Jenkins won't start after Alpine Linux update

I recently upgraded my Jenkins server, running Alpine Linux 13.x, in a VM

It's a straightforward process:

jenkins:~# apk update
jenkins:~# apk upgrade
jenkins:~# reboot

After the VM came back online, the surprise was having Jenkins refusing to start.

I checked the service and it said it's started

jenkins:~# /etc/init.d/jenkins start
 * WARNING: jenkins has already been started

while the status was saying something different

jenkins:~# /etc/init.d/jenkins status
 * status: crashed

I tried to stop/start

jenkins:~# service jenkins stop
 * Stopping jenkins ...   
jenkins:~# service jenkins start
 * Starting jenkins ...

but, the result was the same

jenkins:~# /etc/init.d/jenkins status
 * status: crashed

I've found nothing in Jenkins log file or in system logs.

So, I've tried to start Jenkins manually, and I've found much more helpful information

jenkins:~# java -DJENKINS_HOME=/var/lib/jenkins -jar /usr/share/webapps/jenkins/jenkins.war
Jul 29, 2021 9:41:00 PM Main verifyJavaVersion
SEVERE: Running with Java class version 60 which is not in the list of supported versions: [52, 55]. Run with the --enable-future-java flag to enable such behavior. See https://jenkins.io/redirect/java-support/
java.lang.UnsupportedClassVersionError: 60.0
        at Main.verifyJavaVersion(Main.java:174)
        at Main.main(Main.java:142)

Jenkins requires Java versions [8, 11] but you are running with Java 16 from /usr/lib/jvm/java-16-openjdk
java.lang.UnsupportedClassVersionError: 60.0
        at Main.verifyJavaVersion(Main.java:174)
        at Main.main(Main.java:142)

It was all about the Java version

jenkins:~# java --version
openjdk 16.0.1 2021-04-20
OpenJDK Runtime Environment (build 16.0.1+9-alpine-r0)
OpenJDK 64-Bit Server VM (build 16.0.1+9-alpine-r0, mixed mode, sharing)

I'm quite unsure how it was working before, or how it ended by having OpenJDK 16, buy it looks like it's the right time to install OpenJDK 11 :)

jenkins:~# apk add openjdk11
(1/15) Purging openjdk16-jre-headless (16.0.1_p9-r0)
(2/15) Installing openjdk11-jre-headless (11.0.11_p9-r0)
(3/15) Installing openjdk11-jmods (11.0.11_p9-r0)
(4/15) Installing openjdk11-demos (11.0.11_p9-r0)
(5/15) Installing openjdk11-doc (11.0.11_p9-r0)
(6/15) Installing libxi (1.7.10-r0)
(7/15) Installing libxrender (0.9.10-r3)
(8/15) Installing libxtst (1.2.3-r3)
(9/15) Installing alsa-lib (1.2.5.1-r1)
(10/15) Installing giflib (5.2.1-r0)
(11/15) Installing libjpeg-turbo (2.1.0-r0)
(12/15) Installing lcms2 (2.12-r1)
(13/15) Installing openjdk11-jre (11.0.11_p9-r0)
(14/15) Installing openjdk11-jdk (11.0.11_p9-r0)
(15/15) Installing openjdk11 (11.0.11_p9-r0)
Executing busybox-1.33.1-r4.trigger
Executing java-common-0.4-r0.trigger
OK: 1564 MiB in 247 packages

Now, it's time to try again

jenkins:~# service jenkins restart
 * Caching service dependencies ...                                [ ok ]
 * Stopping jenkins ...
 * start-stop-daemon: no matching processes found                  [ ok ]
 * Starting jenkins ...                                            [ ok ]
jenkins:~# service jenkins status
 * status: started

And, this is how I spent 30 min of my life :)