose objects encapsulate resources that require termination, such as files or threads Just provide an explicit termination method, and require clients of the class to invoke this method on each instance when it is no longer needed One detail worth mentioning is that the instance must keep track of whether it has been terminated: The explicit termination method must record in a private field that the object is no longer valid, and other methods must check this field and throw an IllegalStateException if they are called after the object has been terminated A typical example of an explicit termination method is the close method on InputStream and OutputStream Another example is the cancel method on javautilTimer, which performs the necessary state change to cause the thread associated with a Timer instance to terminate itself gently Examples from javaawt include Graphicsdispose and Windowdispose These methods are often overlooked, with predictably dire performance consequences A related method is Imageflush, which deallocates all the resources associated with an Image instance but leaves it in a state where it can still be used, reallocating the resources if necessary Explicit termination methods are often used in combination with the try-finally construct to ensure prompt termination Invoking the explicit termination method inside the finally clause ensures that it will get executed even if an exception is thrown while the object is being used:
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// try-finally block guarantees execution of termination method Foo foo = new Foo(); try { // Do what must be done with foo } finally { footerminate(); // Explicit termination method }
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So what, if anything, are finalizers good for There are two legitimate uses One is to act as a safety net in case the owner of an object forgets to call the explicit termination method that you provided per the advice in the previous paragraph While there's no guarantee that the finalizer will get invoked promptly, it's better to free the critical resource late than never, in those (hopefully rare) cases when the client fails to hold up its end of the bargain by calling the explicit termination method The three classes mentioned as examples of the explicit
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