WebClient使用exchangeToMono获取ClientResponse时响应体为null问题

我在单元测试中编写代码理解WebClient API时遇到问题：使用exchangeToMono获取的ClientResponse能正确返回200状态码，但响应体始终为null；而使用已废弃的exchange()方法却能正常获取响应内容"Some string response"。

测试代码如下：

@BeforeEach
public void setUpTest() throws JsonProcessingException {
    baseUrl = String.format("http://localhost:%s", mockBackEnd.getPort());
    String response = "Some string response";
    ObjectMapper objectMapper = new ObjectMapper();

    mockBackEnd.enqueue(new MockResponse()
            .setBody(objectMapper.writeValueAsString(response))
            .addHeader(HttpHeaders.CONTENT_TYPE, ContentType.TEXT_PLAIN));
}

@Test
public void test_clientResponse_return_type() {
    WebClient webClient = WebClient.builder()
            .baseUrl(baseUrl)
            .build();

    Mono<ClientResponse> clientResponseMono = webClient.post()
            .uri(baseUrl)
            .accept(MediaType.TEXT_PLAIN)
            .bodyValue("")
            .exchangeToMono(Mono::just);

    ClientResponse clientResponse = clientResponseMono.block();

    log.info("Response StatusCode is :-" + clientResponse.statusCode());
    log.info("Response Body is :-" + clientResponse.bodyToMono(String.class).block());
}

问题原因 #

exchangeToMono的设计初衷是让开发者在操作符内部完成响应体的消费与处理，当你直接用Mono::just返回ClientResponse时，WebClient会认为响应体未被处理，从而提前关闭响应流，导致后续调用bodyToMono无法读取到内容。

而废弃的exchange()方法将ClientResponse直接暴露到外部，允许在外部消费响应体，但这种方式存在资源泄漏风险（如未消费响应体导致连接池资源无法释放），因此Spring官方已不推荐使用。

解决方案 #

方案1：在exchangeToMono内部处理响应体（推荐） #

直接在exchangeToMono中完成状态码日志打印与响应体读取，这是Spring官方推荐的用法：

@Test
public void test_clientResponse_return_type() {
    WebClient webClient = WebClient.builder()
            .baseUrl(baseUrl)
            .build();

    Mono<String> responseBodyMono = webClient.post()
            .uri(baseUrl)
            .accept(MediaType.TEXT_PLAIN)
            .bodyValue("")
            .exchangeToMono(response -> {
                log.info("Response StatusCode is :-" + response.statusCode());
                return response.bodyToMono(String.class);
            });

    String responseBody = responseBodyMono.block();
    log.info("Response Body is :-" + responseBody);
}

方案2：缓存响应体后返回ClientResponse #

如果确实需要获取完整的ClientResponse对象，可以在exchangeToMono内部先缓存响应体，再通过mutate()方法构建新的ClientResponse：

@Test
public void test_clientResponse_return_type() {
    WebClient webClient = WebClient.builder()
            .baseUrl(baseUrl)
            .build();

    Mono<ClientResponse> clientResponseMono = webClient.post()
            .uri(baseUrl)
            .accept(MediaType.TEXT_PLAIN)
            .bodyValue("")
            .exchangeToMono(response -> 
                response.bodyToMono(String.class)
                        .map(body -> response.mutate().body(Mono.just(body)).build())
            );

    ClientResponse clientResponse = clientResponseMono.block();

    log.info("Response StatusCode is :-" + clientResponse.statusCode());
    log.info("Response Body is :-" + clientResponse.bodyToMono(String.class).block());
}

内容的提问来源于stack exchange，提问作者Mark Gargan