Java 9 Multi-Release JAR Files

Java 9 新特性Java 9 New Features

The multi-release JAR feature allows you to create class versions that are only selected when running library programs in a specific version of the Java environment.

via--releasethe parameter to specify the compilation version.

The specific change is that a new attribute has been added to the MANIFEST.MF file in the META-INF directory:

Multi-Release: true

In addition, a versions directory is added under the META-INF directory. If Java 9 is to be supported, there is a 9 directory 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 jar package from the Tester.java file, one for JDK 7 and the other for JDK 9, and then we execute them in different environments.

Step 1

Create the folder c:/test/java7/com/example, and create the Test.java file in this folder. The code is as follows:

Example

package com.example;

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

Step 2

Create the folder c:/test/java9/com/example, and create the Test.java file in this folder. The code is as follows:

Example

package com.example;

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/example/Tester.java

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

Create a multi-release JAR file

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

Execute using JDK 7:

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

Execute using JDK 9:

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

Java 9 新特性Java 9 New Features

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