@Test
public void testOnAddListener() {

  ServletContextDescriptor scd = new ServletContextDescriptor(servletContext);
  when(servletContext.getAttribute(Attributes.SERVLET_CONTEXT_DESCRIPTOR)).thenReturn(scd);
  sessionHelpers.onAddListener(servletContext, "Dummy");
  assertTrue(scd.getHttpSessionListeners().isEmpty());
  assertTrue(scd.getHttpSessionIdListeners().isEmpty());
  assertTrue(scd.getHttpSessionAttributeListeners().isEmpty());
  HttpSessionListener listener = mock(HttpSessionListener.class);
  HttpSessionIdListener idListener = mock(HttpSessionIdListener.class);
  HttpSessionAttributeListener attributeListener = mock(HttpSessionAttributeListener.class);
  HttpSessionListener multiListener = mock(HttpSessionListener.class,
      withSettings().extraInterfaces(HttpSessionAttributeListener.class));
  HttpSessionAttributeListener attributeMultiListener = (HttpSessionAttributeListener)multiListener;
  sessionHelpers.onAddListener(servletContext, listener);
  assertThat(scd.getHttpSessionListeners(), hasItem(listener));
  assertTrue(scd.getHttpSessionIdListeners().isEmpty());
  assertTrue(scd.getHttpSessionAttributeListeners().isEmpty());
  sessionHelpers.onAddListener(servletContext, idListener);
  assertThat(scd.getHttpSessionListeners(), hasItem(listener));
  assertThat(scd.getHttpSessionIdListeners(), hasItem(idListener));
  assertTrue(scd.getHttpSessionAttributeListeners().isEmpty());
  sessionHelpers.onAddListener(servletContext, attributeListener);
  assertThat(scd.getHttpSessionListeners(), hasItem(listener));
  assertThat(scd.getHttpSessionIdListeners(), hasItem(idListener));
  assertThat(scd.getHttpSessionAttributeListeners(), hasItem(attributeListener));
  sessionHelpers.onAddListener(servletContext, multiListener);
  assertThat(scd.getHttpSessionListeners(), hasItem(listener));
  assertThat(scd.getHttpSessionListeners(), hasItem(multiListener));
  assertThat(scd.getHttpSessionIdListeners(), hasItem(idListener));
  assertThat(scd.getHttpSessionAttributeListeners(), hasItem(attributeListener));
  assertThat(scd.getHttpSessionAttributeListeners(), hasItem(attributeMultiListener));
}