What is multi-release JARs in Java?

Multi-release JARs are a feature in Java that allows a single JAR file to contain different versions of classes for different versions of the Java platform. This is particularly useful for providing backward compatibility for applications that need to run on multiple versions of Java.

With multi-release JARs, developers can package different implementations of a class for different Java versions in a single JAR file, thus reducing the complexity of maintaining multiple versions of the same library.

The JAR file structure supports a META-INF/versions directory where specific versions of classes can reside. When a class is loaded, the Java runtime will use the appropriate version based on the running Java platform version.

Here's an example of how a multi-release JAR might be structured:

    com/
    └── example/
        ├── MyClass.class
        └── META-INF/
            └── versions/
                ├── 9/
                │   └── com/
                │       └── example/
                │           └── MyClass.class
                └── 11/
                    └── com/
                        └── example/
                            └── MyClass.class