JUnit测试遇AssertionError：预期<1>实际<30>，求错误排查方案

Hey there! Let's break down why you're hitting this assertion error and fix it step by step.

问题回顾 #

作为JUnit和Mockito新手，你在测试GitHub用户列表获取代码时遇到了这个异常：

java.lang.AssertionError: expected:<1> but was:<30>

对应的堆栈信息：

at org.junit.Assert.fail(Assert.java:88)
at org.junit.Assert.failNotEquals(Assert.java:834)
at org.junit.Assert.assertEquals(Assert.java:645)
at org.junit.Assert.assertEquals(Assert.java:631)

原业务代码 #

public List<GithubModel> getGitHubUsersList(){
    HttpHeaders httpHeaders = new HttpHeaders();
    HttpEntity<GithubModel>httpEntity = new HttpEntity<GithubModel>(httpHeaders);
    ResponseEntity<List<GithubModel>>usersList = restTemplate.exchange("https://api.github.com/repos/git/git/contributors", HttpMethod.GET,httpEntity, new ParameterizedTypeReference<List<GithubModel>>() { });
    return usersList.getBody();
}

测试用例代码 #

@Spy
@InjectMocks
MicroServicesTest1 microServicesTest1;
@Mock
RestTemplate mockRestTemplate;
@Before
public void create(){
    MockitoAnnotations.initMocks(this);
}
@Test
public void getGitHubUsersListTest(){
    ResponseEntity<List<GithubModel>> response = Mockito.mock(ResponseEntity.class);
    Mockito.when(mockRestTemplate.exchange(Mockito.anyString(), Mockito.any(HttpMethod.class), Mockito.any(HttpEntity.class), Mockito.eq(new ParameterizedTypeReference<List<GithubModel>>(){}))).thenReturn(response);
    Mockito.when(response.getBody()).thenReturn(getUsersList());
    List<GithubModel> mocklist = microServicesTest1.getGitHubUsersList();
    assertEquals(1, mocklist.size());
}
/**
 * getUsersList
 * @return
 */
public List<GithubModel>getUsersList(){
    GithubModel githubModel = new GithubModel();
    githubModel.setId(1);
    githubModel.setLogin("hello1");
    githubModel.setNode_id("hello2");
    List<GithubModel>list = new ArrayList<>();
    list.add(githubModel);
    return list;
}

问题根源 #

核心问题出在**Mockito.eq(new ParameterizedTypeReference<List<GithubModel>>(){})**这个匹配逻辑上：

• 你创建的是匿名内部类实例，Java会为每个匿名内部类生成独立的类对象，哪怕代码完全一样，不同实例的equals()判断是基于对象引用的，所以测试里的实例和业务代码里的实例不匹配。
• 再加上你给MicroServicesTest1加了@Spy注解，当mock匹配失败时，Spy会默认调用真实的业务方法——也就是说你的测试实际上请求了GitHub的真实API，拿到了git/git仓库的真实贡献者列表（刚好是30条左右），所以断言assertEquals(1, mocklist.size())才会失败。

解决方法 #

方法一：用any()匹配ParameterizedTypeReference #

最简单的修复是把精确匹配换成模糊匹配，确保mock能被触发：

Mockito.when(mockRestTemplate.exchange(
    Mockito.anyString(), 
    Mockito.any(HttpMethod.class), 
    Mockito.any(HttpEntity.class), 
    Mockito.any(ParameterizedTypeReference.class)
)).thenReturn(response);

方法二：提取ParameterizedTypeReference为常量 #

如果需要精确匹配，可以把业务代码中的ParameterizedTypeReference提取成静态常量，让业务和测试代码共用同一个实例：
首先修改业务代码：

private static final ParameterizedTypeReference<List<GithubModel>> GITHUB_CONTRIBUTORS_TYPE = 
    new ParameterizedTypeReference<List<GithubModel>>() {};

public List<GithubModel> getGitHubUsersList(){
    HttpHeaders httpHeaders = new HttpHeaders();
    HttpEntity<GithubModel>httpEntity = new HttpEntity<GithubModel>(httpHeaders);
    ResponseEntity<List<GithubModel>>usersList = restTemplate.exchange(
        "https://api.github.com/repos/git/git/contributors", 
        HttpMethod.GET,
        httpEntity, 
        GITHUB_CONTRIBUTORS_TYPE
    );
    return usersList.getBody();
}

然后修改测试用例的匹配逻辑：

Mockito.when(mockRestTemplate.exchange(
    Mockito.anyString(), 
    Mockito.any(HttpMethod.class), 
    Mockito.any(HttpEntity.class), 
    Mockito.eq(MicroServicesTest1.GITHUB_CONTRIBUTORS_TYPE)
)).thenReturn(response);

额外优化建议 #

1. 去掉@Spy注解：除非你需要部分mock业务类，否则只用@InjectMocks创建实例即可——这样mock匹配失败时会返回默认值（比如空集合），更容易定位问题。
2. 替换过时方法：用MockitoAnnotations.openMocks(this)替代MockitoAnnotations.initMocks(this)，后者已经被标记为过时。

内容的提问来源于stack exchange，提问作者Krish