Docker run fails with NoClassDefFoundError - even though dependant jar is in classpath

Question

I am a docker newbie. I am trying to run a java class in the container. My main class is depending on a local jar. Am able to add the jar to the docker image and able set the jar in the classpath.

But still, when run the container it fails with "Exception in thread "main" java.lang.NoClassDefFoundError"

My Dockerfile

FROM java:8

WORKDIR /

ADD Test.jar Test.jar

ADD Dependant.jar Dependant.jar

RUN mkdir /usr/myjars

COPY /Dependant.jar /usr/myjars/Dependant.jar

ENV CLASSPATH .:/usr/myjars/Dependant.jar

RUN export CLASSPATH=.:/usr/myjars/Dependant.jar

CMD ["java",  "-jar", "Test.jar"]

Please help me to identify issue

Answer 1

Your Dockerfile is more complex than needed. You could:

FROM java:8

COPY Test.jar /

ENV CLASSPATH=/usr/myjars
RUN mkdir -p ${CLASSPATH}
COPY Dependant.jar ${CLASSPATH}

# You need the SHELL form of CMD or ENTRYPOINT because you're using ENV
ENTRYPOINT java -jar /Test.jar

Answer 2

Normally when we try to run a java class from a JAR file, JVM won't consider the environment CLASSPATH.Instead it will look class path defnition of manifest file in the jar.

Refer: https://docs.oracle.com/javase/tutorial/deployment/jar/downman.html

Considering the case, I have corrected the creation of my CheckSum.jar/Dependant.jar with following manifest.

MANIFEST.MF

Manifest-Version: 1.0
Class-Path: . /usr/myjars/PaytmChecksum.jar
Main-Class: TestChecksumGeneration

following is my corrected Docker file.

Dockerfile

FROM java:8

COPY CheckSum.jar /

ENV JARDIR=/usr/myjars

RUN mkdir -p ${JARDIR}

COPY PaytmChecksum.jar ${JARDIR}

ENTRYPOINT java -jar /CheckSum.jar

Note : I have not provided any CLASSPATH to my docker container.