通过lambda和mockito answer提升单元测试效率

背景

在测试Spring RestTemplate的时候，为了规避上下文带来的影响且保证单元测试的效率与独立性，使用mock技术是常用的手法。但是实际项目中，如果要从RestTemplate转为其他框架的时候，就需要修改所有的单元测试代码，也就是说，目前的代码还是依赖于框架的，还是不够独立：

public class SpringCloudRefactoringTest {

    private static final Long EXISTING_VARIA = 1L;
    private static final Map<String, Long> GOOD_HTTP_PARAMS = Collections.singletonMap("id", EXISTING_VARIA);
    private static final Long NON_EXISTING_VARIA = 15123123L;
    private static final Map<String, Long> NON_EXISTING_HTTP_PARAMS = Collections.singletonMap("id", NON_EXISTING_VARIA);
    private static final Long OVER_VARIA = 99999999L;
    private static final Map<String, Long> BAD_HTTP_PARAMS = Collections.singletonMap("id", BAD_VARIA);

    private static final ResponseEntity<SpringCloudResponse> ERROR_RESPONSE =
            new ResponseEntity<>(new SpringCloudFactResponse("NoSuchQuoteException", "No quote with id=15123123."), HttpStatus.OK);
    private static final ResponseEntity<SpringCloudFactResponse> ITEM_RESPONSE =
            new ResponseEntity<>(new SpringCloudFactResponse("success", new SpringCloudFact(1L, "OK")), HttpStatus.OK);

    @Test
    public void serviceShouldReturnFact() {
        RestTemplate restTemplate = mock(RestTemplate.class);
        when(restTemplate.getForEntity(FACT_URL, SpringCloudFactResponse.class, GOOD_HTTP_PARAMS))
                .thenReturn(ITEM_RESPONSE);
        SpringCloudService myServiceUnderTest = new SpringCloudService(restTemplate);

        SpringCloudFact springCloudFact = myServiceUnderTest.retrieveFact(EXISTING_VARIA);

        assertThat(springCloudFact, is(new SpringCloudFact(EXISTING_VARIA, "OK")));
    }

    @Test
    public void serviceShouldReturnNothing() {
        RestTemplate restTemplate = mock(RestTemplate.class);
        when(restTemplate.getForEntity(FACT_URL, SpringCloudFactResponse.class, NON_EXISTING_HTTP_PARAMS))
                .thenReturn(ERROR_RESPONSE);
        SpringCloudService myServiceUnderTest = new SpringCloudService(restTemplate);

        SpringCloudFact springCloudFact = myServiceUnderTest.retrieveFact(NON_EXISTING_VARIA);

        assertThat(springCloudFact, is(nullValue()));
    }

    @Test(expected = ResourceAccessException.class)
    public void serviceShouldPropagateException() {
        RestTemplate restTemplate = mock(RestTemplate.class);
        when(restTemplate.getForEntity(FACT_URL, SpringCloudFactResponse.class, BAD_HTTP_PARAMS))
                .thenThrow(new ResourceAccessException("I/O error"));
        SpringCloudService myServiceUnderTest = new SpringCloudService(restTemplate);

        myServiceUnderTest.retrieveFact(BAD_VARIA);
    }
}

从上面可以看出，单测代码不仅对RestTemplate有了强依赖，且有很多重复的地方。

优化方案

经过阅读说明文档，发现mockito除了常用的thenReturn、thenThrow方法，还有一个thenAnswer方法。这个方法通过接收参数，实现一个通用的接口，而这个通用接口基本上可以返回任何想要的东西。相比于常用的thenReturn、thenThrow方法（只返回固定的值），Answer可以计算返回值，也就是说它和Java 8新特性中的函数式接口java.util.function.Function的功能基本相同T answer(InvocationOnMock invocation) throws Throwable;，唯一不同可能也就是throws了吧。有了个这个方法，我们就可以进一步优化代码了：

public class SpringCloudServiceStepOneTest {

    private static final Long EXISTING_VARIA = 1L;
    private static final Map<String, Long> GOOD_HTTP_PARAMS = Collections.singletonMap("id", EXISTING_VARIA);
    private static final Long NON_EXISTING_VARIA = 15123123L;
    private static final Map<String, Long> NON_EXISTING_HTTP_PARAMS = Collections.singletonMap("id", NON_EXISTING_VARIA);
    private static final Long BAD_VARIA = 99999999L;
    private static final Map<String, Long> BAD_HTTP_PARAMS = Collections.singletonMap("id", BAD_VARIA);

    private static final ResponseEntity<SpringCloudFactResponse> ERROR_RESPONSE =
            new ResponseEntity<>(new SpringCloudFactResponse("NoSuchQuoteException", "No quote with id=15123123."), HttpStatus.OK);
    private static final ResponseEntity<SpringCloudFactResponse> ITEM_RESPONSE =
            new ResponseEntity<>(new SpringCloudFactResponse("success", new SpringCloudFact(1L, "OK")), HttpStatus.OK);

    @Test
    public void serviceShouldReturnFact() {
        RestTemplate restTemplate = restEndpointShouldAnswer(GOOD_HTTP_PARAMS, (invocation) -> ITEM_RESPONSE);
        SpringCloudService myServiceUnderTest = new SpringCloudService(restTemplate);

        SpringCloudFact springCloudFact = myServiceUnderTest.retrieveFact(EXISTING_VARIA);

        assertThat(springCloudFact, is(new SpringCloudFact(EXISTING_VARIA, "OK")));
    }

    @Test
    public void serviceShouldReturnNothing() {
        RestTemplate restTemplate = restEndpointShouldAnswer(NON_EXISTING_HTTP_PARAMS, (invocation -> ERROR_RESPONSE));
        SpringCloudService myServiceUnderTest = new SpringCloudService(restTemplate);

        SpringCloudFact springCloudFact = myServiceUnderTest.retrieveFact(NON_EXISTING_VARIA);

        assertThat(springCloudFact, is(nullValue()));
    }

    @Test(expected = ResourceAccessException.class)
    public void serviceShouldPropagateException() {
        RestTemplate restTemplate = restEndpointShouldAnswer(BAD_HTTP_PARAMS, (invocation -> {throw new ResourceAccessException("I/O error");}));
        SpringCloudService myServiceUnderTest = new SpringCloudService(restTemplate);

        myServiceUnderTest.retrieveFact(BAD_VARIA);
    }

    private RestTemplate restEndpointShouldAnswer(Map<String, Long> httpParams, Answer<ResponseEntity<SpringCloudFactResponse>> response){
        RestTemplate restTemplate = mock(RestTemplate.class);
        when(restTemplate.getForEntity(FACT_URL, SpringCloudFactResponse.class, httpParams)).thenAnswer(response);
        return restTemplate;
    }
}

优化点：
* 可以直接看到HTTP参数与响应的对照关系
* 可以绕过处理逻辑，不需要URL，HTTP方法这些依赖
* 可以将RestTemplate的mock提取成一个通用的方法，单测代码依赖度更低

问题：
* 代码不优美，每个单测中都有RestTemplate

进一步优化

通过@Before解决上述问题：

public class SpringCloudServiceStepTwoTest {

    private static final Long EXISTING_VARIA = 1L;
    private static final Map<String, Long> GOOD_HTTP_PARAMS = Collections.singletonMap("id", EXISTING_VARIA);
    private static final Long NON_EXISTING_VARIA = 15123123L;
    private static final Map<String, Long> NON_EXISTING_HTTP_PARAMS = Collections.singletonMap("id", NON_EXISTING_VARIA);
    private static final Long BAD_VARIA = 99999999L;
    private static final Map<String, Long> BAD_HTTP_PARAMS = Collections.singletonMap("id", BAD_VARIA);

    private static final ResponseEntity<SpringCloudFactResponse> ERROR_RESPONSE =
            new ResponseEntity<>(new SpringCloudFactResponse("NoSuchQuoteException", "No quote with id=15123123."), HttpStatus.OK);
    private static final ResponseEntity<SpringCloudFactResponse> ITEM_RESPONSE =
            new ResponseEntity<>(new SpringCloudFactResponse("success", new SpringCloudFact(1L, "OK")), HttpStatus.OK);

    private RestTemplate restTemplate;
    private SpringCloudService myServiceUnderTest;

    @Before
    public void setUp(){
        restTemplate = mock(RestTemplate.class);
        myServiceUnderTest = new SpringCloudService(restTemplate);
    }

    @Test
    public void serviceShouldReturnFact() {
        restEndpointShouldAnswer(GOOD_HTTP_PARAMS, (invocation) -> ITEM_RESPONSE);

        SpringCloudFact springCloudFact = myServiceUnderTest.retrieveFact(EXISTING_VARIA);

        assertThat(springCloudFact, is(new SpringCloudFact(EXISTING_VARIA, "OK")));
    }

    @Test
    public void serviceShouldReturnNothing() {
        restEndpointShouldAnswer(NON_EXISTING_HTTP_PARAMS, (invocation -> ERROR_RESPONSE));

        SpringCloudFact springCloudFact = myServiceUnderTest.retrieveFact(NON_EXISTING_VARIA);

        assertThat(springCloudFact, is(nullValue()));
    }

    @Test(expected = ResourceAccessException.class)
    public void serviceShouldPropagateException() {
        restEndpointShouldAnswer(BAD_HTTP_PARAMS, (invocation -> {throw new ResourceAccessException("I/O error");}));

        myServiceUnderTest.retrieveFact(BAD_VARIA);
    }

    private void restEndpointShouldAnswer(Map<String, Long> httpParams, Answer<ResponseEntity<SpringCloudFactResponse>> response){
        when(restTemplate.getForEntity(FACT_URL, SpringCloudFactResponse.class, httpParams)).thenAnswer(response);
    }
}

总结

thenAnswer+lambda的方法虽然不是通用所有的case，但还是能解决大部分的单测依赖度高与代码重复较高的问题，根据说明文档，还有restEndpointShouldAnswer这个方法应该可以更进一步简化代码。无论采用什么方法，核心的思想就是AIR（Automatic, Independent, Repeatable）原则，保证了这个原则，才是高质量的单测代码。

参考资料

Answer (Mockito 2.15.0 API) - Javadoc.IO