Mockito中when(...).thenReturn(...)失效，resendClient.emails()返回null求助

我在编写单元测试时遇到Mockito模拟失效问题，测试代码如下：

@UnitTest
@RunWith(MockitoJUnitRunner.class)
public class ResendMailingStrategyUnitTest {

    @Test
    public void sendTest() throws ResendException {

        Resend resendClient = Mockito.mock(Resend.class);
        Emails mockedEmails = Mockito.mock(Emails.class);

        ResendMailingStrategy resendMailingStrategy = new ResendMailingStrategy();
        ReflectionTestUtils.setField(resendMailingStrategy, "resendClient", resendClient);

        Mockito.when(resendClient.emails()).thenReturn(mockedEmails);
        Mockito.when(mockedEmails.send(Mockito.any(CreateEmailOptions.class))).thenReturn(null);

        resendMailingStrategy.send(StubProvider.getEmailMessageModelStub());

        Mockito.verify(resendClient).emails().send(Mockito.any(CreateEmailOptions.class));

    }

}

设置Mockito.when(resendClient.emails()).thenReturn(mockedEmails)后，resendClient.emails()仍返回null，导致报错：

Cannot invoke "com.resend.services.emails.Emails.send(com.resend.services.emails.model.CreateEmailOptions)" because the return value of "com.resend.Resend.emails()" is null

修复方案 #

• 检查Resend类的emails()方法是否为final
  Mockito默认无法模拟final方法，如果emails()是final的，需启用Mockito的final方法模拟支持：
  在src/test/resources/mockito-extensions目录下创建文件org.mockito.plugins.MockMaker，内容写入mock-maker-inline即可。

• 调整verify语句的写法
  原verify语句会尝试链式调用方法，模拟失效时会触发NPE。应拆分验证逻辑：

  Mockito.verify(resendClient).emails();
  Mockito.verify(mockedEmails).send(Mockito.any(CreateEmailOptions.class));
  

• 验证ReflectionTestUtils字段设置是否成功
  可添加断言确认字段注入正确：

  Assertions.assertSame(resendClient, ReflectionTestUtils.getField(resendMailingStrategy, "resendClient"));
  

  若字段名错误或为private final，会导致注入失败，需核对字段名称与访问权限。

• 改用@Mock注解管理模拟对象
  替换手动创建模拟对象的方式，用注解更简洁可靠：

  @UnitTest
  @RunWith(MockitoJUnitRunner.class)
  public class ResendMailingStrategyUnitTest {
  
      @Mock
      private Resend resendClient;
  
      @Mock
      private Emails mockedEmails;
  
      private ResendMailingStrategy resendMailingStrategy;
  
      @Before
      public void setUp() {
          resendMailingStrategy = new ResendMailingStrategy();
          ReflectionTestUtils.setField(resendMailingStrategy, "resendClient", resendClient);
          Mockito.when(resendClient.emails()).thenReturn(mockedEmails);
          Mockito.when(mockedEmails.send(Mockito.any(CreateEmailOptions.class))).thenReturn(null);
      }
  
      @Test
      public void sendTest() throws ResendException {
          resendMailingStrategy.send(StubProvider.getEmailMessageModelStub());
  
          Mockito.verify(resendClient).emails();
          Mockito.verify(mockedEmails).send(Mockito.any(CreateEmailOptions.class));
      }
  }
  

内容的提问来源于stack exchange，提问作者Nicolas Villacorta