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On the surface, finalization seems pretty straightforward: you create an object and its Finalize method is called when it is collected . But once you dig in, finalization is more complicated than this . When an application creates a new object, the new operator allocates the memory from the heap . If the object s type defines a Finalize method, a pointer to the object is placed on the finalization list just before the type s instance constructor is called . The finalization list is an internal data structure controlled by the garbage collector . Each entry in the list points to an object that should have its Finalize method called before the object s memory can be reclaimed . Figure 21-5 shows a heap containing several objects . Some of these objects are reachable from the application s roots, and some are not . When objects C, E, F, I, and J were created, the system detected that these objects types defined a Finalize method and so added pointers to these objects in the finalization list .
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Core Facilities Roots: Static fields Method parameters Local variables CPU registers
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Finalization list
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Freachable queue
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FIguRE 21-5 The managed heap showing pointers in its finalization list
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Note Even though System.Object defines a Finalize method, the CLR knows to ignore it;
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that is, when constructing an instance of a type, if the type s Finalize method is the one inherited from System.Object, the object isn t considered finalizable . One of the derived types must override Object s Finalize method .
When a garbage collection occurs, objects B, E, G, H, I, and J are determined to be garbage . The garbage collector scans the finalization list looking for pointers to these objects . When a pointer is found, the pointer is removed from the finalization list and appended to the freachable queue . The freachable queue (pronounced F-reachable ) is another of the garbage collector s internal data structures . Each pointer in the freachable queue identifies an object that is ready to have its Finalize method called . After the collection, the managed heap looks like Figure 21-6 . In this figure, you see that the memory occupied by objects B, G, and H has been reclaimed because these objects didn t have a Finalize method . However, the memory occupied by objects E, I, and J couldn t be reclaimed because their Finalize methods haven t been called yet . A special high-priority CLR thread is dedicated to calling Finalize methods . A dedicated thread is used to avoid potential thread synchronization situations that could arise if one of the application s normal-priority threads were used instead . When the freachable queue is empty (the usual case), this thread sleeps . But when entries appear, this thread wakes, removes each entry from the queue, and then calls each object s Finalize method . Because of the way this thread works, you shouldn t execute any code in a Finalize method that makes any assumptions about the thread that s executing the code . For example, avoid accessing thread-local storage in the Finalize method .
21 Roots: Static fields Method parameters Local variables CPU registers
Automatic Memory Management (Garbage Collection)
Managed heap A C D E F I J
Finalization list queue
Freachable queue
FIguRE 21-6 The managed heap showing pointers that moved from the finalization list to the freachable
In the future, the CLR may use multiple finalizer threads . So you should avoid writing any code that assumes that Finalize methods will be called serially . In other words, you will need to use thread synchronization locks if code in a Finalize method touches shared state . With just one finalizer thread, there could be performance and scalability issues in the scenario in which you have multiple CPUs allocating finalizable objects but only one thread executing Finalize methods the one thread might not be able to keep up with the allocations . The interaction between the finalization list and the freachable queue is fascinating . First I ll tell you how the freachable queue got its name . Well, the f is obvious and stands for finalization; every entry in the freachable queue is a reference to an object in the managed heap that should have its Finalize method called . But the reachable part of the name means that the objects are reachable . To put it another way, the freachable queue is considered a root, just as static fields are roots . So if an object is in the freachable queue, the object is reachable and is not garbage . In short, when an object isn t reachable, the garbage collector considers the object to be garbage . Then when the garbage collector moves an object s reference from the finalization list to the freachable queue, the object is no longer considered garbage and its memory can t be reclaimed . As freachable objects are marked, objects referred to by their reference type fields are also marked recursively; all these objects must survive the collection . At this point, the garbage collector has finished identifying garbage . Some of the objects identified as garbage have been reclassified as not garbage in a sense, the object has become resurrected . The garbage collector compacts the reclaimable memory, and the special CLR thread empties the freachable queue, executing each object s Finalize method .
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