Spring Boot pass/upload file from one api to another via WebClient

Question

I have two Spring Boot Apis where I am first sending the multipart file from postman to one spring boot application and from there to the second spring boot application like so

Postman (file) -> API 1 -> API2

When the file is uploaded from postman to API1 everything goes well but when I send this multipart file from API1 to API2 things fall apart. I am using WebClient to send the file from API1 to API2. Below is the related code and the errors I see in the consoles of both the APIs.

Code in API1 where I am calling API2 with the intended file using WebClient

public List<DTO> importPrograms(String urlPath, MultipartFile file){
        try {
            MultipartBodyBuilder builder = new MultipartBodyBuilder();
            builder.part("file", file);
            // also tried 
            //builder.part("file", new ByteArrayResource(file.getBytes()));
            Flux<ProductVersionDTO> flux = webClient.post()
                    .uri(urlPath)
                    .contentType(MediaType.MULTIPART_FORM_DATA)
                    .header(HttpHeaders.CONTENT_TYPE, MediaType.APPLICATION_JSON.toString())
                    .accept(MediaType.APPLICATION_JSON)
                    .bodyValue(builder.build()) // also tried body(BodyInserters.fromMultipartData(builder.build()))
                    .retrieve()
                    .bodyToFlux(DTO.class);
            return flux.collectList().block();
        } catch(Exception e) {
            LOG.error(e.getMessage());
            return null;
        }
    }

Controller code in API2 that fetches the file

@PostMapping(path = "/importPrograms", consumes = MediaType.MULTIPART_FORM_DATA_VALUE)
@ResponseBody
public ResponseEntity<List<DTO>> importPrograms(@RequestPart (required = false) MultipartFile file) {
        LOG.debug("Executing importPrograms.");
        return importPrograms(file);
    }

public ResponseEntity<List<DTO>> importPrograms(MultipartFile file){
        Workbook workbook = new XSSFWorkbook(file.getInputStream());
        //............ more code
}

The error in the console for API1 (where file is being sent to API2) is as below

2021-02-19 00:13:08 WARN  HttpClientConnect:299 - [id: 0xc092ccbb, L:/127.0.0.1:64168 - R:localhost/127.0.0.1:10002] The connection observed an error
java.lang.IllegalStateException: Unexpected I/O error while writing to byte array builder
    at org.springframework.http.codec.json.AbstractJackson2Encoder.encodeValue(AbstractJackson2Encoder.java:220) ~[spring-web-5.3.1.jar:5.3.1]
    at org.springframework.http.codec.json.AbstractJackson2Encoder.lambda$encode$0(AbstractJackson2Encoder.java:146) ~[spring-web-5.3.1.jar:5.3.1]
    at reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.onNext(FluxMapFuseable.java:113) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.Operators$ScalarSubscription.request(Operators.java:2346) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.request(FluxMapFuseable.java:169) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.MonoSingle$SingleSubscriber.request(MonoSingle.java:101) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.Operators$MultiSubscriptionSubscriber.set(Operators.java:2154) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.Operators$MultiSubscriptionSubscriber.onSubscribe(Operators.java:2028) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.MonoSingle$SingleSubscriber.onSubscribe(MonoSingle.java:121) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.onSubscribe(FluxMapFuseable.java:96) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.MonoJust.subscribe(MonoJust.java:54) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.Mono.subscribe(Mono.java:3987) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.FluxConcatArray$ConcatArraySubscriber.onComplete(FluxConcatArray.java:208) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.FluxConcatArray.subscribe(FluxConcatArray.java:80) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.Flux.subscribe(Flux.java:8095) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.FluxConcatArray$ConcatArraySubscriber.onComplete(FluxConcatArray.java:208) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.FluxConcatArray.subscribe(FluxConcatArray.java:80) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.Flux.subscribe(Flux.java:8095) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.FluxConcatMap$ConcatMapImmediate.drain(FluxConcatMap.java:448) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.FluxConcatMap$ConcatMapImmediate.onSubscribe(FluxConcatMap.java:218) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.FluxIterable.subscribe(FluxIterable.java:164) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.FluxIterable.subscribe(FluxIterable.java:86) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.Flux.subscribe(Flux.java:8095) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.FluxConcatMap$ConcatMapImmediate.drain(FluxConcatMap.java:448) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.FluxConcatMap$ConcatMapImmediate.onSubscribe(FluxConcatMap.java:218) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.FluxIterable.subscribe(FluxIterable.java:164) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.FluxIterable.subscribe(FluxIterable.java:86) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.Flux.subscribe(Flux.java:8095) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.FluxConcatArray$ConcatArraySubscriber.onComplete(FluxConcatArray.java:208) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.FluxConcatArray.subscribe(FluxConcatArray.java:80) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.Flux.subscribe(Flux.java:8095) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.netty.channel.MonoSendMany.subscribe(MonoSendMany.java:102) ~[reactor-netty-core-1.0.1.jar:1.0.1]
    at reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.drain(MonoIgnoreThen.java:154) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.MonoIgnoreThen.subscribe(MonoIgnoreThen.java:56) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.Mono.subscribe(Mono.java:3987) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.FluxConcatIterable$ConcatIterableSubscriber.onComplete(FluxConcatIterable.java:147) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.FluxConcatIterable.subscribe(FluxConcatIterable.java:60) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.Mono.subscribe(Mono.java:3987) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.FluxFlatMap.trySubscribeScalarMap(FluxFlatMap.java:202) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.MonoFlatMap.subscribeOrReturn(MonoFlatMap.java:53) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.InternalMonoOperator.subscribe(InternalMonoOperator.java:57) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.core.publisher.MonoDefer.subscribe(MonoDefer.java:52) ~[reactor-core-3.4.0.jar:3.4.0]
    at reactor.netty.http.client.HttpClientConnect$HttpIOHandlerObserver.onStateChange(HttpClientConnect.java:379) ~[reactor-netty-http-1.0.1.jar:1.0.1]
    at reactor.netty.ReactorNetty$CompositeConnectionObserver.onStateChange(ReactorNetty.java:612) ~[reactor-netty-core-1.0.1.jar:1.0.1]
    at reactor.netty.resources.DefaultPooledConnectionProvider$DisposableAcquire.onStateChange(DefaultPooledConnectionProvider.java:194) ~[reactor-netty-core-1.0.1.jar:1.0.1]
    at reactor.netty.resources.DefaultPooledConnectionProvider$PooledConnection.onStateChange(DefaultPooledConnectionProvider.java:465) ~[reactor-netty-core-1.0.1.jar:1.0.1]
    at reactor.netty.channel.ChannelOperationsHandler.channelActive(ChannelOperationsHandler.java:63) ~[reactor-netty-core-1.0.1.jar:1.0.1]
    at io.netty.channel.AbstractChannelHandlerContext.invokeChannelActive(AbstractChannelHandlerContext.java:230) ~[netty-transport-4.1.54.Final.jar:4.1.54.Final]
    at io.netty.channel.AbstractChannelHandlerContext.invokeChannelActive(AbstractChannelHandlerContext.java:216) ~[netty-transport-4.1.54.Final.jar:4.1.54.Final]
    at io.netty.channel.AbstractChannelHandlerContext.fireChannelActive(AbstractChannelHandlerContext.java:209) ~[netty-transport-4.1.54.Final.jar:4.1.54.Final]
    at io.netty.channel.CombinedChannelDuplexHandler$DelegatingChannelHandlerContext.fireChannelActive(CombinedChannelDuplexHandler.java:412) ~[netty-transport-4.1.54.Final.jar:4.1.54.Final]
    at io.netty.channel.ChannelInboundHandlerAdapter.channelActive(ChannelInboundHandlerAdapter.java:69) ~[netty-transport-4.1.54.Final.jar:4.1.54.Final]
    at io.netty.channel.CombinedChannelDuplexHandler.channelActive(CombinedChannelDuplexHandler.java:211) ~[netty-transport-4.1.54.Final.jar:4.1.54.Final]
    at io.netty.channel.AbstractChannelHandlerContext.invokeChannelActive(AbstractChannelHandlerContext.java:230) ~[netty-transport-4.1.54.Final.jar:4.1.54.Final]
    at io.netty.channel.AbstractChannelHandlerContext.invokeChannelActive(AbstractChannelHandlerContext.java:216) ~[netty-transport-4.1.54.Final.jar:4.1.54.Final]
    at io.netty.channel.AbstractChannelHandlerContext.fireChannelActive(AbstractChannelHandlerContext.java:209) ~[netty-transport-4.1.54.Final.jar:4.1.54.Final]
    at io.netty.channel.DefaultChannelPipeline$HeadContext.channelActive(DefaultChannelPipeline.java:1398) ~[netty-transport-4.1.54.Final.jar:4.1.54.Final]
    at io.netty.channel.AbstractChannelHandlerContext.invokeChannelActive(AbstractChannelHandlerContext.java:230) ~[netty-transport-4.1.54.Final.jar:4.1.54.Final]
    at io.netty.channel.AbstractChannelHandlerContext.invokeChannelActive(AbstractChannelHandlerContext.java:216) ~[netty-transport-4.1.54.Final.jar:4.1.54.Final]
    at io.netty.channel.DefaultChannelPipeline.fireChannelActive(DefaultChannelPipeline.java:895) ~[netty-transport-4.1.54.Final.jar:4.1.54.Final]
    at io.netty.channel.nio.AbstractNioChannel$AbstractNioUnsafe.fulfillConnectPromise(AbstractNioChannel.java:305) ~[netty-transport-4.1.54.Final.jar:4.1.54.Final]
    at io.netty.channel.nio.AbstractNioChannel$AbstractNioUnsafe.finishConnect(AbstractNioChannel.java:335) ~[netty-transport-4.1.54.Final.jar:4.1.54.Final]
    at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:707) ~[netty-transport-4.1.54.Final.jar:4.1.54.Final]
    at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:655) ~[netty-transport-4.1.54.Final.jar:4.1.54.Final]
    at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:581) ~[netty-transport-4.1.54.Final.jar:4.1.54.Final]
    at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:493) [netty-transport-4.1.54.Final.jar:4.1.54.Final]
    at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:989) [netty-common-4.1.54.Final.jar:4.1.54.Final]
    at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74) [netty-common-4.1.54.Final.jar:4.1.54.Final]
    at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30) [netty-common-4.1.54.Final.jar:4.1.54.Final]
    at java.lang.Thread.run(Thread.java:834) [?:?]
Caused by: java.io.FileNotFoundException: MultipartFile resource [file] cannot be resolved to absolute file path
    at org.springframework.core.io.AbstractResource.getFile(AbstractResource.java:138) ~[spring-core-5.3.1.jar:5.3.1]
    at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:?]
    at jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:?]
    at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:?]
    at java.lang.reflect.Method.invoke(Method.java:566) ~[?:?]
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:689) ~[jackson-databind-2.11.3.jar:2.11.3]
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:755) ~[jackson-databind-2.11.3.jar:2.11.3]
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:178) ~[jackson-databind-2.11.3.jar:2.11.3]
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:728) ~[jackson-databind-2.11.3.jar:2.11.3]
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:755) ~[jackson-databind-2.11.3.jar:2.11.3]
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:178) ~[jackson-databind-2.11.3.jar:2.11.3]
    at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider._serialize(DefaultSerializerProvider.java:480) ~[jackson-databind-2.11.3.jar:2.11.3]
    at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:319) ~[jackson-databind-2.11.3.jar:2.11.3]
    at com.fasterxml.jackson.databind.ObjectWriter$Prefetch.serialize(ObjectWriter.java:1516) ~[jackson-databind-2.11.3.jar:2.11.3]
    at com.fasterxml.jackson.databind.ObjectWriter.writeValue(ObjectWriter.java:1006) ~[jackson-databind-2.11.3.jar:2.11.3]
    at org.springframework.http.codec.json.AbstractJackson2Encoder.encodeValue(AbstractJackson2Encoder.java:210) ~[spring-web-5.3.1.jar:5.3.1]
    ... 69 more
2021-02-19 00:13:08 ERROR APIService:125 - Unexpected I/O error while writing to byte array builder; nested exception is java.lang.IllegalStateException: Unexpected I/O error while writing to byte array builder

Error in console of API2 where file is being received

2021-02-19 00:13:08 ERROR [dispatcherServlet]:175 - Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is org.springframework.web.multipart.MultipartException: Failed to parse multipart servlet request; nested exception is java.io.IOException: org.apache.tomcat.util.http.fileupload.FileUploadException: Stream ended unexpectedly] with root cause
org.apache.tomcat.util.http.fileupload.MultipartStream$MalformedStreamException: Stream ended unexpectedly
    at org.apache.tomcat.util.http.fileupload.MultipartStream.readHeaders(MultipartStream.java:520) ~[tomcat-embed-core-9.0.41.jar:9.0.41]
    at org.apache.tomcat.util.http.fileupload.impl.FileItemIteratorImpl.findNextItem(FileItemIteratorImpl.java:228) ~[tomcat-embed-core-9.0.41.jar:9.0.41]
    at org.apache.tomcat.util.http.fileupload.impl.FileItemIteratorImpl.<init>(FileItemIteratorImpl.java:127) ~[tomcat-embed-core-9.0.41.jar:9.0.41]
    at org.apache.tomcat.util.http.fileupload.FileUploadBase.getItemIterator(FileUploadBase.java:256) ~[tomcat-embed-core-9.0.41.jar:9.0.41]
    at org.apache.tomcat.util.http.fileupload.FileUploadBase.parseRequest(FileUploadBase.java:280) ~[tomcat-embed-core-9.0.41.jar:9.0.41]
    at org.apache.catalina.connector.Request.parseParts(Request.java:2895) ~[tomcat-embed-core-9.0.41.jar:9.0.41]
    at org.apache.catalina.connector.Request.parseParameters(Request.java:3228) ~[tomcat-embed-core-9.0.41.jar:9.0.41]
    at org.apache.catalina.connector.Request.getParameterValues(Request.java:1191) ~[tomcat-embed-core-9.0.41.jar:9.0.41]
    at org.apache.catalina.connector.RequestFacade.getParameterValues(RequestFacade.java:424) ~[tomcat-embed-core-9.0.41.jar:9.0.41]
    at javax.servlet.ServletRequestWrapper.getParameterValues(ServletRequestWrapper.java:185) ~[tomcat-embed-core-9.0.41.jar:4.0.FR]
    at org.springframework.security.web.firewall.StrictHttpFirewall$StrictFirewalledRequest.getParameterValues(StrictHttpFirewall.java:721) ~[spring-security-web-5.4.2.jar:5.4.2]
    at javax.servlet.ServletRequestWrapper.getParameterValues(ServletRequestWrapper.java:185) ~[tomcat-embed-core-9.0.41.jar:4.0.FR]
    at javax.servlet.ServletRequestWrapper.getParameterValues(ServletRequestWrapper.java:185) ~[tomcat-embed-core-9.0.41.jar:4.0.FR]
    at org.springframework.security.oauth2.server.resource.web.DefaultBearerTokenResolver.resolveFromRequestParameters(DefaultBearerTokenResolver.java:116) ~[spring-security-oauth2-resource-server-5.4.2.jar:5.4.2]
    at org.springframework.security.oauth2.server.resource.web.DefaultBearerTokenResolver.resolve(DefaultBearerTokenResolver.java:52) ~[spring-security-oauth2-resource-server-5.4.2.jar:5.4.2]
    at org.springframework.security.oauth2.server.resource.web.BearerTokenAuthenticationFilter.doFilterInternal(BearerTokenAuthenticationFilter.java:106) ~[spring-security-oauth2-resource-server-5.4.2.jar:5.4.2]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) ~[spring-web-5.3.2.jar:5.3.2]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:336) ~[spring-security-web-5.4.2.jar:5.4.2]
    at org.springframework.security.web.authentication.logout.LogoutFilter.doFilter(LogoutFilter.java:103) ~[spring-security-web-5.4.2.jar:5.4.2]
    at org.springframework.security.web.authentication.logout.LogoutFilter.doFilter(LogoutFilter.java:89) ~[spring-security-web-5.4.2.jar:5.4.2]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:336) ~[spring-security-web-5.4.2.jar:5.4.2]
    at org.springframework.security.web.header.HeaderWriterFilter.doHeadersAfter(HeaderWriterFilter.java:90) ~[spring-security-web-5.4.2.jar:5.4.2]
    at org.springframework.security.web.header.HeaderWriterFilter.doFilterInternal(HeaderWriterFilter.java:75) ~[spring-security-web-5.4.2.jar:5.4.2]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) ~[spring-web-5.3.2.jar:5.3.2]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:336) ~[spring-security-web-5.4.2.jar:5.4.2]
    at org.springframework.security.web.context.SecurityContextPersistenceFilter.doFilter(SecurityContextPersistenceFilter.java:110) ~[spring-security-web-5.4.2.jar:5.4.2]
    at org.springframework.security.web.context.SecurityContextPersistenceFilter.doFilter(SecurityContextPersistenceFilter.java:80) ~[spring-security-web-5.4.2.jar:5.4.2]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:336) ~[spring-security-web-5.4.2.jar:5.4.2]
    at org.springframework.security.web.context.request.async.WebAsyncManagerIntegrationFilter.doFilterInternal(WebAsyncManagerIntegrationFilter.java:55) ~[spring-security-web-5.4.2.jar:5.4.2]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) ~[spring-web-5.3.2.jar:5.3.2]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:336) ~[spring-security-web-5.4.2.jar:5.4.2]
    at org.springframework.security.web.FilterChainProxy.doFilterInternal(FilterChainProxy.java:211) ~[spring-security-web-5.4.2.jar:5.4.2]
    at org.springframework.security.web.FilterChainProxy.doFilter(FilterChainProxy.java:183) ~[spring-security-web-5.4.2.jar:5.4.2]
    at org.springframework.web.filter.DelegatingFilterProxy.invokeDelegate(DelegatingFilterProxy.java:358) ~[spring-web-5.3.2.jar:5.3.2]
    at org.springframework.web.filter.DelegatingFilterProxy.doFilter(DelegatingFilterProxy.java:271) ~[spring-web-5.3.2.jar:5.3.2]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.41.jar:9.0.41]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.41.jar:9.0.41]
    at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:100) ~[spring-web-5.3.2.jar:5.3.2]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) ~[spring-web-5.3.2.jar:5.3.2]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.41.jar:9.0.41]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.41.jar:9.0.41]
    at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:93) ~[spring-web-5.3.2.jar:5.3.2]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) ~[spring-web-5.3.2.jar:5.3.2]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.41.jar:9.0.41]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.41.jar:9.0.41]
    at org.springframework.boot.actuate.metrics.web.servlet.WebMvcMetricsFilter.doFilterInternal(WebMvcMetricsFilter.java:93) ~[spring-boot-actuator-2.4.1.jar:2.4.1]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) ~[spring-web-5.3.2.jar:5.3.2]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.41.jar:9.0.41]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.41.jar:9.0.41]
    at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201) ~[spring-web-5.3.2.jar:5.3.2]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) ~[spring-web-5.3.2.jar:5.3.2]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.41.jar:9.0.41]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.41.jar:9.0.41]
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:202) [tomcat-embed-core-9.0.41.jar:9.0.41]
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:97) [tomcat-embed-core-9.0.41.jar:9.0.41]
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:542) [tomcat-embed-core-9.0.41.jar:9.0.41]
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:143) [tomcat-embed-core-9.0.41.jar:9.0.41]
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92) [tomcat-embed-core-9.0.41.jar:9.0.41]
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:78) [tomcat-embed-core-9.0.41.jar:9.0.41]
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343) [tomcat-embed-core-9.0.41.jar:9.0.41]
    at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:374) [tomcat-embed-core-9.0.41.jar:9.0.41]
    at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65) [tomcat-embed-core-9.0.41.jar:9.0.41]
    at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:888) [tomcat-embed-core-9.0.41.jar:9.0.41]
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1597) [tomcat-embed-core-9.0.41.jar:9.0.41]
    at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) [tomcat-embed-core-9.0.41.jar:9.0.41]
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128) [?:?]
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628) [?:?]
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-9.0.41.jar:9.0.41]
    at java.lang.Thread.run(Thread.java:834) [?:?]
2021-02-19 00:13:08 ERROR [localhost]:175 - Exception Processing ErrorPage[errorCode=0, location=/error]
org.apache.catalina.connector.ClientAbortException: java.io.IOException: An established connection was aborted by the software in your host machine
    at org.apache.catalina.connector.OutputBuffer.doFlush(OutputBuffer.java:309) ~[tomcat-embed-core-9.0.41.jar:9.0.41]
    at org.apache.catalina.connector.OutputBuffer.flush(OutputBuffer.java:272) ~[tomcat-embed-core-9.0.41.jar:9.0.41]
    at org.apache.catalina.connector.CoyoteOutputStream.flush(CoyoteOutputStream.java:118) ~[tomcat-embed-core-9.0.41.jar:9.0.41]
    at org.springframework.security.web.util.OnCommittedResponseWrapper$SaveContextServletOutputStream.flush(OnCommittedResponseWrapper.java:523) ~[spring-security-web-5.4.2.jar:5.4.2]
    at java.io.FilterOutputStream.flush(FilterOutputStream.java:153) ~[?:?]
    at com.fasterxml.jackson.core.json.UTF8JsonGenerator.flush(UTF8JsonGenerator.java:1176) ~[jackson-core-2.11.3.jar:2.11.3]

One thing to note is that when just testing API2 and sending file with postman, it works without giving any error. It makes me believe that there is some problem in the code of either sending or receiving of the files by these APIs

Answer 1

So I resolved this with the help of this post Spring Web Reactive client

When using WebClient to send multipart file "Spring really needs the Content-Disposition header to include a filename for a upload to be serialized to a MultipartFile in the Controller"

Thus, I changed my code in API1 from

builder.part("file", file);

to

builder.part("file", new ByteArrayResource(multipartFile.getBytes())).filename(multipartFile.getName());

I also changed my header in the webClient to multipart form data instead of JSON, since I am not sending any JSON data but multipart file, like so

Flux<DTO> flux = webClient.post()
                .uri(urlPath)
                .contentType(MediaType.MULTIPART_FORM_DATA)
                .header(HttpHeaders.CONTENT_TYPE, MediaType.MULTIPART_FORM_DATA.toString())
                .accept(MediaType.APPLICATION_JSON)
                .body(BodyInserters.fromMultipartData(builder.build()))
                .retrieve()
                .bodyToFlux(DTO.class);

Hope this helps someone who is getting a similar "Stream ended unexpectedly" error