Why Are Jenkins Logs Filled With ALPN Callback Dropped Spam?

Issue

The following INFO level log appears repeatedly:

o.internal.platform.Platform#log: ALPN callback dropped: HTTP/2 is disabled. Is alpn-boot on the boot class path?

Environment

CloudBees CI on modern cloud platforms - operations center
CloudBees CI on modern cloud platforms - managed controller
CloudBees CI on traditional platforms - operations center
CloudBees CI on traditional platforms - client controller
Jenkins

Resolution

This happens when the client talks to a server that supports http2, but the JVM does not have ALPN support. ALPN is supported in JDK 9 and later by default, and was backported to JDK 8u252. Upgrading to 8u252 or later will remove the log. See here for more details.

Workaround

If upgrading the JDK is not possible, set the the log level of com.squareup.okhttp.OkHttpClient and okhttp3.OkHttpClient to WARNING.