I have two applications, A and B, running in Tomcat 10.1.28 on RHEL
8.10 with Java 21 (OpenJDK Runtime Environment Red_Hat-21.0.5.0.10-1).
Application uses an AsyncContext and SSE to send messages back to an
application running in a Chrome browser. Sometimes I get an NPE with
the following stacktrace from app A:
04-Jan-2025 21:58:12.447 SEVERE [http-nio-8080-exec-11]
org.apache.catalina.core.Async
ContextImpl$RunnableWrapper.run Error
during processing of asynchronous Runnable via AsyncContext.start()
java.lang.NullPointerException: Cannot invoke
"org.apache.catalina.connector.OutputBuffer.setErrorException(java.lang.Exception)"
because "this.ob" is null
at
org.apache.catalina.connector.CoyoteOutputStream.flush(CoyoteOutputStream.java:138)
at com.a.b.c.A$2.run(A.java:123)
at
org.apache.catalina.core.AsyncContextImpl$RunnableWrapper.run(AsyncContextImpl.java:562)
at
org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1190)
at
org.apache.tomcat.util.threads.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:659)
at
org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:63)
at java.base/java.lang.Thread.run(Thread.java:1583)
In a development environment, I can reproduce an IOE in
CoyoteOutputStream.flush() if I in the browser switch to another tab
while my application A is trying to send a message to it. But I cannot
reproduce the following NPE shown above.
I've also tried to figure out why ob is set to null while in
CoyoteOutputStream.flush(), but that hasn't helped me much. Anyway,
I'm surprised to get an NPE from the platform.
In various logs from nightly(?) Tomcat test runs, I've been able to
find a similar problem. See this, for instance:
https://nightlies.apache.org/tomcat/tomcat-10.1.x/logs/1309/TEST-org.apache.coyote.http2.TestAsyncError.NIO.txt
.
Now, the next thing that happens is even more surprising to me.
Sometimes the uncaught NPE triggers an HTTP 500 response on a request
to a servlet in application B! This I've been able to reproduce in my
development environment, if I simply throw an NPE in application A
from withing the AsyncContext as shown in the stacktrace above while I
simultaneously keep requests coming in to application B. Then it
sometimes happens. I haven't been able to see what's going on in
application B when this happens, but my trace logs don't show anything
suspicious. The client making requests to application B just get's
back this:
<!doctype html><html lang="en"><head><title>HTTP Status 500 – Internal
Server Error</title><style type="text/css">body
{font-family:Tahoma,Arial,sans-serif;} h1, h2, h3, b
{color:white;background-color:#525D76;} h1 {font-size:22px;} h2
{font-size:16px;} h3 {font-size:14px;} p {font-size:12px;} a
{color:black;} .line
{height:1px;background-color:#525D76;border:none;}</style></head><body><h1>HTTP
Status 500 – Internal Server Error</h1><hr class="line"
/><p><b>Type</b> Exception Report</p><p><b>Description</b> The server
encountered an unexpected condition that prevented it from fulfilling
the request.</p><p><b>Exception</b></p><pre>java.lang.NullPointerException
</pre><p><b>Note</b> The full stack trace of the root cause is
available in the server logs.</p><hr class="line" /><h3>Apache
Tomcat/10.1.28</h3></body></html>
I don't know much about all this, but isn't both the NPE and it's
effect on another application's requests surprising?
For now, I've worked around the problem by explicitly catching NPEs in
this async code in application A.
Kind regards,
Michael
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