title: Spring 5 Reactive fails when extending Flux/implementing Publisher and calling s.onNext() more than once
body: <p>I just started playing with the new Spring 5 reactive support and wanted to simulate some asyncronous data generation, having noticed two faulty behaviours:</p>
  
  <p><strong>1) Calling s.onNext( String ) more than once:</strong></p>
  
  <pre><code>@GetMapping("/strings")
  public Publisher&lt;String&gt; getStrings(){
  
      return new Publisher&lt;String&gt;() {
  
          @Override
          public void subscribe(Subscriber&lt;? super String&gt; s) {
              int i = 0;
              while(++i &lt;= 5){
                  try {
                      Thread.sleep(1000);
                  } catch (InterruptedException e) {
                      e.printStackTrace();
                  }
                  s.onNext("message");
              }
              s.onComplete();
          }
      };
  }
  </code></pre>
  
  <p>In this case the stacktrace is:</p>
  
  <pre><code>2016-08-03 13:35:04.986 DEBUG 5136 --- [nio-8080-exec-1] s.w.r.r.m.a.RequestMappingHandlerMapping : Looking up handler method for path /strings
  2016-08-03 13:35:04.994 DEBUG 5136 --- [nio-8080-exec-1] s.w.r.r.m.a.RequestMappingHandlerMapping : Returning handler method [public org.reactivestreams.Publisher&lt;java.lang.String&gt; com.codependent.spring5.playground.reactive.web.AccountsController.getStrings()]
  2016-08-03 13:35:04.994 DEBUG 5136 --- [nio-8080-exec-1] o.s.b.f.s.DefaultListableBeanFactory     : Returning cached instance of singleton bean 'accountsController'
  2016-08-03 13:35:07.120 DEBUG 5136 --- [nio-8080-exec-1] o.s.w.s.h.ExceptionHandlingWebHandler    : Could not complete request
  
  java.lang.IllegalStateException: RECEIVED
      at org.springframework.http.server.reactive.AbstractResponseBodyProcessor$State.onNext(AbstractResponseBodyProcessor.java:316) ~[spring-web-5.0.0.M1.jar:5.0.0.M1]
      at org.springframework.http.server.reactive.AbstractResponseBodyProcessor.onNext(AbstractResponseBodyProcessor.java:77) ~[spring-web-5.0.0.M1.jar:5.0.0.M1]
      at org.springframework.http.server.reactive.AbstractResponseBodyProcessor.onNext(AbstractResponseBodyProcessor.java:47) ~[spring-web-5.0.0.M1.jar:5.0.0.M1]
      at org.springframework.http.server.reactive.ChannelSendOperator$WriteWithBarrier.doNext(ChannelSendOperator.java:97) ~[spring-web-5.0.0.M1.jar:5.0.0.M1]
      at reactor.core.publisher.OperatorAdapter.onNext(OperatorAdapter.java:88) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.FluxMap$MapSubscriber.onNext(FluxMap.java:123) [reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at com.codependent.spring5.playground.reactive.web.AccountsController$4.subscribe(AccountsController.java:107) [classes/:na]
      at reactor.core.publisher.FluxSource.subscribe(FluxSource.java:59) [reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.FluxMap.subscribe(FluxMap.java:73) [reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at org.springframework.http.server.reactive.ChannelSendOperator.subscribe(ChannelSendOperator.java:54) [spring-web-5.0.0.M1.jar:5.0.0.M1]
      at reactor.core.publisher.MonoOtherwise.subscribe(MonoOtherwise.java:47) [reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.MonoThenApply$MonoThenApplyManager.onNext(MonoThenApply.java:133) [reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.Operators$DeferredScalarSubscriber.complete(Operators.java:797) [reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.MonoThenApply$MonoThenApplyManager$SecondSubscriber.onNext(MonoThenApply.java:203) [reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.FluxResume$ResumeSubscriber.onNext(FluxResume.java:75) [reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.onNext(FluxMapFuseable.java:130) [reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.Operators$ScalarSubscription.request(Operators.java:1293) [reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.request(FluxMapFuseable.java:186) [reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.Operators$MultiSubscriptionSubscriber.set(Operators.java:1010) [reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.FluxResume$ResumeSubscriber.onSubscribe(FluxResume.java:70) [reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.onSubscribe(FluxMapFuseable.java:100) [reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.FluxFlatMap.trySubscribeScalarMap(FluxFlatMap.java:169) [reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.MonoThenApply.subscribe(MonoThenApply.java:51) [reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.MonoMapFuseable.subscribe(MonoMapFuseable.java:69) [reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.MonoOtherwise.subscribe(MonoOtherwise.java:47) [reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.MonoThenApply$MonoThenApplyManager.onNext(MonoThenApply.java:133) [reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.FluxSwitchIfEmpty$SwitchIfEmptySubscriber.onNext(FluxSwitchIfEmpty.java:71) [reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.MonoNext$NextSubscriber.onNext(MonoNext.java:82) [reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.FluxConcatMap$ConcatMapImmediate.drain(FluxConcatMap.java:383) [reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.FluxConcatMap$ConcatMapImmediate.onSubscribe(FluxConcatMap.java:192) [reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.FluxIterable.subscribe(FluxIterable.java:96) [reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.FluxIterable.subscribe(FluxIterable.java:60) [reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.FluxConcatMap.subscribe(FluxConcatMap.java:116) [reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.MonoNext.subscribe(MonoNext.java:45) [reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.MonoOtherwiseIfEmpty.subscribe(MonoOtherwiseIfEmpty.java:47) [reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.MonoThenApply.subscribe(MonoThenApply.java:58) [reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.MonoThenApply.subscribe(MonoThenApply.java:58) [reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.MonoOtherwise.subscribe(MonoOtherwise.java:47) [reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.MonoOtherwise.subscribe(MonoOtherwise.java:47) [reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.MonoOtherwise.subscribe(MonoOtherwise.java:47) [reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.MonoThenSupply$MonoConcatIgnoreManager.drain(MonoThenSupply.java:167) [reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.MonoThenSupply.subscribe(MonoThenSupply.java:55) [reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at org.springframework.http.server.reactive.ServletHttpHandlerAdapter.service(ServletHttpHandlerAdapter.java:93) [spring-web-5.0.0.M1.jar:5.0.0.M1]
      at javax.servlet.http.HttpServlet.service(HttpServlet.java:729) [tomcat-embed-core-8.5.4.jar:8.5.4]
      at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:230) [tomcat-embed-core-8.5.4.jar:8.5.4]
      at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:165) [tomcat-embed-core-8.5.4.jar:8.5.4]
      at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:198) [tomcat-embed-core-8.5.4.jar:8.5.4]
      at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:108) [tomcat-embed-core-8.5.4.jar:8.5.4]
      at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:522) [tomcat-embed-core-8.5.4.jar:8.5.4]
      at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140) [tomcat-embed-core-8.5.4.jar:8.5.4]
      at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79) [tomcat-embed-core-8.5.4.jar:8.5.4]
      at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87) [tomcat-embed-core-8.5.4.jar:8.5.4]
      at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:349) [tomcat-embed-core-8.5.4.jar:8.5.4]
      at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:1110) [tomcat-embed-core-8.5.4.jar:8.5.4]
      at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66) [tomcat-embed-core-8.5.4.jar:8.5.4]
      at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:785) [tomcat-embed-core-8.5.4.jar:8.5.4]
      at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1425) [tomcat-embed-core-8.5.4.jar:8.5.4]
      at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) [tomcat-embed-core-8.5.4.jar:8.5.4]
      at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) [na:1.8.0_45]
      at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) [na:1.8.0_45]
      at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-8.5.4.jar:8.5.4]
      at java.lang.Thread.run(Thread.java:745) [na:1.8.0_45]
  
  2016-08-03 13:35:07.121 DEBUG 5136 --- [nio-8080-exec-1] o.s.h.s.r.ServletServerHttpResponse      : Can't set the status 500 because the HTTP response has already been committed
  2016-08-03 13:35:08.127 ERROR 5136 --- [nio-8080-exec-1] a.c.c.C.[.[.0.0.0.[.[httpHandlerServlet] : Servlet.service() for servlet [httpHandlerServlet] in context with path [] threw exception
  
  reactor.core.Exceptions$BubblingException: java.lang.IllegalStateException: RECEIVED
      at reactor.core.Exceptions.bubble(Exceptions.java:97) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.Exceptions.onErrorDropped(Exceptions.java:263) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.MonoThenApply$MonoThenApplyManager$SecondSubscriber.onError(MonoThenApply.java:209) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.FluxResume$ResumeSubscriber.onError(FluxResume.java:105) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.OperatorAdapter.doOnSubscriberError(OperatorAdapter.java:113) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.OperatorAdapter.onNext(OperatorAdapter.java:91) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.FluxMap$MapSubscriber.onNext(FluxMap.java:123) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at com.codependent.spring5.playground.reactive.web.AccountsController$4.subscribe(AccountsController.java:107) ~[classes/:na]
      at reactor.core.publisher.FluxSource.subscribe(FluxSource.java:59) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.FluxMap.subscribe(FluxMap.java:73) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at org.springframework.http.server.reactive.ChannelSendOperator.subscribe(ChannelSendOperator.java:54) ~[spring-web-5.0.0.M1.jar:5.0.0.M1]
      at reactor.core.publisher.MonoOtherwise.subscribe(MonoOtherwise.java:47) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.MonoThenApply$MonoThenApplyManager.onNext(MonoThenApply.java:133) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.Operators$DeferredScalarSubscriber.complete(Operators.java:797) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.MonoThenApply$MonoThenApplyManager$SecondSubscriber.onNext(MonoThenApply.java:203) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.FluxResume$ResumeSubscriber.onNext(FluxResume.java:75) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.onNext(FluxMapFuseable.java:130) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.Operators$ScalarSubscription.request(Operators.java:1293) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.request(FluxMapFuseable.java:186) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.Operators$MultiSubscriptionSubscriber.set(Operators.java:1010) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.FluxResume$ResumeSubscriber.onSubscribe(FluxResume.java:70) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.onSubscribe(FluxMapFuseable.java:100) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.FluxFlatMap.trySubscribeScalarMap(FluxFlatMap.java:169) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.MonoThenApply.subscribe(MonoThenApply.java:51) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.MonoMapFuseable.subscribe(MonoMapFuseable.java:69) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.MonoOtherwise.subscribe(MonoOtherwise.java:47) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.MonoThenApply$MonoThenApplyManager.onNext(MonoThenApply.java:133) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.FluxSwitchIfEmpty$SwitchIfEmptySubscriber.onNext(FluxSwitchIfEmpty.java:71) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.MonoNext$NextSubscriber.onNext(MonoNext.java:82) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.FluxConcatMap$ConcatMapImmediate.drain(FluxConcatMap.java:383) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.FluxConcatMap$ConcatMapImmediate.onSubscribe(FluxConcatMap.java:192) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.FluxIterable.subscribe(FluxIterable.java:96) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.FluxIterable.subscribe(FluxIterable.java:60) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.FluxConcatMap.subscribe(FluxConcatMap.java:116) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.MonoNext.subscribe(MonoNext.java:45) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.MonoOtherwiseIfEmpty.subscribe(MonoOtherwiseIfEmpty.java:47) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.MonoThenApply.subscribe(MonoThenApply.java:58) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.MonoThenApply.subscribe(MonoThenApply.java:58) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.MonoOtherwise.subscribe(MonoOtherwise.java:47) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.MonoOtherwise.subscribe(MonoOtherwise.java:47) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.MonoOtherwise.subscribe(MonoOtherwise.java:47) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.MonoThenSupply$MonoConcatIgnoreManager.drain(MonoThenSupply.java:167) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at reactor.core.publisher.MonoThenSupply.subscribe(MonoThenSupply.java:55) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      at org.springframework.http.server.reactive.ServletHttpHandlerAdapter.service(ServletHttpHandlerAdapter.java:93) ~[spring-web-5.0.0.M1.jar:5.0.0.M1]
      at javax.servlet.http.HttpServlet.service(HttpServlet.java:729) ~[tomcat-embed-core-8.5.4.jar:8.5.4]
      at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:230) ~[tomcat-embed-core-8.5.4.jar:8.5.4]
      at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:165) ~[tomcat-embed-core-8.5.4.jar:8.5.4]
      at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:198) ~[tomcat-embed-core-8.5.4.jar:8.5.4]
      at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:108) [tomcat-embed-core-8.5.4.jar:8.5.4]
      at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:522) [tomcat-embed-core-8.5.4.jar:8.5.4]
      at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140) [tomcat-embed-core-8.5.4.jar:8.5.4]
      at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79) [tomcat-embed-core-8.5.4.jar:8.5.4]
      at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87) [tomcat-embed-core-8.5.4.jar:8.5.4]
      at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:349) [tomcat-embed-core-8.5.4.jar:8.5.4]
      at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:1110) [tomcat-embed-core-8.5.4.jar:8.5.4]
      at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66) [tomcat-embed-core-8.5.4.jar:8.5.4]
      at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:785) [tomcat-embed-core-8.5.4.jar:8.5.4]
      at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1425) [tomcat-embed-core-8.5.4.jar:8.5.4]
      at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) [tomcat-embed-core-8.5.4.jar:8.5.4]
      at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) [na:1.8.0_45]
      at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) [na:1.8.0_45]
      at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-8.5.4.jar:8.5.4]
      at java.lang.Thread.run(Thread.java:745) [na:1.8.0_45]
  Caused by: java.lang.IllegalStateException: RECEIVED
      at org.springframework.http.server.reactive.AbstractResponseBodyProcessor$State.onNext(AbstractResponseBodyProcessor.java:316) ~[spring-web-5.0.0.M1.jar:5.0.0.M1]
      at org.springframework.http.server.reactive.AbstractResponseBodyProcessor.onNext(AbstractResponseBodyProcessor.java:77) ~[spring-web-5.0.0.M1.jar:5.0.0.M1]
      at org.springframework.http.server.reactive.AbstractResponseBodyProcessor.onNext(AbstractResponseBodyProcessor.java:47) ~[spring-web-5.0.0.M1.jar:5.0.0.M1]
      at org.springframework.http.server.reactive.ChannelSendOperator$WriteWithBarrier.doNext(ChannelSendOperator.java:97) ~[spring-web-5.0.0.M1.jar:5.0.0.M1]
      at reactor.core.publisher.OperatorAdapter.onNext(OperatorAdapter.java:88) ~[reactor-core-3.0.0.BUILD-SNAPSHOT.jar:na]
      ... 57 common frames omitted
  </code></pre>
  
  <p><strong>2) Calling s.onNext( Alert.class -any DTO- ) more than once:</strong></p>
  
  <pre><code>@GetMapping("/alerts")
  public Publisher&lt;Alert&gt; getAlerts(){
  
      return new Publisher&lt;Alert&gt;() {
  
          @Override
          public void subscribe(Subscriber&lt;? super Alert&gt; s) {
              int i = 0;
              while(++i &lt;= 5){
                  try {
                      Thread.sleep(1000);
                  } catch (InterruptedException e) {
                      e.printStackTrace();
                  }
                  s.onNext(new Alert((long)1, "ms"));
              }
              s.onComplete();
          }
      };
  }
  </code></pre>
  
  <p>Now it doesn't show any error on the logs but <strong>the caller gets a 500 response code and the content '['</strong>.</p>
  
  <p>Log:</p>
  
  <pre><code>2016-08-03 13:37:11.834 DEBUG 5136 --- [nio-8080-exec-3] o.s.web.reactive.DispatcherHandler       : Processing GET request for [http://localhost:8080/alerts]
  2016-08-03 13:37:11.835 DEBUG 5136 --- [nio-8080-exec-3] s.w.r.r.m.a.RequestMappingHandlerMapping : Looking up handler method for path /alerts
  2016-08-03 13:37:11.836 DEBUG 5136 --- [nio-8080-exec-3] s.w.r.r.m.a.RequestMappingHandlerMapping : Returning handler method [public org.reactivestreams.Publisher&lt;com.codependent.spring5.playground.reactive.dto.Alert&gt; com.codependent.spring5.playground.reactive.web.AccountsController.getAlerts()]
  2016-08-03 13:37:11.836 DEBUG 5136 --- [nio-8080-exec-3] o.s.b.f.s.DefaultListableBeanFactory     : Returning cached instance of singleton bean 'accountsController'
  </code></pre>
  
  <p>Why can't we invoke onNext() multiple times? How could we do that?</p>
  
  <p><strong>NOTE:</strong> I if just call <code>onNext</code> once it works ok: </p>
  
  <pre><code>@Override
  public void subscribe(Subscriber&lt;? super String&gt; s) {
      s.onNext("my message" + Math.random());
      s.onComplete();
  }
  </code></pre>
  
  <p>or </p>
  
  <pre><code>@Override
  public void subscribe(Subscriber&lt;? super Alert&gt; s) {
      s.onNext(new Alert((long)1, "ms"));
      s.onComplete();
  }
  </code></pre>