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, eg 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.

Answer 2

Thank you it was very helpful for me. I had exactly the same problem, and looking the solution here as well

My fix was:

sudo add-apt-repository ppa:webupd8team/java

sudo apt-get update

then, install the JDK Oracle 8

sudo apt-get install oracle-java8-installer

and then, checking java:

sudo update-alternatives --config java

It is displayed between other choices:

 0            /usr/lib/jvm/java-8-oracle/jre/bin/java          1082 
     auto mode
......

Press to keep the current choice [*] , or type selection number:

I keep the 0 current choice [*]

then, running mqttspy worked just fine :-)

java -jar mqtt-spy-0.5.1-jar-with-dependencies.jar

Answer 3

在我的案例中起作用的解决方案是使用Oracle的HotSpot Java 8。

Answer 4

Just in case you have multiple java versions, you can run the jar file by giving the java file path as well.

brew install --cask liberica-jdk@17-full
brew install --cask liberica-jdk17-full
/Library/Java/JavaVirtualMachines/liberica-jdk-17-full.jdk/Contents/Home/bin/java -jar mqtt-spy-1.0.1-beta-b18-jar-with-dependencies.jar