Just read the docs:
This exception is thrown when no initial context implementation can be created. The policy of how an initial context implementation is selected is described in the documentation of the InitialContext class.
This exception can be thrown during any interaction with the InitialContext, not only when the InitialContext is constructed. For example, the implementation of the initial context might lazily retrieve the context only when actual methods are invoked on it. The application should not have any dependency on when the existence of an initial context is determined.
But this is explained much better in the docs for InitialContext