In this post I would like to explain how to build multi release jar. To provide brief overview of this: It is new feature introduced in jdk9 JEP-238, now you can have 2 versions of class files and can control which version should be picked up for which jdk version.
Here I want to build 2 versions of A.java in one jar, jdk8 should run jdk8 specific version and jdk9 should run jdk9 specific version.
Created dir javacode, inside this java8 and java9. and both have A.java
XXXX@XXX-MacBook-Air.local:~/javacode/java8/com/vipin/exp$ pwd
/Users/XXXX/javacode/java8/com/vipin/exp
XXXX@XXX-MacBook-Air.local:~/javacode/java8/com/vipin/exp$ cat A.java
package com.vipin.exp;
public class A{
public static void main(String[] args){
System.out.println("Inside java8 version");
}
}
XXXX@XXX-MacBook-Air.local:~/javacode/java9/com/vipin/exp$ cat A.java
package com.vipin.exp;
public class A{
public static void main(String[] args){
System.out.println("Inside java9 version");
}
}
This is my directory structure:
XXXX@XXX-MacBook-Air.local:~/javacode$ tree
.
|____java8
| |____com
| | |____vipin
| | | |____exp
| | | | |____A.java
|____java9
| |____com
| | |____vipin
| | | |____exp
| | | | |____A.java
Compiling source:
XXXX@XXX-Air:~/javacode$ javac --release 9 -d /Users/XXXX/javacode/java9/ java9/com/vipin/exp/A.java
XXXX@XXX-Air:~/javacode$ javac --release 8 -d /Users/XXXX/javacode/java8/ java8/com/vipin/exp/A.java
Dir structure after compiling source code:
XXXX@XXX-MacBook-Air.local:~/javacode$ tree
.
|____java8
| |____com
| | |____vipin
| | | |____exp
| | | | |____A.class
| | | | |____A.java
|____java9
| |____com
| | |____vipin
| | | |____exp
| | | | |____A.class
| | | | |____A.java
Creating multi release jar:
XXXX@XXX-MacBook-Air.local:~/javacode$ jar -c -f vipin.jar -C java8 . --release 9 -C java9 .
Warning: entry META-INF/versions/9/com/vipin/exp/A.java, multiple resources with same name
XXXX@XXX-MacBook-Air.local:~/javacode$ jar -tvf vipin.jar
0 Wed Oct 18 19:06:26 IST 2017 META-INF/
82 Wed Oct 18 19:06:26 IST 2017 META-INF/MANIFEST.MF
0 Tue Oct 17 18:02:04 IST 2017 com/
0 Tue Oct 17 18:02:04 IST 2017 com/vipin/
0 Tue Oct 17 23:26:56 IST 2017 com/vipin/exp/
430 Wed Oct 18 19:00:38 IST 2017 com/vipin/exp/A.class
136 Tue Oct 17 22:49:20 IST 2017 com/vipin/exp/A.java
0 Tue Oct 17 20:00:34 IST 2017 META-INF/versions/9/
0 Tue Oct 17 20:00:34 IST 2017 META-INF/versions/9/com/
0 Tue Oct 17 20:00:34 IST 2017 META-INF/versions/9/com/vipin/
0 Tue Oct 17 23:27:04 IST 2017 META-INF/versions/9/com/vipin/exp/
430 Wed Oct 18 19:02:04 IST 2017 META-INF/versions/9/com/vipin/exp/A.class
135 Tue Oct 17 22:49:26 IST 2017 META-INF/versions/9/com/vipin/exp/A.java
Now multi release jar is ready, lets see it if it works as expected.
Running class A with jdk9:
XXXX@XXX-MacBook-Air.local:~/javacode$ java -version
java version "9"
Java(TM) SE Runtime Environment (build 9+181)
Java HotSpot(TM) 64-Bit Server VM (build 9+181, mixed mode)
XXXX@XXX-MacBook-Air.local:~/javacode$ java -cp vipin.jar com.vipin.exp.A
Inside java9 version
Running class A with jdk8:
XXXX@XXX-MacBook-Air.local:~/javacode$ /Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/bin/java -version
java version "1.8.0_111"
XXXX@XXX-MacBook-Air.local:~/javacode$ /Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/bin/java -cp vipin.jar com.vipin.exp.A
Inside java8 version
With output it is clear, version of java class is executed as per jdk version!
At the end
To learn the best java language features and get the best Java Jobs, download my ebook 5 steps to Best Java Jobs for Free.
Follow me on twitter @vipinbit to get daily tips like this on Java Language Features.