remote job monitoring failure after upgrade

[Adrian Nye]

Hi all,
 
I was successfully monitoring cron jobs using the “External Monitor Job Type Plugin” with Jenkins.1.65.
 
I just upgraded to Jenkins 2.51, extracted the jar files from the war file as instructed on the plugin wiki page, and copied them to the remote system where the cron job runs, just as I did with the previous Jenkins version.   My main Jenkins is running on java 1.8 and
the remote system has java 1.7.0_131.
 
Now however, the plugin doesn’t work, it fails with this trace:
 
Exception in thread "main" java.lang.NoClassDefFoundError: javax/servlet/ServletContextListener
                at java.lang.ClassLoader.defineClass1(Native Method)
                at java.lang.ClassLoader.defineClass(ClassLoader.java:803)
                at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
                at java.net.URLClassLoader.defineClass(URLClassLoader.java:442)
                at java.net.URLClassLoader.access$100(URLClassLoader.java:64)
                at java.net.URLClassLoader$1.run(URLClassLoader.java:354)
                at java.net.URLClassLoader$1.run(URLClassLoader.java:348)
                at java.security.AccessController.doPrivileged(Native Method)
                at java.net.URLClassLoader.findClass(URLClassLoader.java:347)
                at java.lang.ClassLoader.loadClass(ClassLoader.java:425)
                at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
                at java.lang.ClassLoader.loadClass(ClassLoader.java:358)
                at hudson.Main.<clinit>(Main.java:222)
Caused by: java.lang.ClassNotFoundException: javax.servlet.ServletContextListener
                at java.net.URLClassLoader$1.run(URLClassLoader.java:359)
                at java.net.URLClassLoader$1.run(URLClassLoader.java:348)
                at java.security.AccessController.doPrivileged(Native Method)
                at java.net.URLClassLoader.findClass(URLClassLoader.java:347)
                at java.lang.ClassLoader.loadClass(ClassLoader.java:425)
                at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
                at java.lang.ClassLoader.loadClass(ClassLoader.java:358)
                ... 13 more
 
From google I learn that ServletContextListener is in the servlet api which does not appear to be present on the system, and it is not listed as a requirement of the plugin.   Anyway I found the servlet-api-2.5.jar available online and tried putting it in the same directory as the other jar files, but the same error occurs.
 
My question is how do I make this work, and should I report this as a bug to the plugin maintainer, or is it some kind of setup mistake I have made?
 
Thanks
Adrian