How do I unit test a Servlet Filter with jUnit?

Question

Implemented doFilter(). How to properly cover Filter with jUnit ?

public void doFilter(ServletRequest servletRequest, ServletResponse servletRes         


        

Answer 1

ServletRequest, ServletResponse and FilterChain are all interfaces, so you can easily create test stubs for them, either by hand or using a mocking framework.

Make the mock objects configurable so that you can prepare a canned response to getRequestURI() and so that you can query the ServletResponse to assert that sendRedirect has been invoked.

Inject a mock ModeService.

Invoke doFilter passing the mock ServletRequest, ServletResponse and FilterChain as its parameters.

@Test
public void testSomethingAboutDoFilter() {
    MyFilter filterUnderTest = new MyFilter();
    filterUnderTest.setModeService(new MockModeService(ModeService.ONLINE));
    MockFilterChain mockChain = new MockFilterChain();
    MockServletRequest req = new MockServletRequest("/maintenance.jsp");
    MockServletResponse rsp = new MockServletResponse();

    filterUnderTest.doFilter(req, rsp, mockChain);

    assertEquals("/maintenance.jsp",rsp.getLastRedirect());
}

In practice you'll want to move the setup into an @Before setUp() method, and write more @Test methods to cover every possible execution path.

... and you'd probably use a mocking framework like JMock or Mockito to create mocks, rather than the hypothetical MockModeService etc. I've used here.

This is a unit testing approach, as opposed to an integration test. You are only exercising the unit under test (and the test code).