Use some mocking framework e.g. Mockito or JMock which comes with mocking capacity of such objects.
In Mockito, you can do mocking as:
HttpServletRequest mockedRequest = Mockito.mock(HttpServletRequest.class);
For details on Mockito, see: How do I drink it? on the Mockito site.
In JMock, you can do mocking as :
Mockery context = new Mockery();
HttpServletRequest mockedRequest = context.mock(HttpServletRequest.class);
For details on jMock, please refer: jMock – Getting Started