Sunday, February 9, 2014

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I am learning Spring framework and currently reading a book about it. In this book it says that a Spring singleton is different from a Java singleton? What does this mean and what are the differences? Thanks
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You already have a good understanding of the difference from other response, but here is my two cents on why one might not want a Java singleton - testability. When you are writing unit tests, would it cause problems that the singleton accumulates state information? –  Ustaman Sangat Mar 7 '13 at 17:53
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marked as duplicate by C. RossJayanataylorGodekeAndrew AylettMar 6 '13 at 17:25

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The Java singleton is scoped by the Java class loader, the Spring singleton is scoped by the container context.
Which basically means that, in Java, you can be sure a singleton is a truly a singleton only within the context of the class loader which loaded it. Other class loaders should be capable of creating another instance of it (provided the class loaders are not in the same class loader hierarchy), despite of all your efforts in code to try to prevent it.
In Spring, if you could load your singleton class in two different contexts and then again we can break the singleton concept.
So, in summary, Java considers something a singleton if it cannot create more than one instance of that class within a given class loader, whereas Spring would consider something a singleton if it cannot create more than one instance of a class within a given container/context.
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As we all know we have beans as singleton by default in Spring container and if we have a web application based on Spring framework then in that case do we really need to implement Singleton design pattern to hold global data rather than just creating a bean through spring.
Please bear with me if I'm not able to explain what I actually meant to ask.
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6 Answers

Singleton bean in Spring and Singleton pattern is quite different. Singleton pattern says that one and only one instance of a particular class will ever be created per classloader.
THe scope of Spring singleton is described as per container per bean. It is the scope of bean definition to a single object instance per Spring IoC container. The default scope in Spring in Singleton.
Eventhough the default scope is singleton, you can change the scope of bean by specifying the scope attribute of  element.
<bean id=".." class=".." scope="prototype" />
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Thanks, this really clears all my doubts. –  Peeyush Apr 15 '10 at 13:33
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@user184794 : per container per bean ,meaning there is only one classloader in spring container.if there are two or more classloader in spring container , then each classloader will have own instance. Does it mean "per container per classloader per bean". kindly clarify!! –  Suresh Sankar Jul 28 '11 at 10:59
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