How to use 3rd party library in Java9 module?

Question

I have some java9 module that uses 3rd party library that is not Java9 module, just a simple utility jar.

However, the compiler complains that it can't find a package from my utility.

What should I do in module-info.java to enable usage of my 3rd party library?

Answer 1

You can use your library as an automatic module. An automatic module is a module that doesn't have a module descriptor (i.e. module-info.class). But what name should you use to refer to this module? The name of the automatic module is derived from the JAR name (unless this JAR contains an Automatic-Module-Name attribute). The full rule is quite long (see Javadoc for ModuleFinder.of), so for simplicity, you just have to drop the version from its name and then replace all non-alphanumeric characters with dots (.).

For example, if you want to use foo-bar-1.2.3-SNAPSHOT.jar, you need to add the following line to module-info.java:

module <name> {
    requires foo.bar;
}

Answer 2

Just to put in simple steps, to use a 3rd party jar(e.g. log4j-api-2.9.1.jar below) in your module:-

1. Execute the descriptor command of jar tool

   jar --file=/path/to/your/jar/log4j-api-2.9.1.jar --describe-module
   

   This would provide you an output similar to

   No module descriptor found. Derived automatic module.

   log4j.api@2.9.1 automatic

2. In your module-info.java, declare a requires to that module name as:-

   module your.module {
       requires log4j.api;
   }
   

   That's it.