public class TestPerson {
    private Injector injector;
    @Before
    public void init(){
        injector = Guice.createInjector(new AbstractModule(){
            @Override
            protected void configure() {
                bind(MessageService.class).to(MockService.class);
            }
        });
    }
    @Test
    public void tetGreetFriend(){
        Person person = injector.getInstance(Person.class);
        person.gretFriends();
        MockService mockService = injector.getInstance(MockService.class);
        assertEquals("Hello my Friend", mockService.msg);
    }
}

java.lang.AssertionError: expected:<hello world> but was:<null>