How to write a jar that is framework agnostic?

I have to write a client API(jar) in java for a web service that is to work with any framework used by the service consumer. And I came across the term 'Framework-Agnostic' in this scenario.

Please explain what it means and how to code in a way that makes a code framework independent( meaning we have n problem calling the code from Spring, struts or any custom framework). Personally I dont get how ordinary(normal) code can be specific to one framework when we are just calling methods in a jar. Maybe language-independent makes more sense in this context, but not this(and we are using only java).