测试用例运行时出现Bean序列化异常，浏览器访问接口正常

问题描述 #

浏览器访问/getEmployee接口功能正常，但运行WebMvc测试用例时抛出Jackson序列化异常，测试失败。

相关代码 #

Controller类 #

@RestController
public class EmployeeController {

    @Autowired
    private EmployeeDTO employeeDTO;

  
    @GetMapping("/getEmployee")
    public EmployeeDTO getEmployee(){
        employeeDTO.setEmpId(1);
        employeeDTO.setEmpName("Bob");
        return employeeDTO;
    }
}

EmployeeDTO类 #

@Getter
@Setter
@NoArgsConstructor
@Component
public class EmployeeDTO {

    private int empId;
    private String empName;

}

EmployeeControllerTest类 #

@WebMvcTest
public class EmployeeControllerTest {
    @Autowired
    private MockMvc mockmvc;

    @MockBean
    private EmployeeDTO employeeDTO;

    @Test
    public void testGetEmployee() throws Exception {
        RequestBuilder request = MockMvcRequestBuilders.get("/getEmployee");
        mockmvc.perform(request)
                .andExpect(status().isOk());
    }
}

pom.xml #

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.7.8</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.spring</groupId>
    <artifactId>tennis</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>tennis</name>
    <description>Demo project</description>
    <properties>
        <java.version>11</java.version>
    </properties>
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>

        <dependency>
            <groupId>org.junit.jupiter</groupId>
            <artifactId>junit-jupiter-api</artifactId>
            <version>5.7.0</version>
            <scope>test</scope>
        </dependency>

        <dependency>
            <groupId>org.junit.jupiter</groupId>
            <artifactId>junit-jupiter-engine</artifactId>
            <version>5.7.0</version>
            <scope>test</scope>
        </dependency>

        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <version>1.18.26</version>
            <scope>provided</scope>
        </dependency>

    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>

</project>

异常信息 #

org.springframework.web.util.NestedServletException: Request processing failed; nested exception is org.springframework.http.converter.HttpMessageConversionException: Type definition error: [simple type, class org.mockito.internal.creation.bytebuddy.ByteBuddyCrossClassLoaderSerializationSupport]; nested exception is com.fasterxml.jackson.databind.exc.InvalidDefinitionException: No serializer found for class org.mockito.internal.creation.bytebuddy.ByteBuddyCrossClassLoaderSerializationSupport and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS) (through reference chain: com.spring.tennis.model.EmployeeDTO$MockitoMock$MCOb7jmM["mockitoInterceptor"]->org.mockito.internal.creation.bytebuddy.MockMethodInterceptor["serializationSupport"])

原因分析 #

1. Mock对象序列化问题：测试时用@MockBean生成的EmployeeDTO是Mockito代理对象，包含Mockito内部的拦截器、序列化支持类等属性，Jackson无法序列化这些未暴露的内部类，导致异常。
2. DTO设计不合理：EmployeeDTO被标记为@Component，成为Spring单例Bean。正常运行时Spring注入真实实例，Jackson可以序列化其属性；但测试时Mock替换了真实实例，触发序列化问题。同时，DTO作为数据传输对象，不应该被Spring管理为单例，应该每次请求创建新实例。

解决方案 #

方案一：修正DTO与Controller的设计（推荐） #

这是从根源解决问题的方式：

1. 移除DTO的@Component注解：

@Getter
@Setter
@NoArgsConstructor
public class EmployeeDTO {
    private int empId;
    private String empName;
}

2. 修改Controller，不再注入DTO，而是方法内创建新实例：

@RestController
public class EmployeeController {
  
    @GetMapping("/getEmployee")
    public EmployeeDTO getEmployee(){
        EmployeeDTO employeeDTO = new EmployeeDTO();
        employeeDTO.setEmpId(1);
        employeeDTO.setEmpName("Bob");
        return employeeDTO;
    }
}

3. 调整测试用例，无需Mock DTO：

@WebMvcTest
public class EmployeeControllerTest {
    @Autowired
    private MockMvc mockmvc;

    @Test
    public void testGetEmployee() throws Exception {
        RequestBuilder request = MockMvcRequestBuilders.get("/getEmployee");
        mockmvc.perform(request)
                .andExpect(status().isOk())
                .andExpect(jsonPath("$.empId").value(1))
                .andExpect(jsonPath("$.empName").value("Bob"));
    }
}

方案二：临时适配测试用例（不推荐，仅用于无法修改原代码的场景） #

如果暂时不能修改Controller和DTO的设计，可通过指定Mock对象的行为让Jackson正常序列化：

@WebMvcTest
public class EmployeeControllerTest {
    @Autowired
    private MockMvc mockmvc;

    @MockBean
    private EmployeeDTO employeeDTO;

    @Test
    public void testGetEmployee() throws Exception {
        // 指定Mock对象的getter返回值，确保Jackson能序列化所需属性
        given(employeeDTO.getEmpId()).willReturn(1);
        given(employeeDTO.getEmpName()).willReturn("Bob");

        RequestBuilder request = MockMvcRequestBuilders.get("/getEmployee");
        mockmvc.perform(request)
                .andExpect(status().isOk())
                .andExpect(jsonPath("$.empId").value(1))
                .andExpect(jsonPath("$.empName").value("Bob"));
    }
}

或者禁用Jackson的FAIL_ON_EMPTY_BEANS特性（治标不治本，可能隐藏其他问题）：

@WebMvcTest
public class EmployeeControllerTest {
    @Autowired
    private MockMvc mockmvc;

    @MockBean
    private EmployeeDTO employeeDTO;

    @Bean
    public ObjectMapper objectMapper() {
        return new ObjectMapper().disable(SerializationFeature.FAIL_ON_EMPTY_BEANS);
    }

    @Test
    public void testGetEmployee() throws Exception {
        RequestBuilder request = MockMvcRequestBuilders.get("/getEmployee");
        mockmvc.perform(request)
                .andExpect(status().isOk());
    }
}

内容的提问来源于stack exchange，提问作者ts009