Spring Boot WebClient block()方法返回错误java.lang.IollegalStateEx

Question

我正在尝试使用Spring WebFluxWebClient获取值(字符串(，(使用SpringBoot 2.4.5版本(

@GetMapping("/test")
public Mono<String> getData(){
WebClient webClient = WebClient.create("http://localhost:9999");
Mono<String> stringMono = webClient.get()
.uri("/some/thing")
.retrieve()
.bodyToMono(String.class);
stringMono.subscribe( System.out::println);
System.out.println("Value : " + stringMono.block()); // this doesn't work,  expecting to return ResponseBody as "Hello World" ,
return stringMono;
}

但是低于错误

2021-05-11 20:02:15.521 ERROR 55613 --- [ctor-http-nio-2] a.w.r.e.AbstractErrorWebExceptionHandler : [19114471-1]  500 Server Error for HTTP GET "/test"
java.lang.IllegalStateException: block()/blockFirst()/blockLast() are blocking, which is not supported in thread reactor-http-nio-2
at reactor.core.publisher.BlockingSingleSubscriber.blockingGet(BlockingSingleSubscriber.java:83) ~[reactor-core-3.4.3.jar:3.4.3]
Suppressed: reactor.core.publisher.FluxOnAssembly$OnAssemblyException: 
Error has been observed at the following site(s):
|_ checkpoint ⇢ HTTP GET "/test" [ExceptionHandlingWebHandler]
Stack trace:
at reactor.core.publisher.BlockingSingleSubscriber.blockingGet(BlockingSingleSubscriber.java:83) ~[reactor-core-3.4.3.jar:3.4.3]
at reactor.core.publisher.Mono.block(Mono.java:1703) ~[reactor-core-3.4.3.jar:3.4.3]
at com.example.demo.DemoApplication.getData(DemoApplication.java:28) ~[main/:na]
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:na]
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:na]
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:na]
at java.base/java.lang.reflect.Method.invoke(Method.java:566) ~[na:na]
at org.springframework.web.reactive.result.method.InvocableHandlerMethod.lambda$invoke$0(InvocableHandlerMethod.java:146) ~[spring-webflux-5.3.4.jar:5.3.4]
at reactor.core.publisher.FluxFlatMap.trySubscribeScalarMap(FluxFlatMap.java:151) ~[reactor-core-3.4.3.jar:3.4.3]

块法参考-https://docs.spring.io/spring-framework/docs/current/reference/html/web-reactive.html#webflux-客户端同步

Answer 1

在使用Webflux时，整个想法是不要阻塞-如果阻塞，会导致巨大的性能问题(请参阅此处以获得解释原因的相关答案(，因此框架明确禁止它，如果尝试，会抛出异常。

你也不应该手动订阅-而不是"；必死之罪；在被动的世界里，就像封锁一样，这无疑是另一个危险信号。订阅由框架处理。您只需要返回Mono，Webflux就会在需要时订阅以处理您的请求。在您的情况下，手动订阅意味着整个链实际上将为每个请求执行两次-一次将结果打印到终端，一次将将结果返回到/test端点。

相反，如果你想要一个"；副作用"；像这样(当你有值的时候打印出来(，那么你需要使用doOnNext()操作符来改变反应链。这意味着你可以做：

return webClient.get()
.uri("/some/thing")
.retrieve()
.bodyToMono(String.class)
.doOnNext(s -> System.out.println("Value: " + s));

这将确保将值打印到终端，但不会阻塞，也不会手动订阅。