blocks|key|2074810|text|这个讨论有帮助吗？(我不知道链接到另一个编程论坛是不是禁忌，但我不想引用整个讨论=)|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2074811|Sun+Discussion+on+this+subject|offset|length|2074812|结论似乎是，在大多数情况下，它不会产生足够的影响，尽管从技术上讲，静态方法更有效。|2074813|entityMap|0|LINK|mutability|MUTABLE|url|http://forums.sun.com/thread.jspa?threadID=5221231&messageID=9898827^0|0|0|U|0|0|0^^$0|@$1|2|3|4|5|6|7|P|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|Q|8|@]|9|@$D|R|E|S|1|T]]|A|$]]|$1|F|3|G|5|6|7|U|8|@]|9|@]|A|$]]|$1|H|3|-4|5|6|7|V|8|@]|9|@]|A|$]]]|I|$J|$5|K|L|M|A|$N|O]]]]

Does this discussion help? (I don't know if it's taboo to link to another programming forum, but I'd rather not just quote the whole discussion =) )

<a href="http://forums.sun.com/thread.jspa?threadID=5221231&amp;messageID=9898827" rel="nofollow noreferrer">Sun Discussion on this subject</a>

The verdict seems to be that it doesn't make enough of a difference to matter in most cases, though technically the static methods are more efficient.

blocks|key|2074883|text|从严格的运行时性能角度来看，差异实际上可以忽略不计。两者之间的主要区别在于“静态”生命周期是链接到类加载器的，而对于单例，它是一个常规的实例生命周期。通常情况下，最好远离ClassLoader业务，避免一些棘手的问题，特别是当您尝试重新加载web应用程序时。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2074884|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

From a strict runtime performance point of view, the difference is really negligible. The main difference between the two lies down in the fact that the "static" lifecycle is linked to the classloader, whereas for the singleton it's a regular instance lifecycle. Usually it's better to stay away from the ClassLoader business, you avoid some tricky problems, especially when you try to reload the web application.

blocks|key|1861102|text|如果需要存储任何状态，我会使用单例，否则使用静态类。实例化一些东西是没有意义的，即使是一个单独的实例，除非它需要存储一些东西。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1861103|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

I would use a singleton if it needed to store any state, and static classes otherwise. There's no point in instantiating something, even a single instance, unless it needs to store something.

blocks|key|1861145|text|根据我的经验，唯一重要的是哪一个更容易在单元测试中模拟。我一直觉得Singleton更容易模仿，也更自然。如果您的组织允许您使用JMockit，这无关紧要，因为您可以克服这些顾虑。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1861146|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

From my experience, the only thing that matters is which one is easier to mock in unit tests. I always felt Singleton is easier and natural to mock out. If your organization lets you use JMockit, it doesn't matter since you can overcome these concerns.

blocks|key|1861195|text|静态不利于可扩展性，因为静态方法和字段不能被子类扩展或覆盖。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1861196|这也不利于单元测试。在单元测试中，您无法避免不同测试的副作用溢出，因为您无法控制类加载器。在一个单元测试中初始化的静态字段将在另一个单元测试中可见，或者更糟糕的是，并发运行测试将产生不可预测的结果。|1861197|在谨慎使用时，Singleton通常是一个可以接受的模式。我更喜欢使用DI框架，让它为我管理我的实例(可能在不同的作用域中，比如Guice)。|1861198|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|H|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|I|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|J|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|G|$]]

Static is bad for extensibility since static methods and fields cannot be extended or overridden by subclasses. 

It's also bad for unit tests. Within a unit test you cannot keep the side effects of different tests from spilling over since you cannot control the classloader. Static fields initialized in one unit test will be visible in another, or worse, running tests concurrently will yield unpredictable results.

Singleton is generally an ok pattern when used sparingly. I prefer to use a DI framework and let that manage my instances for me (possibly within different scopes, as in Guice).

blocks|key|1861305|text|编写一些代码来测量性能。答案将取决于JVM(Sun的执行可能与JRockit不同)和您的应用程序使用的VM标志。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1861306|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

Write some code to measure the performance. The answer is going to be dependent on the JVM(Sun's JDK might perform differently than JRockit) and the VM flags your application uses.

I am merging a CVS branch and one of the larger changes is the replacement wherever it occurs of a Singleton pattern with abstract classes that have a static initialisation block and all static methods.

Is this something that's worth keeping since it will require merging a lot of conflicts, what sort of situation would I be looking at for this refactoring to be worthwhile?

We are running this app under Weblogic 8.1 (so JDK 1.4.2)

<hr>

sorry Thomas, let me clarify..

the HEAD version has the traditional singleton pattern (private constructor, getInstance() etc)

the branch version has no constructor, is a 'public abstract class' and modified all the methods on the object to be 'static'. The code that used to exist in the private constructor is moved into a static block.

Then all usages of the class are changed which causes multiple conflicts in the merge.

There are a few cases where this change was made.

Java Singleton vs static - is there a real performance benefit?

单元测试

我正在合并一个CVS分支，其中一个较大的变化是用具有静态初始化块和所有静态方法的抽象类替换出现的Singleton模式。这是值得保留的东西吗?因为它需要合并许多冲突，我应该考虑什么样的情况才是值得进行这种重构的？我们在Weblogic 8.1 (因此JDK 1.4.2)下运行此应用程序。对不起，托马斯，让我澄清一下.....

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