How to Junit test servlet-filters [duplicate]

Question

Is there anyway to unit test a filter as below?

public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain) throws IOException,
    ServletException {
    HttpServletRequest request = (HttpServletRequest) req;
    HttpServletResponse response = (HttpServletResponse) res;
    String param = request.getParameter("param");

    if (param == null) {
        response.sendError(400, "param can't be null");
        return;
    }

Answer 1

Yes, there is, here is one way. Here I am mocking HttpServletRequest and HttpServletResponse and asserting on methods call by defining expectations.Here I am using JMock.

public class SampleFilterTest {


  private Mockery context = new Mockery();

  private SampleFilter sampleFilter = new SampleFilter();
  private HttpServletRequest request;
  private HttpServletResponse response;


  @BeforeMethod
  public void setUp() throws Exception {
    request = context.mock(HttpServletRequest.class);
    response = context.mock(HttpServletResponse.class);
  }


  @Test
  public void sampleFilterTest() throws IOException, ServletException {
    context.checking(new Expectations() {{
      oneOf(request).getParameter("param");
      will(returnValue(null));
      oneOf(response).sendError(400, "param can't be null");
    }});
    sampleFilter.doFilter(request, response, null);
  }
}

Here I am saying when request.getParmeter is called with "param" then return null, and response.sendError, with 400, and "param can't be null" parameters,should be called.