Occasional NullPointerException when publishing STOMP message using SpringFramework - spring-boot

Question

I am running into some occasional NPE when publishing STOMP messages using Spring (5.2.0).
Unfortunately I cannot precisely provide the steps to reproduce but I was hoping to get some insight into how this could happen.
I looked into the source and the only thing I can think of is that the channel is null. I'm hoping to get a confirmation and possible reason for this, or an alternate explanation.
The client did not disconnect but it seems like this error causes the client to disconnect as it receives a 1002 code.
Attached is the stacktrace. Any help or insight would be greatly appreciated. Thanks!
org.springframework.messaging.MessageDeliveryException: Failed to send message to ExecutorSubscribableChannel[clientInboundChannel]; nested exception is java.lang.NullPointerException
at org.springframework.messaging.support.AbstractMessageChannel.send(AbstractMessageChannel.java:146)
at org.springframework.messaging.support.AbstractMessageChannel.send(AbstractMessageChannel.java:122)
at org.springframework.web.socket.messaging.StompSubProtocolHandler.handleMessageFromClient(StompSubProtocolHandler.java:302)
at org.springframework.web.socket.messaging.SubProtocolWebSocketHandler.handleMessage(SubProtocolWebSocketHandler.java:335)
at org.springframework.web.socket.handler.WebSocketHandlerDecorator.handleMessage(WebSocketHandlerDecorator.java:75)
at org.springframework.web.socket.handler.LoggingWebSocketHandlerDecorator.handleMessage(LoggingWebSocketHandlerDecorator.java:56)
at org.springframework.web.socket.handler.ExceptionWebSocketHandlerDecorator.handleMessage(ExceptionWebSocketHandlerDecorator.java:58)
at org.springframework.web.socket.adapter.standard.StandardWebSocketHandlerAdapter.handleTextMessage(StandardWebSocketHandlerAdapter.java:114)
at org.springframework.web.socket.adapter.standard.StandardWebSocketHandlerAdapter.access$000(StandardWebSocketHandlerAdapter.java:43)
at org.springframework.web.socket.adapter.standard.StandardWebSocketHandlerAdapter$3.onMessage(StandardWebSocketHandlerAdapter.java:85)
at org.springframework.web.socket.adapter.standard.StandardWebSocketHandlerAdapter$3.onMessage(StandardWebSocketHandlerAdapter.java:82)
at org.apache.tomcat.websocket.WsFrameBase.sendMessageText(WsFrameBase.java:395)
at org.apache.tomcat.websocket.server.WsFrameServer.sendMessageText(WsFrameServer.java:119)
at org.apache.tomcat.websocket.WsFrameBase.processDataText(WsFrameBase.java:495)
at org.apache.tomcat.websocket.WsFrameBase.processData(WsFrameBase.java:294)
at org.apache.tomcat.websocket.WsFrameBase.processInputBuffer(WsFrameBase.java:133)
at org.apache.tomcat.websocket.server.WsFrameServer.onDataAvailable(WsFrameServer.java:82)
at org.apache.tomcat.websocket.server.WsFrameServer.doOnDataAvailable(WsFrameServer.java:171)
at org.apache.tomcat.websocket.server.WsFrameServer.notifyDataAvailable(WsFrameServer.java:151)
at org.apache.tomcat.websocket.server.WsHttpUpgradeHandler.upgradeDispatch(WsHttpUpgradeHandler.java:148)
at org.apache.coyote.http11.upgrade.UpgradeProcessorInternal.dispatch(UpgradeProcessorInternal.java:54)
at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:53)
at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:861)
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1579)
at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.NullPointerException: null
at org.springframework.messaging.support.AbstractMessageChannel$ChannelInterceptorChain.applyPreSend(AbstractMessageChannel.java:178)
at org.springframework.messaging.support.AbstractMessageChannel.send(AbstractMessageChannel.java:132)
... 28 common frames omitted```

Answer 1

Not related to this problem; but you should try to keep up with maintenance releases; the current 5.2.x version is 5.2.6.
That NPE implies you have somehow ended up with a null in the interceptor list.
Line 178 in the abstract channel is invoking each interceptor in the list.
https://github.com/spring-projects/spring-framework/blob/927b8c15ef20eaaa4002d4b2170cc536a6d6aa35/spring-messaging/src/main/java/org/springframework/messaging/support/AbstractMessageChannel.java#L178
I think the only way you can get an NPE there is if you have (or something has) added null to the list.
for (ChannelInterceptor interceptor : interceptors) {
Message<?> resolvedMessage = interceptor.preSend(messageToUse, channel); // line 178
if (resolvedMessage == null) {
...