how to show the actual modulepath?

Question

For debugging I have a need to see the actual modulepath. How can I print it out at runtime?

The problem: Using ServiceLoader to load a module against a defined API works fine in normal runtime environment but not in testing. I'd got to find out why.

As I am new to ServiceLoader, it may not to be enough that a provider module can be found on modulepath. However, my first question is: is it on modulepath even in test environment?

Answer 1

When you want to check the presence of a module, you can straight-forwardly check the presence of a module, e.g.

String moduleName = "java.compiler";
ModuleLayer.boot().findModule(moduleName)
    .ifPresentOrElse(
        m -> System.out.println(m + " loaded via " + m.getClassLoader()),
        () -> System.out.println(moduleName + " not found"));

When the module is not present, you can check module path using the non-standard system property,

System.out.println(System.getProperty("jdk.module.path"));

Besides possibility of a mismatching path, it’s worth checking the context loader of each envi­ron­ment if loading a service fails in one environment, as the method ServiceLoader.load(Class) uses the context loader for searching.

Also check that the module’s classes are not accidentally provided on the old class path.