单例/静态会影响多线程Java应用程序的性能吗？

Question

Does use of Singleton / Static slows down Multi-Threaded Application (running on multi-core system)? I was running an application creating 6 threads on 8 core machine. Initially a few singleton objects shared among all the thread. And I was not getting the the expected performance boost for going from single-threaded to multi-threaded model. After this I change my code so that there is no singleton or static (ie tried not to share anything among the threads) and I got a performance boost of approx 6 times. Can somebody please explain the behavior?

Answer 1

1. static has definitely nothing to do with slowdowns. In fact, static fields are the fastest of all to access.

2. Singletons by themselves are not an issue either. You may have lazily initialized singletons, whose accessor methods are synchronized . In this case your code ss just using the wrong idiom. The idiom of choice for static singletons is the lazy initialization holder class , which incurs zero penalty and is completely thread-safe.

3. It may also be the case that the methods you call on singletons need synchronization. Usually, if we are using mutable singletons at all, it is because we have no choice but to use them. If you do have such choice, then by all means avoid singletons.

For optimum performance, uphold these simple rules:

1. share as little mutable state as possible. Mutation needs mutual exclusion and hurts performance;

2. share as much immutable state as possible. This will reduce CPU cache misses and improve performance.

Answer 2

There are many issues with stateful singletons, but what you are describing sounds more like contention for the singleton object. That is, you had a slowdown because your singleton(s) were synchronizing on something. In that case, the multi-threaded version may not be doing the same thing; that is, you may have traded a performance problem for a race condition . You need to show us more detail of your code for a more specific answer. As is, pretty much just guessing.

BTW, IMHO state less singletons are best handled as enum .