[jetty-users] Jetty httpclient threads keeping program from ending

[Shawn Heisey <eclipse@xxxxxxxxxxxx>]

I have a little program utilizing SolrJ's Http2SolrClient which uses the 
Jetty httpclient.

This is my little program:

https://paste.elyograg.org/view/f9bbe53c

If I run this, the program never ends.  If I change Http2SolrClient to 
HttpSolrClient (an older client utilizing Apache httpclient 4.x) and fix 
the imports, then it ends.  With HttpSolrClient, I do not need to 
provide it with SSL configuration.

Background: we switched to the jetty client to gain http2 support.  I 
wasn't involved in that development, but I believe that the author chose 
not to go with the version 5 Apache httpclient because at the time they 
still did not have a stable release, and the jetty client at the time 
had http2 support in stable releases for about three years.

A little bit of poking around yielded this output from jstack:

https://paste.elyograg.org/view/9b9bcc80

I believe the problem is that the threads the Jetty httpcient starts are 
not classified as Daemon threads, so when the main thread exits, they 
keep running.  I think that the jetty client should mark the threads it 
starts as daemon.

If I uncomment the last statement in the program, which closes the solr 
client (and by extension closing the httpclient), then the program ends 
as expected.

I know someone is going to tell me that I'm breaking the rules by not 
cleaning up the things I started.  In a long running program that 
occasionally creates a client, I would completely agree with that 
sentiment.  For quick and dirty things like what I have here, or for a 
long-running program that uses the same client for its entire life, I 
don't think it's unusual to let Java handle the final cleanup as it exits.

Is there something we could have done in Http2SolrClient that would tell 
the Jetty client to use daemon threads, or would it take a feature 
request here for that?  I have not dived into the code to see what's 
available.

Thanks,
Shawn