I have some java classes in which the members variables are all Autowired using Spring beans. I think that guarantees the singleton pattern. But I wonder how does the code run on a production server? Is singleton guaranteed per thread or is it guaranteed globally? How should I think about concurrency in development.
EDIT
An example: A class the takes requests and response the number of requests it has received. In this case we need to guarantee the global singleton of the counter, right? Say no matter how many servers we use, there should only be one counter. Is this guarantee implicitly?
Scope of classic singletons and Spring singletons
Singleton generally means singleton for the whole application running on a JVM.
That is the case for Java classic implementations where you implement it yourself (singleton double check idiom, enum idiom, initialization-on-demand holder idiom, and so for).
In these cases, the singleton is indeed created on the classloading of the singleton class, so a single time by the JVM.
Spring singletons work a little differently.
They indeed depend on the container. So, if you create multiple containers in a same JVM, you may create multiple singletons. Now it is really a corner case and besides these singleton beans are isolated between. So don't focus on it.
About concurrency
Singletons don't have any concurrency issue while these are immutable.
Of course, you can define dependencies and properties in a singleton.
But these should not change after the singleton instantiation.
Indeed if a singleton provides methods that allow to change its state, you are bound to have race conditions for distinct threads that manipulate that.
So as a hint, keep your singletons immutable or as immutable as you can.
If these cannot be completely immutable, you have to ensure that the race conditions are handled by synchronizing the methods/fields that need to.