Example of a multi-release Jar files contents

By setting [Multi-Release: true](<http://download.java.net/java/jdk9/docs/api/java/util/jar/Attributes.Name.html#MULTI_RELEASE>) 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.