Java 9 Multi-Release JARs

Java 9 New Features

The Multi-Release JAR feature allows you to create different versions of classes that are used only when the library runs on specific Java environments.

Specify the compilation version with the --release parameter.

The specific change is the addition of a new attribute in the META-INF directory's MANIFEST.MF file:

Multi-Release: true

Then, a new versions directory is added under the META-INF directory. For support with Java 9, there will be a directory named 9 under the versions directory.

multirelease.jar
├── META-INF
│   └── versions
│       └── 9
│           └── multirelease
│               └── Helper.class
├── multirelease
    ├── Helper.class
    └── Main.class

In the following example, we use the Multi-Release JAR feature to generate two versions of the Tester.java file into a JAR, one for JDK 7 and another for JDK 9, and then execute them in different environments.

Step 1

Create a folder c:/test/java7/com/tutorialpro and create a Test.java file in this folder with the following code:

Example

package com.tutorialpro;

public class Tester {
   public static void main(String[] args) {
      System.out.println("Inside java 7");
   }
}

Step 2

Create a folder c:/test/java9/com/tutorialpro and create a Test.java file in this folder with the following code:

Example

package com.tutorialpro;

public class Tester {
   public static void main(String[] args) {
      System.out.println("Inside java 9");
   }
}

Compile the source code:

C:\test > javac --release 9 java9/com/tutorialpro/Tester.java

C:\JAVA > javac --release 7 java7/com/tutorialpro/Tester.java

Create a multi-release JAR:

C:\JAVA > jar -c -f test.jar -C java7 . --release 9 -C java9 .
Warning: entry META-INF/versions/9/com/tutorialpro/Tester.java, 
   multiple resources with same name

Execute with JDK 7:

C:\JAVA > java -cp test.jar com.tutorialpro.Tester
Inside Java 7

Execute with JDK 9:

C:\JAVA > java -cp test.jar com.tutorialpro.Tester
Inside Java 9

Java 9 New Features

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