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Do Java classes have an instance on machine (JVM) level if they contain only static methods and fields?

And if yes, what are the effects of static methods and fields when doing multithreading? Any rules of thumb?

Marcus
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    Your question is unclear: a static class behaves just as a "normal" class, except that it has been nested into another one. – assylias Feb 07 '14 at 16:29
  • Make sure static classes are thread-safe, if you need it to be thread-safe? Not exactly sure what you're looking for. And what do you mean by a "static class"? Do you really just mean a class with only static methods? – Dave Newton Feb 07 '14 at 16:29
  • static (embedded) classes are just classes. The non-static embedded classes are special: those have a second (third) this of the outer class(es). "static" here has a different meaning than other uses. – Joop Eggen Feb 07 '14 at 16:30
  • @Marcus If you really meant static classes (as opposed to static method/field), then the answer is that it makes no difference vs. non static, top-level classes. – assylias Feb 07 '14 at 16:36
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    Allright, I changed the question, hope it's clear now... – Marcus Feb 07 '14 at 16:42

4 Answers4

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Static methods and variables are at the class level in Java, not instance level.

All shared, writable state needs to be synchronized and thread safe, regardless of static or instance.

duffymo
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Yes, for each loaded class in the JVM there is an instance of java.lang.Class. It does not matter whether they only contain static methods/fields or instance methods/fields as well.

And this does not have any extra impact on multi-threading beyond what instance fields and methods already have. That is, as long as you realise that the value of a static field is shared between all instances. If you want to synchronize, you need to synchronize on the java.lang.Class instance for the class (or if the method is a static method inside said class, it can have the 'synchronized' modifier on the static method to have the same effect as synchronizing on the java.lang.Class instance for the class).

One extra thing to note is that a class with the same name can be loaded by more than one classloader at the same time in the JVM- hence classes in Java are not uniquely identifier by their fully-qualified name, instead they are uniquely identifier by the combination of java.lang.ClassLoader instance used to load the class, and the fully-qualified name.

Erwin Bolwidt
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There are no such thing as "static classes" in java. There are inner static classes, but i presume that your question its not about this type of classes.

Classes are loaded once per classloader not per Virtual Machine, this is an important diference, for example applications server like tomcat have different classloaders per application deployed, this way each application is independent (not completely independent, but better than nothing).

The effects for multithreading are the effects of shared data structures in multithreading, nothing special in java. There are a lot of books in this subject like http://www.amazon.com/Java-Concurrency-Practice-Brian-Goetz/dp/0321349601 (centered in java) or http://pragprog.com/book/pb7con/seven-concurrency-models-in-seven-weeks (that explain difference concurrency models, really interesting book)

AlfredoCasado
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Yes, a class with static fields and methods has exactly one instance, which is accessed through a static call.

If you use static methods, the variables declared within the method are isolated and don't need to be synchronized (the same as in C#: C# : What if a static method is called from multiple threads?).

But when your classes have static variables and you access them within a static method, there is a trade off: When doing multithreading, the access to the static variables must be synchronized. That's the reason why the singleton pattern doesn't work as good as some believe: Instantiation costs, even more if it's only singlethreaded.

Rules of thumb? Static methods with no static class variables are always good. But static classes with variables can become very evil when doing multithreading. Therefore beware of static bottlenecks!

Community
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Marcus
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