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Download and install Okapi Longhorn as shown here: http://okapiframework.org/wiki/index.php?title=Longhorn#Download_and_Installation.
If you have a working and running Tomcat that has autoDeploy set to "true" in the /etc/tomcat9/server.xml file, than to deploy your okapi-longhorn.war file you can simply copy it to the webapps-folder of your Tomcat installation (on Ubuntu this is located under /var/lib/tomcat9/webapps). You may have to set the file access rights of the war file, so that Tomcat can handle the file and you may need to restart Tomcat.
Important hints for OpenJDK
OpenJDK is the Java runtime usually used on Linux for Tomcat. Since May 2022 there is an OpoenJDK update for most java versions, where new limits for certain xpath recursions are introduced.
This leads to problems, when using Okapi with the XML ITS filter.
Therefore you should add the following increased limit to the startup java settings of your tomcat:
-Djdk.xml.xpathExprOpLimit=2000
For doing this, open the setenv.sh file in the bin directory of the CATALINA_HOME directory of your tomcat installation.
Under Ubuntu you should find that directory at /usr/share/tomcat9/bin
If there is no file setenv.sh in this directory, create it as empty file with the rights 755.
Then add to it the line
JAVA_OPTS="-Djdk.xml.xpathExprOpLimit=200"
Restart tomcat.
Important hints for Okapi with Tomcat9
Tomcat9 only has rights to write inside of its working directory, which under Ubuntu 18.04 and TomCat9 is
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Translate5 Configuration of the Okapi Plug-In
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