WebClient can not upload empty file by POST

[fishjam]

Bug Reports

I try to upload lots of files to a file server with POST method, and there are some files are empty(size is 0).
WebClient can not upload them, throw exception: io.netty.handler.codec.EncoderException: java.lang.IllegalStateException: unexpected message type: DefaultFileRegion, state: 0

Reproduce Steps

 @Test
  public void uploadEmptyFileWithOriginalWebClient() throws IOException {
    File emptyFile = new File("prov2_empty.txt");
    if (!emptyFile.exists()) {
      emptyFile.createNewFile();
    }

    try {
      WebClient orgWebClient = WebClient.builder().build();
      Mono<String> monoUploadEmpty = orgWebClient.post()
          .uri("http://any_web_that_support_post_file")
          .body(BodyInserters.fromResource(new FileSystemResource(emptyFile)))
          .retrieve()
          .bodyToMono(String.class);

      StepVerifier.create(monoUploadEmpty)
          .expectNextCount(1)
          .verifyComplete();

    }finally {
      emptyFile.delete();
    }

  }

Exception:

• jdk-8.0.232 ,
• spring-boot-starter-webflux-2.7.18
• reactor-netty-http-1.0.39

2025-09-23 13:43:04.515  WARN --- [reactor-http-nio-3] r.netty.http.client.HttpClientConnect    : [90baf8b6-1, L:/192.168.1.10:54724 - R:192.168.1.10:8000] The connection observed an error
io.netty.handler.codec.EncoderException: java.lang.IllegalStateException: unexpected message type: DefaultFileRegion, state: 0
	at io.netty.handler.codec.http.HttpObjectEncoder.write(HttpObjectEncoder.java:108)
	at io.netty.channel.CombinedChannelDuplexHandler.write(CombinedChannelDuplexHandler.java:346)
	at io.netty.channel.AbstractChannelHandlerContext.invokeWrite0(AbstractChannelHandlerContext.java:879)
	at io.netty.channel.AbstractChannelHandlerContext.invokeWriteAndFlush(AbstractChannelHandlerContext.java:940)
	at io.netty.channel.AbstractChannelHandlerContext.write(AbstractChannelHandlerContext.java:966)
	at io.netty.channel.AbstractChannelHandlerContext.writeAndFlush(AbstractChannelHandlerContext.java:934)
	at io.netty.channel.AbstractChannelHandlerContext.writeAndFlush(AbstractChannelHandlerContext.java:984)
	at io.netty.channel.DefaultChannelPipeline.writeAndFlush(DefaultChannelPipeline.java:1025)
	at io.netty.channel.AbstractChannel.writeAndFlush(AbstractChannel.java:306)
	at reactor.netty.channel.ChannelOperations.lambda$sendUsing$4(ChannelOperations.java:329)
	at reactor.core.publisher.MonoUsing.subscribe(MonoUsing.java:85)
	at reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.subscribeNext(MonoIgnoreThen.java:240)
	at reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.onComplete(MonoIgnoreThen.java:203)
	at reactor.core.publisher.Operators.complete(Operators.java:137)
	at reactor.netty.FutureMono.doSubscribe(FutureMono.java:122)
	at reactor.netty.FutureMono$DeferredFutureMono.subscribe(FutureMono.java:114)
	at reactor.core.publisher.Mono.subscribe(Mono.java:4490)
	at reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.subscribeNext(MonoIgnoreThen.java:263)
	at reactor.core.publisher.MonoIgnoreThen.subscribe(MonoIgnoreThen.java:51)
	at reactor.core.publisher.Mono.subscribe(Mono.java:4490)
	at reactor.core.publisher.FluxConcatIterable$ConcatIterableSubscriber.onComplete(FluxConcatIterable.java:147)
	at reactor.core.publisher.FluxConcatIterable.subscribe(FluxConcatIterable.java:60)
	at reactor.core.publisher.Mono.subscribe(Mono.java:4490)
	at reactor.core.publisher.FluxFlatMap.trySubscribeScalarMap(FluxFlatMap.java:203)
	at reactor.core.publisher.MonoFlatMap.subscribeOrReturn(MonoFlatMap.java:53)
	at reactor.core.publisher.InternalMonoOperator.subscribe(InternalMonoOperator.java:57)
	at reactor.core.publisher.MonoDefer.subscribe(MonoDefer.java:53)
	at reactor.netty.http.client.HttpClientConnect$HttpIOHandlerObserver.onStateChange(HttpClientConnect.java:445)
	at reactor.netty.ReactorNetty$CompositeConnectionObserver.onStateChange(ReactorNetty.java:707)
	at reactor.netty.resources.DefaultPooledConnectionProvider$DisposableAcquire.onStateChange(DefaultPooledConnectionProvider.java:193)
	at reactor.netty.resources.DefaultPooledConnectionProvider$PooledConnection.onStateChange(DefaultPooledConnectionProvider.java:454)
	at reactor.netty.channel.ChannelOperationsHandler.channelActive(ChannelOperationsHandler.java:62)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelActive(AbstractChannelHandlerContext.java:262)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelActive(AbstractChannelHandlerContext.java:238)
	at io.netty.channel.AbstractChannelHandlerContext.fireChannelActive(AbstractChannelHandlerContext.java:231)
	at io.netty.channel.CombinedChannelDuplexHandler$DelegatingChannelHandlerContext.fireChannelActive(CombinedChannelDuplexHandler.java:412)
	at io.netty.channel.ChannelInboundHandlerAdapter.channelActive(ChannelInboundHandlerAdapter.java:69)
	at io.netty.channel.CombinedChannelDuplexHandler.channelActive(CombinedChannelDuplexHandler.java:211)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelActive(AbstractChannelHandlerContext.java:260)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelActive(AbstractChannelHandlerContext.java:238)
	at io.netty.channel.AbstractChannelHandlerContext.fireChannelActive(AbstractChannelHandlerContext.java:231)
	at io.netty.channel.DefaultChannelPipeline$HeadContext.channelActive(DefaultChannelPipeline.java:1398)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelActive(AbstractChannelHandlerContext.java:258)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelActive(AbstractChannelHandlerContext.java:238)
	at io.netty.channel.DefaultChannelPipeline.fireChannelActive(DefaultChannelPipeline.java:895)
	at io.netty.channel.nio.AbstractNioChannel$AbstractNioUnsafe.fulfillConnectPromise(AbstractNioChannel.java:305)
	at io.netty.channel.nio.AbstractNioChannel$AbstractNioUnsafe.finishConnect(AbstractNioChannel.java:335)
	at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:776)
	at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:724)
	at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:650)
	at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:562)
	at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:997)
	at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)
	at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
	at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.IllegalStateException: unexpected message type: DefaultFileRegion, state: 0
	at io.netty.handler.codec.http.HttpObjectEncoder.throwUnexpectedMessageTypeEx(HttpObjectEncoder.java:348)
	at io.netty.handler.codec.http.HttpObjectEncoder.encodeNotHttpMessageContentTypes(HttpObjectEncoder.java:267)
	at io.netty.handler.codec.http.HttpObjectEncoder.encode(HttpObjectEncoder.java:181)
	at io.netty.handler.codec.http.HttpClientCodec$Encoder.encode(HttpClientCodec.java:202)
	at io.netty.handler.codec.http.HttpObjectEncoder.write(HttpObjectEncoder.java:97)
	... 54 common frames omitted

BTW:

• 1. if add any contents( example: 1 bytes in the file ), then success.

[fishjam]

After more check, there is same issue in latest version:

• jdk-21.0.3.9
• spring-boot-starter-webflux-3.5.6
• reactor-netty-http-1.2.10

[fishjam]

After debug, I think it's related to HttpObjectEncoder::state .
I don't know why the state in encodeFullHttpMessage is local variable( not class member variable as encodeInitHttpMessage

I will check in netty too. and try fix it.

[violetagg]

@fishjam Please report this issue to Reactor Netty project

[bclozel]

Closing in favor of reactor/reactor-netty#3935