Design Patterns and Best Practices in Java

Synchronized singletons

The code for synchronized singletons is simple and efficient, but there is a situation we should take into consideration. If we use our code in a multithreading application, it may be the case that two threads invoke the getInstance method at the same time when the instance is null. When this happens, it may be the case that the first thread proceeds to instantiate the singleton using the new operator, and, before finishing it, the second thread checks whether the singleton is null. Since the first thread didn't finish instantiating it, the second thread will find that the instance is null, so it will start instantiating it too.

This scenario may seem almost impossible, but if it takes a long time to instantiate the ...