问题

我已经定义了一个CustomHttpMessageReader（它实现了HttpMessageReader<CustomClass>），它能够从服务器读取多部分响应，并将接收到的部分转换为特定类的对象。CustomHttpMessageReader在内部使用DefaultPartHttpMessageReader来实际读取/解析多部分响应。
CustomHttpMessageReader累积DefaultReader读取的部分，并将它们转换为所需的类CustomClass。
我已经创建了一个CustomHttpMessageConverter，它对RestTemplate做了同样的事情，但我很难对WebClient做同样的事情。
我总是得到以下异常：

block()/blockFirst()/blockLast() are blocking, which is not supported in thread reactor-http-nio-2
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)
    at reactor.core.publisher.Flux.blockFirst(Flux.java:2600)
    at com.company.project.deserializer.multipart.CustomHttpMessageReader.readMultipartData(CustomHttpMessageReader.java:116)
    at com.company.project.deserializer.multipart.CustomHttpMessageReader.readMono(CustomHttpMessageReader.java:101)
    at org.springframework.web.reactive.function.BodyExtractors.lambda$readToMono$14(BodyExtractors.java:211)
    at java.base/java.util.Optional.orElseGet(Optional.java:369)
    ...

请注意，我对异步运行WebClient并不感兴趣，我只是为了将来验证我的应用程序，因为RestTemplate显然是only in maintenance mode，Pivotal/Spring的人建议使用WebClient。

我的尝试

据我所知，有些线程是不允许被阻塞的，也就是异常中的netty-nio线程。我试着从我的依赖项中删除netty，这样我就可以只依赖Tomcat了。然而，这似乎没有帮助，因为我得到了另一个异常，解释了我，没有合适的HttpConnector存在（WebClient.Builder抛出的异常）

No suitable default ClientHttpConnector found
java.lang.IllegalStateException: No suitable default ClientHttpConnector found
    at org.springframework.web.reactive.function.client.DefaultWebClientBuilder.initConnector(DefaultWebClientBuilder.java:297)
    at org.springframework.web.reactive.function.client.DefaultWebClientBuilder.build(DefaultWebClientBuilder.java:266)
    at com.company.project.RestClientUsingWebClient.getWebclient(RestClientUsingWebClient.java:160)

我也尝试过在单元测试中执行我的代码，就像启动整个Spring上下文一样。不幸的是结果是一样的。

设置

为了提供更多的细节，下面是前面提到的类的片段。为了更好地理解正在发生的事情，这些类没有完全显示。所有必要的方法都被实现了（例如，阅读器中的canRead()）。

CustomHttpMessageReader的第一个字符

我还在类中包括了CustomPart的用法（除了CustomClass），只是为了说明Part的内容也是读的，即被阻止的。

public class CustomHttpMessageReader  implements HttpMessageReader<CustomClass> {

    private final DefaultPartHttpMessageReader defaultPartHttpMessageReader = new DefaultPartHttpMessageReader();

    @Override
    public Flux<CustomClass> read(final ResolvableType elementType, final ReactiveHttpInputMessage message,
                                  final Map<String, Object> hints) {
        return Flux.merge(readMono(elementType, message, hints));
    }

    @Override
    public Mono<CustomClass> readMono(final ResolvableType elementType, final ReactiveHttpInputMessage message,
                                      final Map<String, Object> hints) {
        final List<CustomPart> customParts = readMultipartData(message);
        return convertToCustomClass(customParts);
    }

    private List<CustomPart> readMultipartData(final ReactiveHttpInputMessage message) {
        final ResolvableType resolvableType = ResolvableType.forClass(byte[].class);
        return Optional.ofNullable(
                        defaultPartHttpMessageReader.read(resolvableType, message, Map.of())
                                .buffer()
                                .blockFirst()) // <- EXCEPTION IS THROWN HERE!
                .orElse(new ArrayList<>())
                .stream()
                .map(part -> {
                    final byte[] content = Optional.ofNullable(part.content().blockFirst()) //<- HERE IS ANOTHER BLOCK
                            .map(DataBuffer::asByteBuffer)
                            .map(ByteBuffer::array)
                            .orElse(new byte[]{});

                    // Here we cherry pick some header fields
                    return new CustomPart(content, someHeaderFields);
                }).collect(Collectors.toList());
    }
}

WebClient的用法

class RestClientUsingWebClient  {

    /**
     * The "Main" Method for our purposes
     */
    public Optional<CustomClass> getResource(final String baseUrl, final String id) {
        final WebClient webclient = getWebclient(baseUrl);

        //curl -X GET "http://BASE_URL/id" -H  "accept: multipart/form-data"
        return webclient.get()
                .uri(uriBuilder -> uriBuilder.path(id).build()).retrieve()
                .toEntity(CustomClass.class)
                .onErrorResume(NotFound.class, e -> Mono.empty())
                .blockOptional() // <- HERE IS ANOTHER BLOCK
                .map(ResponseEntity::getBody);

    }

    //This exists also as a Bean definition
    private WebClient getWebclient(final String baseUrl) {
         final ExchangeStrategies exchangeStrategies = ExchangeStrategies.builder() 
                 .codecs(codecs -> {
                     codecs.defaultCodecs().maxInMemorySize(16 * 1024 * 1024);
                     codecs.customCodecs().register(new CustomHttpMessageReader()); // <- Our custom reader
                 }) 
                 .build();

         return WebClient.builder()
                 .baseUrl(baseUrl)
                 .exchangeStrategies(exchangeStrategies)
                 .build();
    }
}

build.gradle的用法

为了完整起见，下面是我认为是我的build.gradle的相关部分

plugins {
    id 'org.springframework.boot' version '2.7.2'
    id 'io.spring.dependency-management' version '1.0.13.RELEASE'
    ...
}

dependencies {
    implementation 'org.springframework.boot:spring-boot-starter-actuator'
    implementation 'org.springframework.boot:spring-boot-starter-web'  // <- This 
    implementation 'org.springframework.boot:spring-boot-starter-webflux'
    // What I tried:
    // implementation ('org.springframework.boot:spring-boot-starter-webflux'){
    //    exclude group: 'org.springframework.boot', module: 'spring-boot-starter-reactor-netty'
    //}
...
}