Spring Cloud Gateway中WebClient使用遇NullPointerException问题求助

问题场景 #

在Spring Cloud Gateway中使用WebClient发起请求时无法获取响应，触发NullPointerException，核心代码及错误信息如下：

WebClient配置类 #

@Configuration
public class WebClientConfig {

    @Bean
    public WebClient.Builder webClientBuilder() {
        return WebClient.builder();
    }

}

业务请求代码 #

@Autowired
public WebClient webClient;

public Mono<Response> getResponse(......) {

    HttpHeaders headers = new HttpHeaders();
    headers.setContentType(MediaType.APPLICATION_JSON);
    headers.add(HttpHeaders.AUTHORIZATION, authHeader);

    MultiValueMap<String, String> requestBody = new LinkedMultiValueMap<>();
    requestBody.add(.....);

    Mono<Response> responseMono = webClient.method(HttpMethod.POST)
            .uri("some usrl")
            .headers(httpHeaders -> httpHeaders.addAll(headers))
            .body(BodyInserters.fromValue(requestBody))
            .retrieve()
            .bodyToMono(Response.class);

    Mono<Response> customObjMono = responseMono.map(customObject -> {
        Response customObj = new Response();
        customObj.setAccessToken(customObject.getAccessToken());
        return customObj;
    });

    return customObjMono;
}

错误的响应获取方式 #

Mono<Response> token = client.getResponse(........);
AtomicReference<Response> plainJavaObject = new AtomicReference<>();
token.subscribe(transformedToken -> {
    plainJavaObject.set(transformedToken);
});
Response token = plainJavaObject.get();

错误日志 #

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! null
2023-07-22 19:12:05.533 ERROR 2668 --- [ctor-http-nio-3] a.w.r.e.AbstractErrorWebExceptionHandler : [b59e780c-3]  500 Server Error for HTTP POST "/greeting"

java.lang.NullPointerException: The Mono returned by the supplier is null
    at java.base/java.util.Objects.requireNonNull(Objects.java:233) ~[na:na]
    Suppressed: reactor.core.publisher.FluxOnAssembly$OnAssemblyException: 
Error has been observed at the following site(s):
    *__checkpoint ⇢ org.springframework.cloud.gateway.filter.WeightCalculatorWebFilter [DefaultWebFilterChain]
    *__checkpoint ⇢ HTTP POST "/greeting" [ExceptionHandlingWebHandler]
Original Stack Trace:

问题原因 #

1. WebClient注入失败：配置类仅声明了WebClient.Builder的Bean，但直接注入WebClient实例，Spring容器中不存在该类型的Bean，导致webClient变量为null。调用webClient.method()时触发空指针异常，进而导致返回的Mono为null，触发日志中的错误。
2. 违背响应式编程原则：使用AtomicReference配合subscribe()同步获取响应的方式错误。Reactor是异步非阻塞模型，subscribe()后立刻调用get()时，异步请求尚未完成，plainJavaObject仍为null，无法获取有效响应。

解决方案 #

1. 修复WebClient注入问题 #

修改配置类，直接声明WebClient类型的Bean：

@Configuration
public class WebClientConfig {

    @Bean
    public WebClient webClient() {
        return WebClient.builder().build();
    }

}

或者通过WebClient.Builder构建实例后注入：

@Autowired
private WebClient.Builder webClientBuilder;

private WebClient webClient;

@PostConstruct
public void init() {
    this.webClient = webClientBuilder.build();
}

2. 修正响应式调用逻辑 #

放弃阻塞式获取结果的方式，保持响应式流的连续性。例如在Gateway Filter中正确处理：

// 调整getResponse方法，直接返回响应式流
public Mono<Response> getResponse(...) {
    HttpHeaders headers = new HttpHeaders();
    headers.setContentType(MediaType.APPLICATION_JSON);
    headers.add(HttpHeaders.AUTHORIZATION, authHeader);

    MultiValueMap<String, String> requestBody = new LinkedMultiValueMap<>();
    requestBody.add(.....);

    // 修正URL拼写错误（原代码中为"some usrl"）
    return webClient.method(HttpMethod.POST)
            .uri("your-actual-url")
            .headers(httpHeaders -> httpHeaders.addAll(headers))
            .body(BodyInserters.fromValue(requestBody))
            .retrieve()
            .bodyToMono(Response.class)
            .map(customObject -> {
                Response customObj = new Response();
                customObj.setAccessToken(customObject.getAccessToken());
                return customObj;
            });
}

// 在Gateway Filter中使用示例
public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
    return client.getResponse(...)
            .flatMap(response -> {
                // 将响应结果存入Exchange属性供后续使用
                exchange.getAttributes().put("ACCESS_TOKEN", response.getAccessToken());
                return chain.filter(exchange);
            })
            .onErrorResume(ex -> {
                // 异常处理：返回500错误
                exchange.getResponse().setStatusCode(HttpStatus.INTERNAL_SERVER_ERROR);
                return exchange.getResponse().setComplete();
            });
}

3. 额外注意事项 #

• 修正原代码中URL的拼写错误（"some usrl"改为实际有效的请求地址）。
• 确保Response类具备正确的Jackson序列化/反序列化注解（如@Data、@JsonProperty等），避免响应解析失败。

内容的提问来源于stack exchange，提问作者Peter Penzov