ERROR ExceptionWebSocketHandlerDecorator

i have met some problem when using websocket:

2017-07-08 17:01:17 ERROR ExceptionWebSocketHandlerDecorator:82 - Unhandled error for ExceptionWebSocketHandlerDecorator [delegate=LoggingWebSocketHandlerDecorator [delegate=cn.ehai.onlineservice.controller.ChatWSHandler@548580b4]]
java.lang.NullPointerException
at cn.ehai.onlineservice.controller.ChatWSHandler.afterConnectionClosed(ChatWSHandler.java:106)
at org.springframework.web.socket.handler.WebSocketHandlerDecorator.afterConnectionClosed(WebSocketHandlerDecorator.java:85)
at org.springframework.web.socket.handler.LoggingWebSocketHandlerDecorator.afterConnectionClosed(LoggingWebSocketHandlerDecorator.java:72)
at org.springframework.web.socket.handler.ExceptionWebSocketHandlerDecorator.afterConnectionClosed(ExceptionWebSocketHandlerDecorator.java:78)
at org.springframework.web.socket.adapter.standard.StandardWebSocketHandlerAdapter.onClose(StandardWebSocketHandlerAdapter.java:141)
at org.apache.tomcat.websocket.WsSession.fireEndpointOnClose(WsSession.java:549)
at org.apache.tomcat.websocket.WsSession.doClose(WsSession.java:495)
at org.apache.tomcat.websocket.WsSession.close(WsSession.java:459)
at org.springframework.web.socket.adapter.standard.StandardWebSocketSession.closeInternal(StandardWebSocketSession.java:217)
at org.springframework.web.socket.adapter.AbstractWebSocketSession.close(AbstractWebSocketSession.java:138)
at org.springframework.web.socket.adapter.AbstractWebSocketSession.close(AbstractWebSocketSession.java:129)
at cn.ehai.onlineservice.controller.ChatWSHandler.afterConnectionEstablished(ChatWSHandler.java:60)
at org.springframework.web.socket.handler.WebSocketHandlerDecorator.afterConnectionEstablished(WebSocketHandlerDecorator.java:70)
at org.springframework.web.socket.handler.LoggingWebSocketHandlerDecorator.afterConnectionEstablished(LoggingWebSocketHandlerDecorator.java:48)
at org.springframework.web.socket.handler.ExceptionWebSocketHandlerDecorator.afterConnectionEstablished(ExceptionWebSocketHandlerDecorator.java:48)
at org.springframework.web.socket.adapter.standard.StandardWebSocketHandlerAdapter.onOpen(StandardWebSocketHandlerAdapter.java:100)
at org.apache.tomcat.websocket.server.WsHttpUpgradeHandler.init(WsHttpUpgradeHandler.java:133)
at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:844)
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1434)
at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
at java.lang.Thread.run(Thread.java:745)

i have no idea with how it appending. When tomcat restart, the problem will be solved, but repeat after few days.

Start with the reported stack trace and inspect the application's ChatWSHandler.afterConnectionClosed at line 106, then trace the connection lifecycle through afterConnectionEstablished at line 60. Reproduce the failure around WebSocket close handling and identify a reliable cause for the NullPointerException before proposing a fix.