public void add(final ProxyBackendHandler handler) {
        backendHandlers.add(handler);
    }
@Test
    void assertAddDatabaseConnector() {
        ProxyBackendHandler expectedEngine = mock(DatabaseConnector.class);
        databaseConnectionManager.add(expectedEngine);
        Collection<ProxyBackendHandler> actual = getBackendHandlers();
        assertThat(actual.size(), is(1));
        assertThat(actual.iterator().next(), is(expectedEngine));
    }