如何用Mockito模拟方法？被测方法内部实例的Mock问题

原因分析 #

你没法Mock MyHttpClient 的核心问题是：MyDataSourceImpl 的 execute 方法里直接通过 new MyHttpClient(...) 创建了实例，Mockito无法拦截这种硬编码的对象实例化，测试类里@Mock的MyHttpClient和代码里实际运行的完全是两个对象，所以Mock规则不会生效。

方案一：通过依赖注入重构代码（推荐） #

遵循依赖反转原则，把MyHttpClient的创建权交给外部，通过构造方法注入工厂类，这样测试时可以直接传入Mock实例。

1. 修改MyDataSourceImpl类 #

public class MyDataSourceImpl implements MyDataSource {
     private final PropertieService propertieService;
     private final MyHttpClientFactory httpClientFactory;

     // 构造方法注入依赖
     @Autowired
     public MyDataSourceImpl(PropertieService propertieService, MyHttpClientFactory httpClientFactory) {
         this.propertieService = propertieService;
         this.httpClientFactory = httpClientFactory;
     }

     public HttpResponse execute(String url){
            MyHttpClient myClient = httpClientFactory.create(url, propertieService.getProperties());
            HttpResponse response = myClient.performCall();
            this.someOtherStuff(response);
            return response;
     }
}

// 新增HttpClient工厂接口
public interface MyHttpClientFactory {
    MyHttpClient create(String url, Map<String, String> properties);
}

// 生产环境的工厂实现
public class DefaultMyHttpClientFactory implements MyHttpClientFactory {
    @Override
    public MyHttpClient create(String url, Map<String, String> properties) {
        return new MyHttpClient(url, properties);
    }
}

2. 修改测试类 #

@RunWith(MockitoJUnitRunner.class)
public class MyDataSourceImplTest {
      @InjectMocks
      private MyDataSourceImpl myDataSourceImpl;
      @Mock
      private PropertieService propertieService;
      @Mock
      private MyHttpClientFactory httpClientFactory;
      @Mock
      private MyHttpClient myHttpClient;

      @Test
      public void simpleCall(){
           // Mock配置参数返回
           when(propertieService.getProperties()).thenReturn(new HashMap<String,String>());
           // Mock工厂返回Mock的HttpClient
           when(httpClientFactory.create(anyString(), anyMap())).thenReturn(myHttpClient);
           // Mock请求返回结果
           HttpResponse mockResponse = new HttpResponse(); // 替换为你需要的模拟响应
           when(myHttpClient.performCall()).thenReturn(mockResponse);

           HttpResponse response = myDataSourceImpl.execute("https://nothing.com");
           assertEquals(response, mockResponse);
           // 验证方法调用次数
           verify(myHttpClient, times(1)).performCall();
      }
}

方案二：通过Spy重构代码（最小改动） #

如果不想引入工厂类，可将创建MyHttpClient的逻辑抽成protected方法，再用Mockito Spy来Mock这个方法，返回预设的Mock实例。

1. 修改MyDataSourceImpl类 #

public class MyDataSourceImpl implements MyDataSource {
     @Autowired
     PropertieService propertieService;

     public HttpResponse execute(String url){
            MyHttpClient myClient = createHttpClient(url, propertieService.getProperties());
            HttpResponse response = myClient.performCall();
            this.someOtherStuff(response);
            return response;
     }

     // 抽离创建逻辑为protected方法，方便测试Mock
     protected MyHttpClient createHttpClient(String url, Map<String, String> properties) {
         return new MyHttpClient(url, properties);
     }
}

2. 修改测试类 #

@RunWith(MockitoJUnitRunner.class)
public class MyDataSourceImplTest {
      // 使用Spy而非@InjectMocks，用于Mock类内部方法
      @Spy
      private MyDataSourceImpl myDataSourceImpl;
      @Mock
      private PropertieService propertieService;
      @Mock
      private MyHttpClient myHttpClient;

      @Test
      public void simpleCall(){
           // 手动注入Mock的配置服务
           Whitebox.setInternalState(myDataSourceImpl, "propertieService", propertieService);
           // Mock配置参数返回
           when(propertieService.getProperties()).thenReturn(new HashMap<String,String>());
           // MockHttpClient创建方法返回预设实例
           doReturn(myHttpClient).when(myDataSourceImpl).createHttpClient(anyString(), anyMap());
           // Mock请求返回结果
           HttpResponse mockResponse = new HttpResponse();
           when(myHttpClient.performCall()).thenReturn(mockResponse);

           HttpResponse response = myDataSourceImpl.execute("https://nothing.com");
           assertEquals(response, mockResponse);
           verify(myHttpClient, times(1)).performCall();
      }
}

注：Whitebox是Mockito内置的反射工具，若使用Spring测试环境，也可替换为ReflectionTestUtils。

方案三：使用PowerMock（不推荐） #

如果完全不想修改业务代码，可通过PowerMock拦截new操作，但该工具会破坏Mockito原有机制，增加测试复杂度，仅作为临时方案使用。

测试类示例 #

@RunWith(PowerMockRunner.class)
@PrepareForTest(MyDataSourceImpl.class) // 声明需要拦截new操作的类
public class MyDataSourceImplTest {
      @InjectMocks
      private MyDataSourceImpl myDataSourceImpl;
      @Mock
      private PropertieService propertieService;
      @Mock
      private MyHttpClient myHttpClient;

      @Test
      public void simpleCall() throws Exception {
           when(propertieService.getProperties()).thenReturn(new HashMap<String,String>());
           // 拦截new MyHttpClient操作，返回Mock实例
           whenNew(MyHttpClient.class).withAnyArguments().thenReturn(myHttpClient);
           // Mock请求返回结果
           HttpResponse mockResponse = new HttpResponse();
           when(myHttpClient.performCall()).thenReturn(mockResponse);

           HttpResponse response = myDataSourceImpl.execute("https://nothing.com");
           assertEquals(response, mockResponse);
      }
}

内容的提问来源于stack exchange，提问作者niconosave