#Vaadin 14 Websocket PushHandler#connectionLost demo This is a demonstration project of how Vaadin's PushHandler is leaving CurrentInstances in the connectionLost method when the UI is reloaded in a Chromium based browser and push transport is WEBSOCKET(+XHR).

Using

• Vaadin v14.2.0 and also v14.1.x with automatic PUSH mode and WEBSOCKET+XHR PUSH transport (no issues with LONG_POLLING)
• Spring-Boot v2.3.0-RELEASE and also v2.2.x-RELEASE
• Tomcat 9.0.35 (from Boot's BOM)

Build and run with maven, or your favourite IDE as usual. Load up http://localhost:8080/ in a Chromium based browser then hit F5. Check the application log for LoggingTomcatEndpointThreadPoolExecutor messages like this:

2020-06-11 11:57:11.468  WARN 6940 --- [    demo-exec-8] $LoggingTomcatEndpointThreadPoolExecutor : Vaadin CurrentInstance is not empty after execution, clearing now. UI: A57D87D7B3AF3EB1DBF696F2457DC1C9/0
2020-06-11 11:57:16.265  WARN 6940 --- [    demo-exec-2] $LoggingTomcatEndpointThreadPoolExecutor : Vaadin CurrentInstance is not empty after execution, clearing now. UI: A57D87D7B3AF3EB1DBF696F2457DC1C9/1

Repeat this in Firefox, or old MS Edge, and you will not see these warnings.

Got log warnings on:

• Chrome v83.0.4103.97 (64 bit)
• Microsoft Edge v84.0.522.9 dev (64 bit)

No issues on:

• Firefox v76.0.1 (64-bit)
• Microsoft Edge 44.18362.449.0 (the old one)

---

Reloading Vaadin UI-s with Websocket Push enabled in Chromium based browsers works differently than eg. Firefox/old MS Edge. The PushHandler#connectionLost method is not executed by VaadinService#handleRequest but Tomcat's UpgradeProcessorInternal and Atmosphere's JSR356Endpoint and PushAtmosphereHandler classes, like this:

connectionLost:294, PushHandler (com.vaadin.flow.server.communication)
onStateChange:60, PushAtmosphereHandler (com.vaadin.flow.server.communication)
invokeAtmosphereHandler:551, AsynchronousProcessor (org.atmosphere.cpr)
completeLifecycle:493, AsynchronousProcessor (org.atmosphere.cpr)
endRequest:604, AsynchronousProcessor (org.atmosphere.cpr)
executeClose:710, DefaultWebSocketProcessor (org.atmosphere.websocket)
close:658, DefaultWebSocketProcessor (org.atmosphere.websocket)
onClose:310, JSR356Endpoint (org.atmosphere.container)
fireEndpointOnClose:555, WsSession (org.apache.tomcat.websocket)
onClose:533, WsSession (org.apache.tomcat.websocket)
processDataControl:347, WsFrameBase (org.apache.tomcat.websocket)
processData:289, WsFrameBase (org.apache.tomcat.websocket)
processInputBuffer:133, WsFrameBase (org.apache.tomcat.websocket)
onDataAvailable:82, WsFrameServer (org.apache.tomcat.websocket.server)
doOnDataAvailable:171, WsFrameServer (org.apache.tomcat.websocket.server)
notifyDataAvailable:151, WsFrameServer (org.apache.tomcat.websocket.server)
upgradeDispatch:148, WsHttpUpgradeHandler (org.apache.tomcat.websocket.server)
dispatch:54, UpgradeProcessorInternal (org.apache.coyote.http11.upgrade)
process:59, AbstractProcessorLight (org.apache.coyote)
process:868, AbstractProtocol$ConnectionHandler (org.apache.coyote)
doRun:1590, NioEndpoint$SocketProcessor (org.apache.tomcat.util.net)
run:49, SocketProcessorBase (org.apache.tomcat.util.net)
runWorker:1128, ThreadPoolExecutor (java.util.concurrent)
run:628, ThreadPoolExecutor$Worker (java.util.concurrent)
run:61, TaskThread$WrappingRunnable (org.apache.tomcat.util.threads)
run:834, Thread (java.lang)

The same use-case with Firefox (via VaadinService#handleRequest):

connectionLost:294, PushHandler (com.vaadin.flow.server.communication)
onDisconnect:111, PushAtmosphereHandler$AtmosphereResourceListener (com.vaadin.flow.server.communication)
onDisconnect:759, AtmosphereResourceImpl (org.atmosphere.cpr)
notifyListeners:650, AtmosphereResourceImpl (org.atmosphere.cpr)
notifyListeners:630, AtmosphereResourceImpl (org.atmosphere.cpr)
completeLifecycle:499, AsynchronousProcessor (org.atmosphere.cpr)
inspect:77, OnDisconnectInterceptor (org.atmosphere.interceptor)
invokeInterceptors:343, AsynchronousProcessor (org.atmosphere.cpr)
action:200, AsynchronousProcessor (org.atmosphere.cpr)
suspended:114, AsynchronousProcessor (org.atmosphere.cpr)
service:67, Servlet30CometSupport (org.atmosphere.container)
doCometSupport:2297, AtmosphereFramework (org.atmosphere.cpr)
handleRequest:249, PushRequestHandler (com.vaadin.flow.server.communication)
handleRequest:1545, VaadinService (com.vaadin.flow.server)
service:247, VaadinServlet (com.vaadin.flow.server)
service:120, SpringServlet (com.vaadin.flow.spring)
service:741, HttpServlet (javax.servlet.http)
internalDoFilter:231, ApplicationFilterChain (org.apache.catalina.core)
doFilter:166, ApplicationFilterChain (org.apache.catalina.core)
doFilter:53, WsFilter (org.apache.tomcat.websocket.server)
internalDoFilter:193, ApplicationFilterChain (org.apache.catalina.core)
doFilter:166, ApplicationFilterChain (org.apache.catalina.core)
doFilterInternal:100, RequestContextFilter (org.springframework.web.filter)
doFilter:119, OncePerRequestFilter (org.springframework.web.filter)
internalDoFilter:193, ApplicationFilterChain (org.apache.catalina.core)
doFilter:166, ApplicationFilterChain (org.apache.catalina.core)
doFilterInternal:93, FormContentFilter (org.springframework.web.filter)
doFilter:119, OncePerRequestFilter (org.springframework.web.filter)
internalDoFilter:193, ApplicationFilterChain (org.apache.catalina.core)
doFilter:166, ApplicationFilterChain (org.apache.catalina.core)
doFilterInternal:201, CharacterEncodingFilter (org.springframework.web.filter)
doFilter:119, OncePerRequestFilter (org.springframework.web.filter)
internalDoFilter:193, ApplicationFilterChain (org.apache.catalina.core)
doFilter:166, ApplicationFilterChain (org.apache.catalina.core)
invoke:202, StandardWrapperValve (org.apache.catalina.core)
invoke:96, StandardContextValve (org.apache.catalina.core)
invoke:541, AuthenticatorBase (org.apache.catalina.authenticator)
invoke:139, StandardHostValve (org.apache.catalina.core)
invoke:92, ErrorReportValve (org.apache.catalina.valves)
invoke:74, StandardEngineValve (org.apache.catalina.core)
service:343, CoyoteAdapter (org.apache.catalina.connector)
service:373, Http11Processor (org.apache.coyote.http11)
process:65, AbstractProcessorLight (org.apache.coyote)
process:868, AbstractProtocol$ConnectionHandler (org.apache.coyote)
doRun:1590, NioEndpoint$SocketProcessor (org.apache.tomcat.util.net)
run:49, SocketProcessorBase (org.apache.tomcat.util.net)
runWorker:1128, ThreadPoolExecutor (java.util.concurrent)
run:628, ThreadPoolExecutor$Worker (java.util.concurrent)
run:61, TaskThread$WrappingRunnable (org.apache.tomcat.util.threads)
run:834, Thread (java.lang)

The com.vaadin.flow.server.communication.PushHandler.connectionLost executes service.findVaadinSession(vaadinRequest) and service.findUI(vaadinRequest) and this populates the com.vaadin.flow.internal.CurrentInstance.instances ThreadLocal. This Session/UI context holder is not cleared properly when the caller is PushAtmosphereHandler (no one calls com.vaadin.flow.internal.CurrentInstance.clearAll). This leads to leftover Vaadin CurrentInstance state on Tomcat managed (pooled) execution threads.

This could be a serious problem if the application also uses "non Vaadin" code like Spring RestControllers, or custom Servlets, etc.

In our application we are using a generic Aspect that uses an optional UI scoped Bean that stores a user selected entity filter, and applies the selection to JPA queries. When no such Bean exists, or it doesn't contain selection, the aspect doesn't touch the queries. We also have "non Vaadin" code (like a Spring Security UserDetailsService implementation) that calls some otherwise aspect enveloped method. Normally one would expect that in "non Vaadin-pure Spring" code no UI-s exist, hence the aspect won't do anything to the queries. Unfortunately when leftover CurrentInstances remain from a former PushHandler thread this is not the case. :(

There was a similar problem not too long ago with PushHandler, please check vaadin/flow#7116 .

P.S. AFAIK LONG_POLLING PUSH request are unaffected by this because they reach PushHandler via VaadinService#handleRequest, which cleans CurrentInstance ThreadLocals in the requestEnd method.