如何为Json.mapper().constructType(type)编写Mockito.when()存根并解决报错？

场景代码 #

待测试的resolveProperty方法：

@Override
public Property resolveProperty(Type type, ModelConverterContext context, Annotation[] annotations, Iterator<ModelConverter> chain) {
   JavaType jType = Json.mapper().constructType(type);
   ...
}

尝试编写的存根代码：

when(Json.mapper()).thenReturn(objectMapper);
when(objectMapper.constructType(type)).thenReturn(jType);

报错信息 #

org.mockito.exceptions.misusing.MissingMethodInvocationException:
when() requires an argument which has to be 'a method call on a mock'. For example:

when(mock.getArticles()).thenReturn(articles);

Also, this error might show up because:

• you stub either of: final/private/equals()/hashCode() methods. Those methods cannot be stubbed/verified.
• inside when() you don't call method on mock but on some other object.
• the parent of the mocked class is not public. It is a limitation of the mock engine.

解决方案 #

核心原因 #

报错本质是Json.mapper()为静态方法（或final实例方法），Mockito默认无法对这类方法存根——when()要求参数必须是mock对象的方法调用。

方案1：使用Mockito静态方法Mock（Mockito 3.4+） #

若使用Mockito 3.4及以上版本，可通过MockedStatic实现静态方法的mock：

// 创建ObjectMapper的mock实例
ObjectMapper mockObjectMapper = mock(ObjectMapper.class);
when(mockObjectMapper.constructType(type)).thenReturn(jType);

// 用try-with-resources包裹静态mock，确保测试后自动清理
try (MockedStatic<Json> mockedJson = mockStatic(Json.class)) {
    mockedJson.when(Json::mapper).thenReturn(mockObjectMapper);
    
    // 执行你的测试逻辑
}

方案2：重构代码，依赖注入ObjectMapper（推荐） #

避免直接依赖静态方法，改为构造函数注入ObjectMapper，降低代码耦合度：

1. 修改目标类，注入ObjectMapper：

public class YourModelConverter implements ModelConverter {
    private final ObjectMapper objectMapper;

    // 构造函数注入
    public YourModelConverter(ObjectMapper objectMapper) {
        this.objectMapper = objectMapper;
    }

    @Override
    public Property resolveProperty(Type type, ModelConverterContext context, Annotation[] annotations, Iterator<ModelConverter> chain) {
        JavaType jType = objectMapper.constructType(type);
        // 原有业务逻辑
    }
}

2. 测试时直接mock并注入：

// 创建mock的ObjectMapper
ObjectMapper mockObjectMapper = mock(ObjectMapper.class);
when(mockObjectMapper.constructType(type)).thenReturn(jType);

// 初始化测试对象，注入mock实例
YourModelConverter converter = new YourModelConverter(mockObjectMapper);

// 执行测试逻辑

额外注意事项 #

• 若Json.mapper()是final实例方法，需引入mockito-inline依赖，让Mockito支持final方法的mock。
• 确保所有存根对象都是通过mock()方法创建的mock实例，而非真实对象。

内容的提问来源于stack exchange，提问作者Mohitkumar Shah