... I heard I should know at least a framework to get job as fresher...
It vastly depends on the company/product stack you will be hired to work on. Some may require solid core Java knowledge but some in addition (I guess most of them) may need certain framework/library knowledge to get going.
Having said that if you have solid core Java knowledge it would not be that hard to learn Java related framework/library. You can take a one framework and start with basic tutorials (from the documentation/project wiki etc...) and then move to advanced use cases or even internals if you like to master it. Most of the open source frameworks would let you do that.
Choosing which framework to study is mostly up to your choice of problem domain/tech stack.