running java fie on docker returns errors

Question

I try to run a simple java class with docker making the following steps:

1. create java file (HelloWorld.java)

2. compile it (javac HelloWorld.java)

3. create dockerfile (above the content of the Dockerfile)

   FROM alpine:latest
   ADD HelloWorld.class HelloWorld.class
   RUN apk --update add openjdk8-jre
   ENTRYPOINT ["java", "-Djava.security.egd=file:/dev/./urandom", "HelloWorld"]
   

then I build it with this command:

   docker build --tag "docker-hello-world:latest" .

and I got the following:

    C:\Gradle>javac HelloWorld.java

    C:\Gradle>docker build --tag "docker-hello-world:latest" .
    => [internal] load build definition from Dockerfile
    => => transferring dockerfile: 209B
    => [internal] load .dockerignore
    => => transferring context: 2B
    => [internal] load metadata for docker.io/library/alpine:latest
    => [internal] load build context
    => => transferring context: 471B
    => [1/3] FROM docker.io/library/alpine:latest
    => CACHED [2/3] ADD HelloWorld.class HelloWorld.class
    => CACHED [3/3] RUN apk --update add openjdk8-jre
    => exporting to image
    => => exporting layers
   => => writing image sha256:e3450ba2f444321b7d9965d95443e1d394ef8c62549268059e9786bb882992ec
   => => naming to docker.io/library/docker-hello-world:latest   

I think everything is Ok, and I tried to run it with this command:

    docker run docker-hello-world:latest

an I get this Error:

    C:\Gradle>docker run docker-hello-world:latest
    Error: A JNI error has occurred, please check your installation and try again
    Exception in thread "main" java.lang.UnsupportedClassVersionError: HelloWorld has been compiled by a more recent version of the Java Runtime (class file version 59.0), this version of the Java Runtime only recognizes class file versions up to 52.0
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClass(ClassLoader.java:757)
    at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
    at java.net.URLClassLoader.defineClass(URLClassLoader.java:468)
    at java.net.URLClassLoader.access$100(URLClassLoader.java:74)
    at java.net.URLClassLoader$1.run(URLClassLoader.java:369)
    at java.net.URLClassLoader$1.run(URLClassLoader.java:363)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:362)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:419)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:352)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:352)
    at sun.launcher.LauncherHelper.checkAndLoadMain(LauncherHelper.java:601)

this is my java version on my machine :

    C:\Gradle>java -version
    openjdk version "15.0.1" 2020-10-20
    OpenJDK Runtime Environment (build 15.0.1+9-18)
    OpenJDK 64-Bit Server VM (build 15.0.1+9-18, mixed mode, sharing)

and by the way I build my java file whit the same java version on my machine,therefore I don't know why the error said,the HelloWorld java file has been compiled by more recent version of java Runtime... My intention is just to run this Java file with docker and get <Hello,World> in the output

Answer 1

The error UnsupportedClassVersionError means that you are trying to execute a class compiled with the wrong version of java.

You have compiled the class with a recent java version (15), but you are running java 8 inside docker.

Upgrade the java version inside docker to java 15 will fix the error.

Edit: On linux Alpine openjdk15-jre is only available on testing repository.

FROM alpine:latest
ADD HelloWorld.class HelloWorld.class
RUN apk --update add openjdk15-jre --repository=http://dl-cdn.alpinelinux.org/alpine/edge/testing/
ENTRYPOINT ["java", "-Djava.security.egd=file:/dev/./urandom", "HelloWorld"]

Edit

Another possbile solution is to compile the java class to be compatible with java8.

To do this compile with the following command:

javac -source 8 -target 8 HelloWorld.java

Remember that if you compile to java8 you cannot use any new feature introduced after java8 like the new switch expression or records.

After that create the docker with the original configuration:

FROM alpine:latest
ADD HelloWorld.class HelloWorld.class
RUN apk --update add openjdk8-jre
ENTRYPOINT ["java", "-Djava.security.egd=file:/dev/./urandom", "HelloWorld"]