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performing operation sequences atomically Thread-safe classes, however, may be protected from this attack by the use of the private lock object idiom Using internal objects for locking is particularly suited to classes designed for inheritance (Item 15) such as the WorkQueue class in Item 49 If the superclass were to use its instances for locking, a subclass could unintentionally interfere with its operation By using the same lock for different purposes, the superclass and the subclass could end up stepping on each others' toes To summarize, every class should clearly document its thread-safety properties The only way to do this is to provide carefully worded prose descriptions The synchronized modifier plays no part in documenting the thread safety of a class It is, however, important for conditionally thread-safe classes to document which object must be locked to allow sequences of method invocations to execute atomically The description of a class's thread safety generally belongs in the class's documentation comment, but methods with special threadsafety properties should describe these properties in their own documentation comments
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Item 53: Avoid thread groups
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Along with threads, locks, and monitors, a basic abstraction offered by the threading system is thread groups Thread groups were originally envisioned as a mechanism for isolating applets for security purposes They never really fulfilled this promise, and their security importance has waned to the extent that they aren't even mentioned in the seminal work on the Java 2 platform security model [Gong99] Given that thread groups don't provide any security functionality to speak of, what functionality do they provide To a first approximation, they allow you to apply Thread primitives to a bunch of threads at once Several of these primitives have been deprecated, and the remainder are infrequently used On balance, thread groups don't provide much in the way of useful functionality In an ironic twist, the ThreadGroup API is weak from a thread safety standpoint To get a list of the active threads in a thread group, you must invoke the enumerate method, which takes as a parameter an array large enough to hold all the active threads The activeCount method returns the number of active threads in a thread group, but there is no guarantee that this count will still be accurate once an array has been allocated and passed to the enumerate method If the array is too small, the enumerate method silently ignores any extra threads The API to get a list of the subgroups of a thread group is similarly flawed While these problems could have been fixed with the addition of new methods, they haven't been fixed because there is no real need; thread groups are largely obsolete To summarize, thread groups don't provide much in the way of useful functionality, and much of the functionality they do provide is flawed Thread groups are best viewed as an unsuccessful experiment, and you may simply ignore their existence If you are designing a class that deals with logical groups of threads, just store the Thread references comprising each logical group in an array or collection The alert reader may notice that this advice appears to contradict that of Item 30, Know and use the libraries In this instance, Item 30 is wrong
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There is a minor exception to the advice that you should simply ignore thread groups One small piece of functionality is available only in the ThreadGroup API The ThreadGroupuncaughtException method is automatically invoked when a thread in the group throws an uncaught exception This method is used by the execution environment to respond appropriately to uncaught exceptions The default implementation prints a stack trace to the standard error stream You may occasionally wish to override this implementation, for example, to direct the stack trace to an application-specific log
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