@Test
public void compare() throws Exception {

    assertThat(PriorityServiceComparator.getInstance().compare("a", "b")==0).isTrue();
    assertThat(PriorityServiceComparator.getInstance().compare(getClass(), getClass())==0).isTrue();
    assertThat(PriorityServiceComparator.getInstance().compare(new A(), new SystemPropertySource())==-1).isTrue();
    assertThat(PriorityServiceComparator.getInstance().compare(new SystemPropertySource(), new A())==1).isTrue();
}