Java Multi-Release JAR Files

Overview

Java Multi-Release JAR (MRJAR) files enable developers to include multiple versions of classes or resources within a single JAR file. Introduced in Java 9, this feature supports backward compatibility and allows developers to leverage new enhancements in the Java platform while maintaining support for older versions.

Key Concepts

• Multi-Release JAR (MRJAR): A JAR file containing classes targeted for different Java versions.
• Versioned Directories: Within an MRJAR, classes for various Java versions are organized in version-specific directories. For instance, classes for Java 9 and later are located in META-INF/versions/9/, while classes for earlier versions are placed in the default package structure.

Structure of an MRJAR

• Standard JAR Contents: Contains classes and resources for the base version (e.g., Java 8).
• Versioned Subdirectories: Holds classes and resources for newer versions, including:
  • META-INF/versions/9/ for Java 9
  • META-INF/versions/10/ for Java 10
  • And so forth...

Example Directory Structure

my-library.jar
│
├── META-INF/
│   └── versions/
│       ├── 9/
│       │   └── com/
│       │       └── example/
│       │           └── MyClass.class
│       └── 10/
│           └── com/
│               └── example/
│                   └── MyClass.class
│
└── com/
    └── example/
        └── MyClass.class  (for Java 8 and below)

How to Use Multi-Release JARs

1. Creating an MRJAR: Use tools like javac and jar commands to compile and package your classes into an MRJAR.
2. Loading Classes: During application runtime, the Java runtime automatically loads the appropriate class version depending on the Java version in use.

Benefits of Using MRJARs

• Backward Compatibility: Enables applications to function on older Java versions while also utilizing new features in newer releases.
• Simplified Distribution: Developers can distribute a single JAR file instead of multiple JAR files for various versions.

Conclusion

Multi-Release JAR files are a powerful feature in Java that ensures compatibility across different versions while allowing developers to take advantage of the latest enhancements in the language. By organizing classes into version-specific directories, MRJARs provide a streamlined approach to managing Java applications across various runtime environments.