SpringBoot WebClientRequestException executor not accepting a task, when web is disabled using WebApplicationType.NONE

Using these dependencies

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.7.1</version>
    <relativePath/> <!-- lookup parent from repository -->
</parent>

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-webflux</artifactId>
</dependency>

And this simple application

package com.example.issuespringwebclient;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.WebApplicationType;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.builder.SpringApplicationBuilder;
import org.springframework.web.reactive.function.client.WebClient;

@SpringBootApplication
public class Application implements CommandLineRunner {

    @Autowired
    private WebClient.Builder webClientBuilder;

    public static void main(String[] args) {
        new SpringApplicationBuilder(Application.class)
                .web(WebApplicationType.NONE)
                .run(args);
    }

    @Override
    public void run(String... args) {
        var webClient = webClientBuilder.baseUrl("https://www.google.com").build();

        webClient.get()
                .exchangeToMono(clientResponse -> clientResponse.bodyToMono(String.class))
                .doOnNext(s -> System.out.println("response output length " + s.length()))
                .subscribe();
    }

}

results in the following exception

2022-07-11 10:57:08.412 ERROR 18597 --- [ctor-http-nio-3] reactor.core.publisher.Operators         : Operator called default onErrorDropped

reactor.core.Exceptions$ErrorCallbackNotImplemented: org.springframework.web.reactive.function.client.WebClientRequestException: executor not accepting a task; nested exception is java.lang.IllegalStateException: executor not accepting a task
Caused by: org.springframework.web.reactive.function.client.WebClientRequestException: executor not accepting a task; nested exception is java.lang.IllegalStateException: executor not accepting a task
    at org.springframework.web.reactive.function.client.ExchangeFunctions$DefaultExchangeFunction.lambda$wrapException$9(ExchangeFunctions.java:141) ~[spring-webflux-5.3.21.jar:5.3.21]
    Suppressed: reactor.core.publisher.FluxOnAssembly$OnAssemblyException: 
Error has been observed at the following site(s):
    *__checkpoint ⇢ Request to GET null [DefaultWebClient]
Original Stack Trace:
        at org.springframework.web.reactive.function.client.ExchangeFunctions$DefaultExchangeFunction.lambda$wrapException$9(ExchangeFunctions.java:141) ~[spring-webflux-5.3.21.jar:5.3.21]
        at reactor.core.publisher.MonoErrorSupplied.subscribe(MonoErrorSupplied.java:55) ~[reactor-core-3.4.19.jar:3.4.19]
        at reactor.core.publisher.Mono.subscribe(Mono.java:4397) ~[reactor-core-3.4.19.jar:3.4.19]
        at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onError(FluxOnErrorResume.java:103) ~[reactor-core-3.4.19.jar:3.4.19]
        at reactor.core.publisher.FluxPeek$PeekSubscriber.onError(FluxPeek.java:222) ~[reactor-core-3.4.19.jar:3.4.19]
        at reactor.core.publisher.FluxPeek$PeekSubscriber.onError(FluxPeek.java:222) ~[reactor-core-3.4.19.jar:3.4.19]
        at reactor.core.publisher.FluxPeek$PeekSubscriber.onError(FluxPeek.java:222) ~[reactor-core-3.4.19.jar:3.4.19]
        at reactor.core.publisher.MonoNext$NextSubscriber.onError(MonoNext.java:93) ~[reactor-core-3.4.19.jar:3.4.19]
        at reactor.core.publisher.MonoFlatMapMany$FlatMapManyMain.onError(MonoFlatMapMany.java:204) ~[reactor-core-3.4.19.jar:3.4.19]
        at reactor.core.publisher.SerializedSubscriber.onError(SerializedSubscriber.java:124) ~[reactor-core-3.4.19.jar:3.4.19]
        at reactor.core.publisher.FluxRetryWhen$RetryWhenMainSubscriber.whenError(FluxRetryWhen.java:225) ~[reactor-core-3.4.19.jar:3.4.19]
        at reactor.core.publisher.FluxRetryWhen$RetryWhenOtherSubscriber.onError(FluxRetryWhen.java:274) ~[reactor-core-3.4.19.jar:3.4.19]
        at reactor.core.publisher.FluxConcatMap$ConcatMapImmediate.drain(FluxConcatMap.java:415) ~[reactor-core-3.4.19.jar:3.4.19]
        at reactor.core.publisher.FluxConcatMap$ConcatMapImmediate.onNext(FluxConcatMap.java:251) ~[reactor-core-3.4.19.jar:3.4.19]
        at reactor.core.publisher.EmitterProcessor.drain(EmitterProcessor.java:537) ~[reactor-core-3.4.19.jar:3.4.19]
        at reactor.core.publisher.EmitterProcessor.tryEmitNext(EmitterProcessor.java:343) ~[reactor-core-3.4.19.jar:3.4.19]
        at reactor.core.publisher.SinkManySerialized.tryEmitNext(SinkManySerialized.java:100) ~[reactor-core-3.4.19.jar:3.4.19]
        at reactor.core.publisher.InternalManySink.emitNext(InternalManySink.java:27) ~[reactor-core-3.4.19.jar:3.4.19]
        at reactor.core.publisher.FluxRetryWhen$RetryWhenMainSubscriber.onError(FluxRetryWhen.java:190) ~[reactor-core-3.4.19.jar:3.4.19]
        at reactor.core.publisher.MonoCreate$DefaultMonoSink.error(MonoCreate.java:201) ~[reactor-core-3.4.19.jar:3.4.19]
        at reactor.netty.http.client.HttpClientConnect$MonoHttpConnect$ClientTransportSubscriber.onError(HttpClientConnect.java:308) ~[reactor-netty-http-1.0.20.jar:1.0.20]
        at reactor.core.publisher.MonoCreate$DefaultMonoSink.error(MonoCreate.java:201) ~[reactor-core-3.4.19.jar:3.4.19]
        at reactor.netty.resources.DefaultPooledConnectionProvider$DisposableAcquire.onError(DefaultPooledConnectionProvider.java:158) ~[reactor-netty-core-1.0.20.jar:1.0.20]
        at reactor.netty.internal.shaded.reactor.pool.AbstractPool$Borrower.fail(AbstractPool.java:475) ~[reactor-netty-core-1.0.20.jar:1.0.20]
        at reactor.netty.internal.shaded.reactor.pool.SimpleDequePool.lambda$drainLoop$9(SimpleDequePool.java:431) ~[reactor-netty-core-1.0.20.jar:1.0.20]
        at reactor.core.publisher.FluxDoOnEach$DoOnEachSubscriber.onError(FluxDoOnEach.java:186) ~[reactor-core-3.4.19.jar:3.4.19]
        at reactor.core.publisher.MonoCreate$DefaultMonoSink.error(MonoCreate.java:201) ~[reactor-core-3.4.19.jar:3.4.19]
        at reactor.netty.resources.DefaultPooledConnectionProvider$PooledConnectionAllocator$PooledConnectionInitializer.onError(DefaultPooledConnectionProvider.java:548) ~[reactor-netty-core-1.0.20.jar:1.0.20]
        at reactor.core.publisher.MonoFlatMap$FlatMapMain.secondError(MonoFlatMap.java:192) ~[reactor-core-3.4.19.jar:3.4.19]
        at reactor.core.publisher.MonoFlatMap$FlatMapInner.onError(MonoFlatMap.java:259) ~[reactor-core-3.4.19.jar:3.4.19]
        at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onError(FluxOnErrorResume.java:106) ~[reactor-core-3.4.19.jar:3.4.19]
        at reactor.core.publisher.Operators.error(Operators.java:198) ~[reactor-core-3.4.19.jar:3.4.19]
        at reactor.core.publisher.MonoError.subscribe(MonoError.java:53) ~[reactor-core-3.4.19.jar:3.4.19]
        at reactor.core.publisher.Mono.subscribe(Mono.java:4397) ~[reactor-core-3.4.19.jar:3.4.19]
        at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onError(FluxOnErrorResume.java:103) ~[reactor-core-3.4.19.jar:3.4.19]
        at reactor.core.publisher.Operators.error(Operators.java:198) ~[reactor-core-3.4.19.jar:3.4.19]
        at reactor.core.publisher.MonoError.subscribe(MonoError.java:53) ~[reactor-core-3.4.19.jar:3.4.19]
        at reactor.core.publisher.InternalMonoOperator.subscribe(InternalMonoOperator.java:64) ~[reactor-core-3.4.19.jar:3.4.19]
        at reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:157) ~[reactor-core-3.4.19.jar:3.4.19]
        at reactor.netty.transport.TransportConnector$MonoChannelPromise._subscribe(TransportConnector.java:596) ~[reactor-netty-core-1.0.20.jar:1.0.20]
        at reactor.netty.transport.TransportConnector$MonoChannelPromise.lambda$subscribe$0(TransportConnector.java:516) ~[reactor-netty-core-1.0.20.jar:1.0.20]
        at io.netty.util.concurrent.AbstractEventExecutor.runTask(AbstractEventExecutor.java:174) ~[netty-common-4.1.78.Final.jar:4.1.78.Final]
        at io.netty.util.concurrent.AbstractEventExecutor.safeExecute(AbstractEventExecutor.java:167) ~[netty-common-4.1.78.Final.jar:4.1.78.Final]
        at io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:470) ~[netty-common-4.1.78.Final.jar:4.1.78.Final]
        at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:503) ~[netty-transport-4.1.78.Final.jar:4.1.78.Final]
        at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:997) ~[netty-common-4.1.78.Final.jar:4.1.78.Final]
        at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74) ~[netty-common-4.1.78.Final.jar:4.1.78.Final]
        at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30) ~[netty-common-4.1.78.Final.jar:4.1.78.Final]
        at java.base/java.lang.Thread.run(Thread.java:833) ~[na:na]
Caused by: java.lang.IllegalStateException: executor not accepting a task
    at io.netty.resolver.AddressResolverGroup.getResolver(AddressResolverGroup.java:61) ~[netty-resolver-4.1.78.Final.jar:4.1.78.Final]
    at reactor.netty.transport.TransportConnector.doResolveAndConnect(TransportConnector.java:278) ~[reactor-netty-core-1.0.20.jar:1.0.20]
    at reactor.netty.transport.TransportConnector.lambda$connect$6(TransportConnector.java:127) ~[reactor-netty-core-1.0.20.jar:1.0.20]
    at reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:125) ~[reactor-core-3.4.19.jar:3.4.19]
    at reactor.netty.transport.TransportConnector$MonoChannelPromise._subscribe(TransportConnector.java:596) ~[reactor-netty-core-1.0.20.jar:1.0.20]
    at reactor.netty.transport.TransportConnector$MonoChannelPromise.lambda$subscribe$0(TransportConnector.java:516) ~[reactor-netty-core-1.0.20.jar:1.0.20]
    at io.netty.util.concurrent.AbstractEventExecutor.runTask(AbstractEventExecutor.java:174) ~[netty-common-4.1.78.Final.jar:4.1.78.Final]
    at io.netty.util.concurrent.AbstractEventExecutor.safeExecute(AbstractEventExecutor.java:167) ~[netty-common-4.1.78.Final.jar:4.1.78.Final]
    at io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:470) ~[netty-common-4.1.78.Final.jar:4.1.78.Final]
    at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:503) ~[netty-transport-4.1.78.Final.jar:4.1.78.Final]
    at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:997) ~[netty-common-4.1.78.Final.jar:4.1.78.Final]
    at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74) ~[netty-common-4.1.78.Final.jar:4.1.78.Final]
    at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30) ~[netty-common-4.1.78.Final.jar:4.1.78.Final]
    at java.base/java.lang.Thread.run(Thread.java:833) ~[na:na]

Comment From: wilkinsona

When the main method exits, there are no non-daemon threads running so the JVM automatically shuts down. This shutdown causes the application context to be closed which shuts down the executor. If you want the JVM to wait until the response has been received, you need to keep a non-daemon thread, such as the main thread, alive. One (slightly primitive) way to do that is to block() rather than subscribe():

webClient.get()
        .exchangeToMono(clientResponse -> clientResponse.bodyToMono(String.class))
        .doOnNext(s -> System.out.println("response output length " + s.length()))
        .block();

Comment From: sergproua

@wilkinsona thank you so much!