使用 Mockito 模拟接口时出现空指针异常答案

【问题标题】：Null pointer exception when using Mockito to mock interface使用 Mockito 模拟接口时出现空指针异常
【发布时间】：2020-10-16 20:44:42
【问题描述】：

我正在使用 Mockito 测试以下方法：

  protected void produceProducedFieldValue(Target target) throws ValueProducerCreationException {
    String mappedValue = (String) valueProducerFactory.createValueProducer("IdProducer", Collections.emptyList()).get();
    target.setProducedField(mappedValue);
  }

这是我的测试：

     @Test
    public void TestProducingProducedFieldValue() throws ValueProducerCreationException {

        Target mockedTarget = Mockito.mock(Target.class);
        ValueProducerFactory valueProducerFactory = Mockito.mock(ValueProducerFactory.class);
        List<Object> creationParameters = new ArrayList<>();

        String mappedValue= "SomeString";

        Mockito.when(valueProducerFactory.createValueProducer("IdProducer", Collections.emptyList()).get()).thenReturn(mappedValue);
        SourceToTargetMapper sttm = new SourceToTargetMapper(targetModelObjectFactory,mappingOperatorFactory,mapperFactory,valueProducerFactory);
        sttm.produceProducedFieldValue(mockedTarget);
        Mockito.verify(mockedTarget, Mockito.times(1)).setProducedField(Mockito.any());
    }

但我不断收到 NullPointerExeption 这条线路：

    Mockito.when(valueProducerFactory.createValueProducer("IdProducer", Collections.emptyList()).get()).thenReturn(mappedValue);

ValueProducerFactory 是一个接口

而createValueProducer方法签名如下：

    public interface ValueProducerFactory {

        <T> Supplier<T> createValueProducer(String producerName, List<Object> creationParameters) throws ValueProducerCreationException;

    }

我有一个名为 CachingValueProducerFactory 的类，它实现了接口

    @Override
    public <T> Supplier<T> createValueProducer(String producerName, List<Object> creationParameters) throws ValueProducerCreationException {
        Map<List<Object>, Supplier<?>> parametersToOperatorsMap = producersCache.computeIfAbsent(producerName, s -> new HashMap<>());

        List<Object> key = Optional.ofNullable(creationParameters).orElse(Collections.emptyList());

        if (!parametersToOperatorsMap.containsKey(key)) {
            parametersToOperatorsMap.put(key, nestedProducersFactory.createValueProducer(producerName, creationParameters));
        }

        return (Supplier<T>) parametersToOperatorsMap.get(key);
    }

似乎在测试中使用Collections.emptyList() 是问题所在，但我没有看到任何其他解决方案。

【问题讨论】：

标签： java unit-testing nullpointerexception mockito

【解决方案1】：

您正试图同时模拟两种方法。在这一行：

Mockito.when(valueProducerFactory.createValueProducer("IdProducer", Collections.emptyList()).get()).thenReturn(mappedValue);

您在 createValueProducer() 调用的结果上调用 get()，这是 null，因为您还没有模拟它。

有两种解决方案：

使用深度存根：

ValueProducerFactory valueProducerFactory = Mockito.mock(ValueProducerFactory.class, Mockito.RETURNS_DEEP_STUBS);
Mockito.when(valueProducerFactory.createValueProducer("IdProducer", Collections.emptyList()).get()).thenReturn(mappedValue);

先Mock the Supplier（注意被删除的get()）：

Supplier<Object> supplier = () -> mappedValue;
ValueProducerFactory valueProducerFactory = Mockito.mock(ValueProducerFactory.class);
Mockito.when(valueProducerFactory.createValueProducer("IdProducer", Collections.emptyList())).thenReturn(supplier);

【讨论】：