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In ASP.NET the default lifecycle of objects created in IoC container is per web request. While learning SpringBoot + Webflux, I found that the default lifecycle (Bean, Repository, Service, etc) created by the IoC container is singleton. I know I can change the default scope like this:

@Scope("prototype")

but I have not yet found an example where it would be used. So if IoC creates all object as singletons, how come there are no problems with concurrency. Can someone please explain this to me.

Michal
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    spring also has request scoped objects which are useful for classes like RequestContext ie. for storing things about the request/response, Singletons are generally used for stateless classes and as a result do not have issues with concurrency. They perform logic based on the objects passed to them. Try and think of singletons as stateless and protoypes as stateful. Quite often the objects passed to singleton bean methods will be stateful protoypes. – Michael McFadyen Sep 02 '20 at 20:59

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It's a good question. Generally speaking where concurrency is an issue, for example a transaction context in the database layer, springbook uses a thread based locking mechanism. See for example 1.2. Understanding the Spring Framework Transaction Abstraction. Otherwise, yes, anything injected with CDI is a singleton unless specifically annotated otherwise. That means that you should not keep state variables in your @Component or @Service classes. As long as the methods use only parameters passed in or variable local to the method concurrency isn't an issue because ever variable is created on the stack which is unique for each call. I have seen an application work great up until the day two people log in at once.

If you have to have a class with state variables you need to do a new of that class.

Each spring-context is created with a unique thread, so where objects are created or injected that are not stateless then state information is attached to the spring-context which runs in its own thread.

See also How does Spring bean Handle concurrency

K.Nicholas
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