What is Multi-Release jar (mrjar) in Java 9?

A multi-version jar file (also known as mrjar) contains multiple ## of the same library Release of #Jdk version. This means we can have a mrjar library for Jdk 9. The code in mrjar contains class files compiled in Jdk 9. Classes compiled with Jdk 9 can take advantage of the APIs provided by Jdk 9.

mrjar can expand the existing directory structure of a jar file. It contains a root directory, which contains all the contents, and a META-INF directory used to store metadata about the jar file. Typically, a jar file contains a META-INF/MANIFEST.MF file, which contains the attributes.

<strong>- jar-root
- C1.class
- C2.class
- C3.class
- C4.class
- META-INF
- MANIFEST.MF</strong>

MANIFEST.MF file. The mrjar extends the META- INF directory to store classes that can be specific to the Jdk version. The META-INF directory contains version sub-directory that contains many sub-directories, each of them named the same as Jdk major version. For instance, the classes specific to Jdk 9, there is a META-INF/versions/9 directory. For classes specific to Jdk 10, there is META-INF/versions/10 .

<strong>- jar-root
 - C1.class
 - C2.class
 - C3.class
 - C4.class
- META-INF
 - MANIFEST.MF
 - versions
 - 9
  - C2.class
  - C5.class
 - 10
  - C1.class
  - C2.class
  - C6.class</strong>

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