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Accommodations for failure infiltrate the design of multithreaded programs Concurrency introduces the possibility that one part of a program will fail while others continue But without care, a failing action may leave objects in states such that other threads cannot succeed Methods may throw exceptions (as well as set status indicators or issue notifications) when they have detected that their intended effects or postconditions cannot be attained There are six general responses to such failed actions: abrupt termination, continuation (ignoring failures), rollback, rollforward, retry, and delegation to handlers Abrupt termination and continuation are the two most extreme responses Rollback and roll-forward are intermediate options that ensure that objects maintain consistent states Retries locally contain failure points Delegation allows cooperative responses to failure across objects and activities Choices among these options must be agreed upon and advertised It is sometimes possible to support multiple policies and let client code decide which one to use for example via dialogs asking users whether to retry reading from a disk Additional examples of these options are illustrated throughout this book 3111 Abrupt termination An extreme response to failure is to let a method die immediately, returning (usually via an exception) regardless of the state of the current object or status of the current activity This may apply if you are certain that local failure forces failure of the entire activity and that the objects engaged in the activity will never be used again (for example if they are completely confined within a session see 231) For example, this might be the case in a file-conversion component that fails to open the file to be converted Abrupt termination is also the default strategy for uncaught (and undeclared) RuntimeExceptions, such as NullPointerException, that most often indicate programming errors When a normally recoverable failure cannot be dealt with, you can force more extreme responses by escalating it to a throw of a RuntimeException or Error Short of full program termination (via Systemexit), options for further recovery from such errors are often very limited When objects are intrinsically shared across activities, and there is no way to re-establish consistent object states upon failure, and there is no possible (or practical) way to back out of a failing operation, then the only recourse is to set a broken or corrupted flag in the object encountering the failure and then abruptly terminate Such a flag should cause all future operations to fail until the object is somehow repaired, perhaps via the actions of an error handler object 3112 Continuation If a failed invocation has no bearing on either the state of the caller object or the overall functionality requirements of the current activity, then it may be acceptable just to ignore the exception and continue forward While it is ordinarily too irresponsible to contemplate, this option may apply in event frameworks and oneway messaging protocols (see 41) For example, a failed invocation of a change-notification method on a listener object might at worst cause some parts of an animation sequence to be skipped, without any other long-term consequences
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Continuation policies are also seen within other error handlers (and inside most finally clauses) that ignore other incidental exceptions occurring while they are trying to deal with the failure that triggered them, for example ignoring exceptions while closing files They may also be used in threads that should never shut down, and thus try their best to continue in the face of exceptions 3113 Rollback The most desirable semantics in optimistic designs are clean-fail guarantees: Either the operation succeeds completely, or it fails in a way that leaves the object in exactly the same state as before the operation was attempted The optimistic update techniques in 2442 demonstrate one form of this approach in which the success criterion is lack of interference by other threads trying to perform updates There are two complementary styles for maintaining state representations that can be used in rollbacks: Provisional action Before attempting updates, construct a new representation that will, upon success, be swapped in as the current state Methods perform updates on the tentative new version of the state representations, but do not commit to the new version until success is assured This way, nothing needs to be undone upon failure Checkpointing Before attempting updates, record the current state of the object in a history variable, perhaps in the form of a Memento (see the Design Patterns book) Methods directly perform updates on the current representation But upon failure, fields can be reverted to the old values Provisional action is usually necessary when actions are not otherwise fully synchronized Provisional action eliminates the possibility that other threads will see inconsistent, partially updated representations It is also more efficient when reads are much more common than writes Checkpointing is usually simpler to arrange and is thus often preferable in other situations In either approach, it is not always necessary to create new representation objects to record state: often, a few extra fields in the object, or local variables inside the methods, suffice Situation-specific rollback techniques are needed for actions other than state updates that must be undone upon failure, including actions resulting from sending other messages Every message sent within such a method should have an inverse antimessage For example, a credit operation might be undone via debit This idea can be extended to maintaining undo-lists associated with sequences of actions, in order to allow rollback to any given point Some kinds of operations can neither be provisionally attempted nor undone via antimessages, and thus cannot employ rollback techniques This rules out methods with externally visible effects that irrevocably change the real world by performing IO or actuating physical devices unless it is possible to undo the actions without harm In the case of IO, conventions can be adopted to allow the conceptual equivalent of rollback For example, if methods log actions in a log file and the log file supports a "please disregard log entry XYZ" option, then this can be invoked in case of failure However, as discussed further in 3122, rollback of most IO objects (such as InputStreams) themselves is typically not possible There are no control methods to revert the internal buffers or other fields of most IO objects back to the values they held at some arbitrary point Typically, the best you can do is close the IO objects and construct new ones bound to the same files, devices, or network connections
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3114 Roll-forward When rollback is impossible or undesirable but full continuation is also impossible, you may instead push ahead as conservatively as possible to re-establish some guaranteed legal, consistent state that may be different from the one holding upon entry to the method Roll-forward (sometimes known simply as recovery) is often perfectly acceptable as far as other objects, methods, and threads are concerned; in many cases, they cannot even distinguish it from rollback Some such actions may be placed in finally clauses that perform minimal cleanup (for example closing files, cancelling other activities) necessary to reach safe points of program execution Most roll-forward techniques otherwise take forms similar to rollback techniques But because they do not require full representations of saved or provisional state, they are usually slightly easier to arrange Some methods can be divided into two conceptual parts: a preliminary part that can roll back easily (for example, by either returning or rethrowing the exception immediately), and the part occurring after a point of no return, at which some unrecoverable action has already begun, that must be advanced to a safe point even upon failure For example, a method may reach a point in a protocol at which an acknowledgment must be sent or received (see 3414) 3115 Retry You can contain local failure to the current method, rather than throwing exceptions back to clients, if you have reason to believe that retrying an action will succeed Retries are in general only possible when local rollback options can be applied, so that the state of the object and status of the activity remain the same at the beginning of each retry attempt Retry-based tactics may be used when failure is due to other independent objects that may have been in temporarily bad or undesired states; for example, when dealing with IO devices and remote machines As seen in 2442, optimistic state update methods also typically rely on retries, since interference patterns are extremely unlikely to persist indefinitely Retries are also common in polling designs, for example those discussed in 415 Variants of retries are seen in cascading algorithms that first try the most desirable of several alternative actions, and if that fails, try a series of less desirable alternatives until one succeeds Without care, retries can consume unbounded amounts of CPU time (see 326) You can minimize the likelihood of repeated contention-based failures, as well as reduce CPU wastage, by inserting heuristic delays between attempts One popular strategy (seen for example in Ethernet protocols) is exponential backoff, in which each delay is proportionally longer than the last one For example, you could use the following method to connect to a server that sometimes refuses connections because it is overloaded The retry loop backs off for a longer time after each failure However, it fails upon thread interruption (see 312) since there is no point in continuing if the current thread has been cancelled (As noted in 3122, on some releases of JDK, you may need to modify this to catch InterruptedIOException and rethrow InterrruptedException)
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for (;;) { try { return new Socket(server, portnumber); } catch (IOException ex) { Threadsleep(delayTime); delayTime = delayTime * 3 / 2 + 1; // increase 50% } } }
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3116 Handlers Calls, callbacks, or notifications to error-handling objects can be useful when you need to offload error processing operations to centralized handlers because an exception in one thread or one part of a system requires compensating actions in other threads or other parts of a system that wouldn't otherwise be known to the method catching the exception They can also be used to make code more extensible and more resilient when used by clients that cannot be expected to know how to respond to failures However, some care is needed when replacing exceptions with callbacks, events, and related notification techniques When they escape the stack-based flow-of-control rules of exceptions, their use can make it more difficult to predict and manage responses to failure across different parts of a system One way to set up a handler is to create a before/after class (see 14) that deals with exceptions as its after-action For example, suppose you have an interface describing a service that can throw a ServiceException, and an interface describing handlers for the resulting exceptions Implementations of ServiceExceptionHandler serve here as Strategy objects, as discussed in the Design Patterns book You can then make a proxy for use by clients that do not handle ServiceException themselves For example:
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interface ServerWithException { void service() throws ServiceException; } interface ServiceExceptionHandler { void handle(ServiceException e); } class HandledService implements ServerWithException { final ServerWithException server = new ServerImpl(); final ServiceExceptionHandler handler = new HandlerImpl(); public void service() { // no throw clause try { serverservice(); } catch (ServiceException e) { handlerhandle(e); }
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Note that while it is legal to declare that HandledService implements ServerWithException, all usages that rely on handlers would need to be statically typed to use HandledService, not the generic ServerWithException type
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A handler object can perform any action that any code in a catch clause can, including shutting down processing in one or more threads or starting up other cleanup threads The handler call can also somehow communicate the problem to error handling facilities occurring in a different thread, engage in some interactive protocol, rethrow the exception as a RuntimeException or Error, wrap it in an InvocationTargetException to indicate cascaded failures (see 4331), and so on You can set up services in which clients always use handlers by supplying callback arguments to service methods Callback-based handling may also apply when the service itself does not even know which exception it should throw upon failure This can be set up via:
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interface ServerUsingCallback { void anotherservice(ServiceFailureHandler handler); }
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Here all callers must supply a callback target (which may just be themselves) to be invoked in exceptional situations Further details, alternatives, and variants are discussed in 431 Handlers may also be used when converting one style of messaging protocol to another (see 411) For example, when using event-based frameworks, a service may generate and issue a new ExceptionEvent that is processed by an ExceptionEventListener The following ServiceIssuingExceptionEvent class shows one way to set this up It uses the CopyOnWriteArrayList from 244 for managing lists of handlers Alternatively, the events could be issued asynchronously (see 41)
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class ExceptionEvent extends javautilEventObject { public final Throwable theException; public ExceptionEvent(Object src, Throwable ex) { super(src);
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theException = ex; } } class ExceptionEventListener { // Incomplete public void exceptionOccured(ExceptionEvent ee) { // respond to exception } } class ServiceIssuingExceptionEvent { // Incomplete // private final CopyOnWriteArrayList handlers = new CopyOnWriteArrayList(); public void addHandler(ExceptionEventListener h) { handlersadd(h); } public void service() { // if ( /* failed */ ) { Throwable ex = new ServiceException(); ExceptionEvent ee = new ExceptionEvent(this, ex); for (Iterator it = handlersiterator(); ithasNext();) { ExceptionEventListener l = (ExceptionEventListener)(itnext()); lexceptionOccured(ee); } } } }
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An inverse style of conversion, of events to exceptions, is used in the javabeans package, as described in 364
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