The Singleton pattern we discussed earlier contains two good examples of theSingle Thread Execution pattern. The problem motivating this pattern first arisesbecause this example uses lazy instantiation -- delaying instantiating until necessary-- thereby creating the possibility that two different threads may callgetInstance() at the same time:public static synchronized Sequence getInstance(){if(instance==null) // Lazy instantiation{instance = new Sequence();}return instance;}If this method were not protected against simultaneous access with synchronized,each thread might enter the method, test and find that the static instance referenceis null, and each might try to create a new instance. The last thread to finish wins,overwriting the first thread's reference. In this particular example, that might not beso bad -- it only creates an orphaned object that garbage collector will eventuallyclean up -- but had there been a shared resource that enforced single access, suchas opening a port or opening a log file for read/write access, the second thread'sattempt to create an instance would have failed because the first thread would have
