Java 9 Modules split "sub-package" demonstration

com-example-foo-impl1_src_com_example_foo_implone_StandardOutHelloer.java

package com.example.foo.implone;

import com.example.foo.SalutationProvider;

public final class StandardOutHelloer
{
    public static void main(String... arguments)
    {
        System.out.println(new SalutationProvider().getSalutation());
        System.out.flush();
    }
}

com-example-foo-impl1_src_module-info.java

module com.example.foo.implone {
    requires com.example.foo;
    exports com.example.foo.implone;
}

com-example-foo-impl2_src_com_example_foo_impltwo_StandardErrHelloer.java

package com.example.foo.impltwo;

import com.example.foo.SalutationProvider;

public final class StandardErrHelloer
{
    public static void main(String... arguments)
    {
        System.err.println(new SalutationProvider().getSalutation());
        System.err.flush();
        System.exit(15);
    }
}

com-example-foo-impl2_src_module-info.java

module com.example.foo.impltwo {
    requires com.example.foo;
    exports com.example.foo.impltwo;
}

com-example-foo_src_com_example_foo_SalutationProvider.java

package com.example.foo;

public class SalutationProvider
{
    public SalutationProvider()
    {

    }

    public String getSalutation()
    {
        return "Hello, World!";
    }
}

com-example-foo_src_module-info.java

module com.example.foo {
    exports com.example.foo;
}

Running

$ java -Dfile.encoding=UTF-8 -p out/production/com-example-foo-impl1:out/production/com-example-foo -m com.example.foo.implone/com.example.foo.implone.StandardOutHelloer
Hello, World!

$ echo $?
0

$ java -Dfile.encoding=UTF-8 -p out/production/com-example-foo-impl2:out/production/com-example-foo -m com.example.foo.impltwo/com.example.foo.impltwo.StandardErrHelloer
Hello, World!

$ echo $?
15