Can I require Java 9 module via MANIFEST.MF?

Question

My Java library should be compatible with Java 8 and Java 9. For running with Java 9 we need some of the Java 9 modules.

I know that I can add it via command line wi

Answer 1

Unfortunately, there is no equivalent of --add-modules in MANIFEST.MF. However, you can create module-info.java and declare your dependency there:

module  {
    requires ;
    ...
}

However, if you compile module-info.java and simply put module-info.class to your JAR, it may not work on some platforms (e.g. Android). So what to do? There is a new feature in Java 9: multi-release JAR files (JEP 238). The idea is that you can put Java 9 class files to a special directory (META-INF/version/9/) and Java 9 will properly handle them (while Java 8 will ignore them).

So, these are the steps that you should perform:

• Compile all classes except module-info.java with javac --release 8
• Compile module-info.java with javac --release 9.
• Build a JAR file so it will have the following structure:

JAR root
  - A.class
  - B.class
  - C.class
   ...
  - META-INF
    - versions
      - 9
        - module-info.class

The resulting JAR should be compatible with Java 8 and Java 9.

Also, you can just put module-info.class to the META-INF folder. This will have the same effect. This, however, may not work for some tools and platforms. So I think the first way is more preferable.