How to Junit test servlet filter which has specific response

Question

What is the best way to unit test this code? I need to establish a consistent check for httpResponse which sendError() when condition is true. Thanks in advance!

Edit: Unfortunately, this filter is not with Spring MVC so my choice is limited.

    public void doFilter(ServletRequest request, ServletResponse response,
        FilterChain filterchain) throws IOException, ServletException {

    String ipAddress = request.getRemoteAddr();
    if( SomeParameterCheckingFunction ((request)) ) {
        logger.error("Error detected! Time: " + new Date().toString() + ", Originating IP: " + ipAddress);
        if (response instanceof HttpServletResponse){
            HttpServletResponse httpResponse = (HttpServletResponse) response;
            httpResponse.sendError(HttpServletResponse.SC_FORBIDDEN,"You are not allowed to access the server!");
        }
    }
    filterchain.doFilter(request, response);
}

Solution

For example when using the Mockito mock framework, the provided doFilter() method could be JUnit tested using below test case:

@Test
public void testDoFilter() throws IOException, ServletException {
    // create the objects to be mocked
    HttpServletRequest httpServletRequest = mock(HttpServletRequest.class);
    HttpServletResponse httpServletResponse = mock(HttpServletResponse.class);
    FilterChain filterChain = mock(FilterChain.class);
    // mock the getRemoteAddr() response
    when(httpServletRequest.getRemoteAddr()).thenReturn("198.252.206.16");

    AccessFilter accessFilter = new AccessFilter();
    accessFilter.doFilter(httpServletRequest, httpServletResponse,
            filterChain);

    // verify if a sendError() was performed with the expected values
    verify(httpServletResponse).sendError(HttpServletResponse.SC_FORBIDDEN,
            "You are not allowed to access the server!");
}

Comments

[hb5fa]

I already have another unit test against the SomeParameterCheckingFunction() so I can easily mock it with Mockito. Having the ability to verify response.sendError() will be an ideal test case but I am completely stumped on how to do that. Can you show me the technique on how I might be able to verify response.sendError() in junit after the filter is called? Tnx!