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Java Singleton Design Pattern : Questions

Asked 2010-08-07T00:10:14.750
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I had an interview recently and he asked me about Singleton Design Patterns about how are they implemented and I told him that using static variables and static methods we can implement Singleton Design Patterns.

He seems to be half satisfied with the answer but I want to know

  1. How many different ways we can implement Singleton Design Pattern in Java ?
  2. What is the scope of Singleton Object and how does it actually work inside JVM ? I know we would always have one instance of Singleton Object but what is the actual scope of that object, is it in JVM or if there are multiple application running than it's scope is per context basis inside the JVM, I was really stumped at this and was unable to give satisfying explanation ?
  3. Lastly he asked if it is possible to used Singleton Object with Clusters with explanation and is there any way to have Spring not implement Singleton Design Pattern when we make a call to Bean Factory to get the objects ?

Any inputs would be highly appreciated about Singleton and what are the main things to keep in mind while dealing with Singletons ?

Thanks.

java spring design-patterns singleton
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1 Answer

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Singleton is commonly implemented by having a static instance object (private SingletonType SingletonType.instance) that is lazily instantiated via a static SingletonType SingletonType.getInstance() method. There are many pitfalls to using singletons, so many, in fact, that many consider singleton to be a design anti-pattern. Given the questions about Spring, the interviewer probably was looking for an understanding not only of singletons but also their pitfalls as well as a workaround for these pitfalls known as dependency injection. You may find the video on the Google Guice page particularly helpful in understanding the pitfalls of singletons and how DI addresses this.

answered 2010-08-07T00:27:24.177

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