Make sure spring component is stateless

MutabilityDetector seems able to do exactly what you need:

Mutability Detector is designed to analyse Java classes and report on whether instances of a given class are immutable. It can be used:

• In a unit test, with an assertion like assertImmutable(MyClass.class). Is your class actually immutable? What about after that change you just made?
• As a FindBugs plugin. Those classes you annotated with @Immutable, are they actually? At runtime. Does your API require being given immutable objects? From the command line. Do you want to quickly run Mutability Detector over an entire code base?

I would anyway advise to add a clear contract stating that the class is supposed to be immutable either via javadoc or via @Immutable annotation on the class itself, to allow (sensible) developers to maintain the class requisites. (In case Mutability Detector fails to detect specific types of immutability eg: Are String, Date really immutable?)