Last week I upgraded Jenkins to the latest version on the server infrastructure at work. Starting with the Jenkins 2.357 release, Java 11 or Java 17 will be required to run Jenkins. Also, the upcoming LTS release will require Java 11.

This means that I also needed to update Java on our Jenkins servers. Here are the steps that I did to perform the Jenkins and Java upgrade.

SSH into the server and stop the service. Then get the latest upgrades for your server, which is good practice:

service jenkins stop
apt-get update
apt-get upgrade

Depending on your setup, the apt-get upgrade command might upgrade Jenkins to the latest version that does not require Java 11+. In my case, that was 3.346.

When you get a question about updating your current config file, take the default option. This option keeps your current configuration.

However, if your Jenkins is installed from a binary or another source, you might need to upgrade Jenkins to 3.346 using the Jenkins.war file:

cd /usr/share/jenkins
mv jenkins.war jenkins.war.old
wget https://updates.jenkins-ci.org/latest/jenkins.war
service jenkins start

When you start Jenkins, it will be updated to the latest version that does not require Java 11 or higher. You will notice that there will be a new folder called migrate in /usr/share/jenkins , and the jenkins.war is now located in /usr/share/java

This is where I got confused because it did not patch to the latest version, only up to 3.346 and the jenkins.war file was no longer being updated from the /usr/share/jenkins folder.

The reason is that this update moves the .war file to the /usr/share/java directory.

java

To get Jenkins to the latest version, we need to install or update Java and check if it has worked:

apt-get install default-jre
java -version

Now that you have updated the java version, you are ready to update Jenkins to the latest version.

Notice that we use the /usr/share/java folder now, instead of /usr/share/jenkins

service jenkins stop
cd /usr/share/java
mv jenkins.war jenkins.war.old
wget https://updates.jenkins-ci.org/latest/jenkins.war
service jenkins start

nodes

When I accessed the Jenkins GUI, everything seemed fine, and my version was up to 3.358.

However, I noticed that the build nodes were all offline. When inspecting the logs, I saw the following error:

java.io.EOFException
	at java.base/java.io.ObjectInputStream$PeekInputStream.readFully(ObjectInputStream.java:2905)
	at java.base/java.io.ObjectInputStream$BlockDataInputStream.readShort(ObjectInputStream.java:3400)
	at java.base/java.io.ObjectInputStream.readStreamHeader(ObjectInputStream.java:936)
	at java.base/java.io.ObjectInputStream.<init>(ObjectInputStream.java:379)
	at hudson.remoting.ObjectInputStreamEx.<init>(ObjectInputStreamEx.java:49)
	at hudson.remoting.Command.readFrom(Command.java:142)
	at hudson.remoting.Command.readFrom(Command.java:128)
	at hudson.remoting.AbstractSynchronousByteArrayCommandTransport.read(AbstractSynchronousByteArrayCommandTransport.java:35)
	at hudson.remoting.SynchronousCommandTransport$ReaderThread.run(SynchronousCommandTransport.java:61)
Caused: java.io.IOException: Unexpected termination of the channel
	at hudson.remoting.SynchronousCommandTransport$ReaderThread.run(SynchronousCommandTransport.java:75)

Observing that the error had something to do with Java, I ssh’d into the build nodes and updated Java there as well with the same command:

apt-get install default-jre

After updating Java on the build node, head back to the GUI on the master node and restart the build node.

It should now be online again.