Kent Beck on Singletons in Java

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Kent Beck on Singletons
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The real problem with Singletons is that they give you such a good excuse not to think carefully about the appropriate visibility of an object Finding the right balance of exposure and protection for an object is critical for maintaining flexibility Massimo Arnoldi and I once worked on a system that had a Singleton for storing exchange rates Every time we wrote a test that handled multiple currencies, we had to make sure to save the old exchange rates, store some new ones, run the test, then restore the old exchange rates One day we got fed up with test mistakes that were caused by using the wrong exchange rates "But the exchange rates are used all over the system!" we whined A good idea is a good idea, though, so we looked at all the places where the exchange rates were used We added parameters as necessary to pass the exchange rates explicitly We thought it would be a huge amount of work, but it only took us half an hour Sometimes it was a little difficult to get the exchange rates to where they were needed, but it was obvious how to refactor to make it easy These refactorings also resolved several chronic design problems that had bothered us but we hadn't known how to tackle The result of the half hour:
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Martin Fowler also acknowledges the need for a few globals, though he uses them only as a last resort His Registry pattern, from Patterns of Enterprise Application Architecture, is a slight variation on Singleton Martin describes a Registry as "a well-known object that other objects can use to find common objects and services" [Fowler, PEAA, 480] Regarding when to use this pattern, he writes:
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There are alternatives to using a Registry One is to pass around any widely needed data in parameters The problem with this is that parameters are added to method calls when they aren't needed by the called method but only by some other method that's called several layers deep in the call tree Passing a parameter around when it's not needed 90 percent of the time is what leads me to use a Registry instead So there are times when it is right to use a Registry, but remember that any global data is always guilty until proven innocent [Fowler, PEAA, 482 483] My rather close reading of Design Patterns [DP] infected me with Singletonitis Every pattern in that book contains a Related Patterns section, and in many of those sections you'll find sentences that mention Singleton For example, in the section on the State pattern, the authors write, "State objects are often Singletons" [DP, 313], and in the section on the Abstract Factory pattern, they write, "A concrete factory is often a Singleton" [DP, 95] In defense of the authors, these sentences simply observe that State and Abstract Factory classes often are Singletons The book doesn't say they have to be If there is a good reason to make a class a Singleton or Registry, do so The refactoring Limit Instantiation with Singleton (296) describes a good reason to refactor to a Singleton: real performance improvement It also cautions against premature optimization One thing is certain: you need to think and explore really hard before you implement a Singleton And if you encounter a Singleton that shouldn't be a Singleton, by all means inline it!
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+ + Makes object collaborations more visible and explicit Requires no special code to protect a single instance Complicates a design when passing an object instance through many layers is awkward or difficult
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