How to find root of "Could not find or load main class" error when running JAR

Question

I have seen this question asked so many times, but there was no precise answer to my problem. My story is simple - I downloaded a legitimate JAR file, for a functioning and advertised application, from here: https://github.com/kamilfb/mqtt-spy/wiki/Downloads.

The application brings the obvious instruction to start it as java -jar mqtt-spy-0.2.0-jar-with-dependencies.jar but that results in the well known error, being in my case Error: Could not find or load main class pl.baczkowicz.mqttspy.Main

My system is Ubuntu 15.04, Java version is openjdk version "1.8.0_45-internal" OpenJDK Runtime Environment (build 1.8.0_45-internal-b14) OpenJDK 64-Bit Server VM (build 25.45-b02, mixed mode)

I don't have another computer to test it elsewhere. I did some checks and the JAR file looks pretty much normal. I even tried de-compressing it and running the class directly, it won't budge, same error message. I tried some online .class analysis tools, nothing caught my attention. Also I reviewed about a dozen of pages referring to "Could not find or load" problem. No luck so far.

Not sure if anyone bothers, but I did my Java Programmers certification with Sun yet in 2005... and I feel frustrated now. While I hope there is an easy solution for my problem, e.g. some nice Ubuntu issue ;-), I am ready to dig deeper, and looking for any hints that would guide me to finding the root cause of this issue.

Answer 1

Additionally install Java FX, since this is required and worked for me (Ubuntu 16: package openfx):

apt install openjfx

See also this issue: https://github.com/kamilfb/mqtt-spy/issues/88

I currently successfully run it on openjdk version "1.8.0_121" here.