@Test
public void testDoFilterInternal_leave_value() throws ServletException, IOException {

    // do setup for test case.
    MDC.put("dummyKey", "dummyValue");
    // do test.
    testTarget.doFilterInternal(mockRequest, mockResponse, mockFilterChain);
    // do assert.
    // not remove other value from MDC.
    assertThat(MDC.getCopyOfContextMap().size(), is(2));
    assertThat(MDC.get("dummyKey"), is("dummyValue"));
    assertThat(MDC.get("key"), is("value"));
}