Spring Boot RestClient：单例或请求级实例？测试最优用法探讨

根据Spring官方文档，RestClient的推荐用法是注入RestClient.Builder，在服务类构造方法中构建RestClient实例——这种方式在生产环境运行高效，但在多自定义Builder的测试场景下会遇到问题。

文档明确：当应用仅使用一个Builder时，@RestClientTest和@AutoConfigureMockRestServiceServer可以正常工作；但如果需要为不同外部服务配置多个自定义Builder（比如不同的baseUrl、拦截器），就需要为每个服务绑定独立的Builder，因此将这些Builder注册为Spring Bean是合理选择。

初始实现（存在测试问题） #

配置类 #

@RequiredArgsConstructor
@Configuration
public class RestClientConfiguration {
    private final RestClientBuilderConfigurer restClientBuilderConfigurer;

    @Bean
    RestClient.Builder microservice1RestClientBuilder() {
        return createDefaultRestClientBuilder()
                .baseUrl("http://localhost:8081");
    }

    /**
     * 逻辑复制自RestClientAutoConfiguration
     */
    private RestClient.Builder createDefaultRestClientBuilder() {
        RestClient.Builder builder = RestClient
                .builder()
                .requestFactory(ClientHttpRequestFactories.get(ClientHttpRequestFactorySettings.DEFAULTS));
        return restClientBuilderConfigurer.configure(builder);
    }
}

控制器类 #

@RequiredArgsConstructor
@RestController
public class MainController {
    private final MainService mainService;

    @GetMapping
    public String hello() {
        return mainService.helloService();
    }
}

服务类（生产友好，但测试失败） #

@Service
public class MainService {
    private final RestClient restClient;
    
    public MainService(RestClient.Builder microservice1RestClientBuilder) {
        this.restClient = microservice1RestClientBuilder.build();
    }

    public String helloService() {
        return restClient.get().uri("/").retrieve().body(String.class);
    }
}

测试类 #

@SpringBootTest
@AutoConfigureMockMvc
class MainControllerTest {
    private MockRestServiceServer microservice1Server;

    @Autowired
    private RestClient.Builder microservice1RestClientBuilder;

    @Autowired
    private MockMvc mvc;

    @BeforeEach
    void init() {
        microservice1Server = MockRestServiceServer.bindTo(microservice1RestClientBuilder).build();
    }

    @Test
    void test() throws Exception {
        microservice1Server
                .expect(requestTo("http://localhost:8081/"))
                .andRespond(withSuccess("Hello world", MediaType.TEXT_PLAIN));
        mvc.perform(get("/")).andExpect(status().is2xxSuccessful()).andExpect(content().string("Hello world"));
    }
}

问题分析 #

上述代码测试无法运行，核心原因是：RestClient实例在服务类初始化时就已创建，而MockRestServiceServer是在@BeforeEach阶段才绑定到Builder——此时RestClient已经使用了Builder原来的请求工厂，Mock无法拦截请求。

如果修改服务类每次请求都构建RestClient（如下），测试可以通过，但会导致每次请求创建新实例，造成不必要的资源消耗：

@RequiredArgsConstructor
@Service
public class MainService {
    private final RestClient.Builder microservice1RestClientBuilder;

    public String helloService() {
        return microservice1RestClientBuilder.build().get().uri("/").retrieve().body(String.class);
    }
}

最优实现方案 #

核心思路是让RestClient使用的请求工厂可被Spring管理，测试时替换为Mock工厂，同时保持生产环境中RestClient的单例特性。

1. 调整配置类：绑定可管理的请求工厂 #

为每个自定义Builder单独配置请求工厂Bean，方便测试时替换：

@RequiredArgsConstructor
@Configuration
public class RestClientConfiguration {
    private final RestClientBuilderConfigurer restClientBuilderConfigurer;

    // 为microservice1定义独立的请求工厂Bean
    @Bean
    ClientHttpRequestFactory microservice1RequestFactory() {
        return ClientHttpRequestFactories.get(ClientHttpRequestFactorySettings.DEFAULTS);
    }

    @Bean
    RestClient.Builder microservice1RestClientBuilder(ClientHttpRequestFactory microservice1RequestFactory) {
        return createDefaultRestClientBuilder()
                .baseUrl("http://localhost:8081")
                .requestFactory(microservice1RequestFactory);
    }

    private RestClient.Builder createDefaultRestClientBuilder() {
        RestClient.Builder builder = RestClient.builder();
        return restClientBuilderConfigurer.configure(builder);
    }
}

2. 服务类保持单例RestClient（生产友好） #

服务类依然在构造方法中构建RestClient，保持单例特性，避免重复创建：

@Service
public class MainService {
    private final RestClient restClient;
    
    public MainService(RestClient.Builder microservice1RestClientBuilder) {
        this.restClient = microservice1RestClientBuilder.build();
    }

    public String helloService() {
        return restClient.get().uri("/").retrieve().body(String.class);
    }
}

3. 测试类：替换请求工厂并绑定MockServer #

通过@MockBean替换目标请求工厂，再绑定MockRestServiceServer，此时RestClient会使用Mock工厂拦截请求：

@SpringBootTest
@AutoConfigureMockMvc
class MainControllerTest {
    private MockRestServiceServer microservice1Server;

    @Autowired
    private RestClient.Builder microservice1RestClientBuilder;

    @Autowired
    private MockMvc mvc;

    // 替换microservice1的请求工厂为Mock实现
    @MockBean
    private ClientHttpRequestFactory microservice1RequestFactory;

    @BeforeEach
    void init() {
        microservice1Server = MockRestServiceServer.bindTo(microservice1RestClientBuilder).build();
    }

    @Test
    void test() throws Exception {
        microservice1Server
                .expect(requestTo("http://localhost:8081/"))
                .andRespond(withSuccess("Hello world", MediaType.TEXT_PLAIN));
        mvc.perform(get("/")).andExpect(status().is2xxSuccessful()).andExpect(content().string("Hello world"));
    }
}

4. 单独测试服务类的简化方案 #

如果不需要启动完整Spring上下文，仅测试服务类，可以使用@RestClientTest配合@Import导入配置类：

@RestClientTest(MainService.class)
@Import(RestClientConfiguration.class)
class MainServiceTest {
    @Autowired
    private MainService mainService;

    @Autowired
    private MockRestServiceServer mockServer;

    @Test
    void helloServiceTest() {
        mockServer.expect(requestTo("http://localhost:8081/"))
                .andRespond(withSuccess("Hello world", MediaType.TEXT_PLAIN));
        String result = mainService.helloService();
        assertEquals("Hello world", result);
    }
}

方案优势 #

• 生产环境：RestClient是单例，请求工厂为默认实现，无性能损耗，符合官方推荐用法。
• 测试环境：通过替换请求工厂实现Mock拦截，无需修改服务类逻辑，测试用例简洁可靠。
• 多Builder场景：每个服务的Builder绑定独立的请求工厂Bean，测试时可分别替换，互不干扰。

内容的提问来源于stack exchange，提问作者Anthony Lofton