Be Careful When Implementing Singletons in Multi-Threaded Environments

Singletons are builder design patterns where the number of instances is limited to one (according to the Gang of Four – DesignPatterns). The most common way of implementing them is as follows:

     public class Singleton{        private static Singleton theInstance = null;        protected Singleton(){        }        public static Singleton getInstance(){            if (theInstance == null)                theInstance = new Singleton();            return theInstance;        }    }

This works fine in a single threaded environment, but this code is unsafe for multithreaded environments. There can be multiple threads accessing getInstance() at the same time which can end up creating multiple objects, even though the intent is a one instance limit.

To fix this, synchronized the getInstance method to ensure that only one thread will be accessing the method at a time. Changing the getInstance() to as described below will make it safe to work in multi threaded environments.

     public static synchronized Singleton getInstance(){        if (theInstance == null)            theInstance = new Singleton();        return theInstance;    }

Synchronizing getInstance does have a performance penalty, since all threads should now get the lock and release the lock. But synchronization is the only safe solution, since Java does not have a clean memory model (this is the same case in C++).

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