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Another difference between .NET and Java is that the methods suspend and resume are available for use in a C# application. In Java, these methods (and the stop method) have been deprecated because they are considered inherently unsafe. While the same potential problems with the Suspend and Resume methods exist in a .NET application, Microsoft decided not to eliminate this functionality but rather to allow developers to take their own precautions. It's recommended that Java threads be controlled by modifying a variable that is periodically checked during execution, a process known as polling. This is commonly achieved by setting the thread reference to null and checking that the reference matches the current thread during iteration:
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13. Threading and Synchronization private volatile Thread o_blinker; public void stop() { o_blinker = null; } public void run() { Thread x_this_thread = Thread.currentThread(); while (o_blinker == x_this_thread) { // perform some operations } }
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Besides reducing flexibility and introducing complexity, this approach has one major consequence: for methods that take a long time to complete, the responsiveness of the thread is dependent on the frequency with which the thread reference is checked. The programmer has to decide between checking the state frequently and accepting that the call to stop might not result in the thread being halted immediately. By contrast, .NET supports almost immediate thread control using the Suspend and Resume methods. These methods work as their names indicate. Resuming a thread that has been suspended causes execution to continue from the point at which it stopped. Watch out for the exceptions that can be thrown; it's easy to forget about them since C# doesn't enforce declaring exceptions on method signatures. Both methods will throw a ThreadStateException if the thread hasn't been started or because the thread is dead. In addition, Resume will throw this exception if the thread hasn't been suspended. Checking the thread state before making one of these calls can help minimize these exceptions. See the "Thread States" section later in this chapter. Also note that these calls will throw a SecurityException if the caller doesn't have the required permission. See 17, "Security and Cryptography," for more information on security.
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The .NET Thread class provides an overloaded method, named Abort, for stopping the execution of a thread. Calling this method causes a ThreadAbortException to be raised in the method that was passed to the ThreadStart instance when the thread was instantiated. There are two overloads on this method:
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public void Abort(); public void Abort(object);
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The first version takes no parameters, causes the exception to be thrown, and begins the process of terminating the thread. The second overload takes an object as an argument, which is made available through the exception, and can be used to tidy up. It's possible to catch a ThreadAbortException, but this doesn't prevent the thread from being killed. The exception will be thrown again at the end of the try catch block, but only after any finally blocks have been executed. This presents an opportunity to clean up any incomplete state and to ensure that any resources are correctly released.
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13. Threading and Synchronization
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public void run() { try { while (true) { try { // do some important operation that can result // in state problems if stopped suddenly } catch (ThreadAbortException) { Console.WriteLine("Got inner abort exception."); } finally { Console.WriteLine("Tidy up."); } } } catch (ThreadAbortException) { Console.WriteLine("Got outer abort exception."); } }
The results of passing a method into a ThreadStart delegate and then calling Abort on the running thread are shown below:
Got inner abort exception. Tidy up. Got outer abort exception.
The opportunity to tidy up occurs at each point in the code where the ThreadAbortException is caught. If the exception is not caught, it will unwind up the stack to the thread class instance and will not be seen again. The try catch finally block is not required since there is no state to maintain and notifications about thread deaths are not required. Although calling Abort seems neater and more intuitive than the Java approach detailed earlier, there is no way to determine where the exception will be thrown. It falls to the developer to ensure that no unusual state is left behind if, for example, the exception is thrown during iteration. Although calls to Abort usually result in threads being stopped, the CLR makes no guarantees. An unbounded computation (one that will never complete) can be started in the catch block for ThreadAbortException. For this reason, it's important to use the catch block only as intended and not to perform further calculations. If the caller of the Abort method needs to ensure that the thread has stopped, the Join method can be used, which will block until the thread has been unraveled and stopped. Another reason that a thread might not stop is that Thread.ResetAbort might be called in the catch statement. This doesn't stop the exception from being thrown again but does prevent the thread from terminating. This method should be used with care because any other threads blocking through the Join method won't return as expected. Time should be taken to consider why a thread should refuse to die when asked.
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