Java 9: Generating a runtime image with JLink using 3rd party jars

Question

I'd like to create a Java 9 runtime image that contains 3rd party jars. I have made a simple Java project (let's call this Example) to call a utility jar (let's call this ExampleUtil.jar). Example contains the module-info.java in the src directory and runs fine in Eclipse (I had added ExampleUtil.jar as a module dependency). If I call:

jlink -v
    --module-path "C:\Program Files\Java\jdk-9.0.4\jmods";C:\Temp
    --add-modules com.example.steven
    --output C:\Temp\image.steven
    --launcher launch=com.example.steven/com.example.steven`

... I get the error message:

Error: module-info.class not found for ExampleUtil module

Is there a way to create a runtime image using jars that aren't modules? Thank you.

Answer 1

No, jlink requires all included modules to be explicit, meaning they need to have a module descriptor. Here's what the documentation says about the jlink's module path:

The path where the jlink tool discovers observable modules. These modules can be modular JAR files, JMOD files, or exploded modules.

Note the absence of "plain JARs" (i.e. JARs without descriptor).

You can upgrade existing third-party JARs to modular JARs, though (with some effort). The steps are:

• get the JAR and all its dependencies
• create a module-info.java for it (either manually or with JDeps)
• compile it to module-info.class with --patch-module to tell the compiler about the sources
• use jar --update to add the module declaration to the existing JAR

Alternatively, you can use a tool like ModiTect that does these things for you.

Answer 2

jlink directly can not do this. The trick is to call jdeps on all external libs to collect their system module dependencies and than modify the parameters to jlink accordingly. Generating a module-info.java will not always work to solve the problem but you can use jdeps to see what system modules you need and than create a custom Java Runtime with jlink which includes all these system dependencies and put the external libs on the classpath or module path dependening on if they are non-modular jars or automatic module jars.

You can use something like a special maven plugin to automate that task.