使用Mockito when().thenReturn()打桩时出现NullPointerException问题排查

你遇到的NullPointerException是因为只Mock了BindingStubHelper，但没有Mock它返回的BindingStub对象。测试代码执行bindingStubHelper.getBindingStub()时，默认返回null，后续调用null.generatePayments()自然抛出空指针异常。

你的核心代码回顾 #

定义的接口

public interface BindingStubHelper {
  public BindingStub getBindingStub() throws java.rmi.RemoteException;
}

实现类

@Configuration
public class BindingStubImpl implements BindingStubHelper {
  @Value("${my.url}")
  private URL url;

  @Override
  public BindingStub getBindingStub() throws java.rmi.RemoteException {
    BindingStub BindingStub = new BindingStub(url, null);
    return BindingStub;
  }
}

业务代码调用

@Autowired private BindingStubHelper bindingStub;
...
payments = bindingStub.getBindingStub().generateSchedule(paymentRequest);

测试代码（存在问题的版本）

Payment payments =
        new Payment(
            34.97,
            22.58304,
            0.0,
            0.0,
            534.97,
            0,
            0,
            7.0,
            installmentList,
            500,
            500,
            500,
            500,
            10,
            500,
            0.0,
            20.00);
PaymentRequest paymentRequest =
        new PaymentRequest(
            500.0,
            startDate,
            renewalDate,
            Source.Annex,
            "838",
            "065714",
            "MQ",
            Type.New);
when(bindingStubHelper
        .getBindingStub()
        .generatePayments(paymentRequest))
    .thenReturn(payments);

修正后的测试代码 #

需要先MockBindingStub实例，让bindingStubHelper.getBindingStub()返回这个Mock对象，再对generatePayments方法进行Mock：

// 1. 先Mock BindingStub对象
BindingStub mockBindingStub = Mockito.mock(BindingStub.class);

// 2. 指定BindingStubHelper返回这个Mock的BindingStub
when(bindingStubHelper.getBindingStub()).thenReturn(mockBindingStub);

// 3. 最后Mock generatePayments方法的返回结果
Payment payments = new Payment(
            34.97,
            22.58304,
            0.0,
            0.0,
            534.97,
            0,
            0,
            7.0,
            installmentList,
            500,
            500,
            500,
            500,
            10,
            500,
            0.0,
            20.00);
PaymentRequest paymentRequest = new PaymentRequest(
            500.0,
            startDate,
            renewalDate,
            Source.Annex,
            "838",
            "065714",
            "MQ",
            Type.New);
when(mockBindingStub.generatePayments(paymentRequest)).thenReturn(payments);

简洁链式写法（可选） #

如果追求代码简洁，也可以用链式调用完成：

when(bindingStubHelper.getBindingStub()).thenReturn(Mockito.mock(BindingStub.class));
when(bindingStubHelper.getBindingStub().generatePayments(paymentRequest)).thenReturn(payments);

不过分开定义Mock对象的方式更清晰，便于后续扩展测试逻辑。

内容的提问来源于stack exchange，提问作者runnerpaul