By setting Multi-Release: true (opens new window) in the MANIFEST.MF file, the Jar file becomes a multi-release Jar and the Java runtime (as long as it supports the MRJAR format) will pick the appropriate versions of classes depending on the current major version.

The structure of such a Jar is the following:

jar root
  - A.class
  - B.class
  - C.class
  - D.class
  - META-INF
     - versions
        - 9
           - A.class
           - B.class
        - 10
           - A.class

• On JDKs < 9, only the classes in the root entry are visible to the Java runtime.
• On a JDK 9, the classes A and B will be loaded from the directory root/META-INF/versions/9, while C and D will be loaded from the base entry.
• On a JDK 10, class A would be loaded from the directory root/META-INF/versions/10.

The jar command can be used to create a multi-release Jar containing two versions of the same class compiled for both Java 8 and Java 9, albeit with a warning telling that the classes are identical:

C:\Users\manouti>jar --create --file MR.jar -C sampleproject-base demo --release 9 -C sampleproject-9 demo
Warning: entry META-INF/versions/9/demo/SampleClass.class contains a class that
is identical to an entry already in the jar

The --release 9 option tells jar to include everything that follows (the demo package inside the sampleproject-9 directory) inside a versioned entry in the MRJAR, namely under root/META-INF/versions/9. The result is the following contents:

jar root
  - demo
     - SampleClass.class
  - META-INF
     - versions
        - 9
           - demo
              - SampleClass.class

Let us now create a class called Main that prints the URL of the SampleClass, and add it for the Java 9 version:

package demo;

import java.net.URL;

public class Main {

    public static void main(String[] args) throws Exception {
        URL url = Main.class.getClassLoader().getResource("demo/SampleClass.class");
        System.out.println(url);
    }
}

If we compile this class and re-run the jar command, we get an error:

C:\Users\manouti>jar --create --file MR.jar -C sampleproject-base demo --release 9 -C sampleproject-9 demoentry: META-INF/versions/9/demo/Main.class, contains a new public class not found in base entries
Warning: entry META-INF/versions/9/demo/Main.java, multiple resources with same name
Warning: entry META-INF/versions/9/demo/SampleClass.class contains a class that
is identical to an entry already in the jar
invalid multi-release jar file MR.jar deleted

The reason is that the jar tool prevents adding public classes to versioned entries if they are not added to the base entries as well. This is done so that the MRJAR exposes the same public API for the different Java versions. Note that at runtime, this rule is not required. It may be only applied by tools like jar. In this particular case, the purpose of Main is to run sample code, so we can simply add a copy in the base entry. If the class were part of a newer implementation that we only need for Java 9, it could be made non-public.

To add Main to the root entry, we first need to compile it to target a pre-Java 9 release. This can be done using the new --release option of javac:

C:\Users\manouti\sampleproject-base\demo>javac --release 8 Main.java
C:\Users\manouti\sampleproject-base\demo>cd ../..
C:\Users\manouti>jar --create --file MR.jar -C sampleproject-base demo --release 9 -C sampleproject-9 demo

Running the Main class shows that the SampleClass gets loaded from the versioned directory:

C:\Users\manouti>java --class-path MR.jar demo.Main
jar:file:/C:/Users/manouti/MR.jar!/META-INF/versions/9/demo/SampleClass.class

Given the following multi-release Jar:

jar root
  - demo
     - SampleClass.class
  - META-INF
     - versions
        - 9
           - demo
              - SampleClass.class

The following class prints the URL of the SampleClass:

package demo;

import java.net.URL;

public class Main {

    public static void main(String[] args) throws Exception {
        URL url = Main.class.getClassLoader().getResource("demo/SampleClass.class");
        System.out.println(url);
    }
}

If the class is compiled and added on the versioned entry for Java 9 in the MRJAR, running it would result in:

C:\Users\manouti>java --class-path MR.jar demo.Main
jar:file:/C:/Users/manouti/MR.jar!/META-INF/versions/9/demo/SampleClass.class