Class#getResource spec has following assertion which mentions that ,

"If this class is in a named Module then this method will attempt to find the resource in the module by means of the absolute resource name, subject to the rules for encapsulation specified in the Module getResourceAsStream method. "

Module getResourceAsStream says:
If the caller is in a seperate module and calling this method, then the package of the resource in named module should be
opened to the caller.

But what is observed is if the resource path name
* is prefixed with "./" (for resource in same directory as class package)
* is prefixed with "../" (for resource in parent directory of the class package)

Then even whenthe package in named module is not opened to the caller(unnamed) ,the resource is found by Class#getResource.

For E.g: (Attached the files)

getresource\Test.java
 System.out.println(Layer.boot().findModule("m2").get().getClassLoader().loadClass("p2.C2").getResource(
                  "./abc.txt"));

getresource\m2
getresource\m2\module-info.java
module m2{}

getresource\m2\p2
getresource\m2\p2\C2.java
package p2;
public class C2{}

compile:
"C:\Program Files\java\jdk-b148\jdk-9\bin\javac" --module-source-path . m2/*.java m2/p2/*.java -d classes
"C:\Program Files\java\jdk-b148\jdk-9\bin\javac" Test.java

classes output:
getresource\classes\m2\p2\abc.txt --> resource in the same pacakge as class
getresource\classes\m2\p2\C2.class
getresource\Test.class

execute:
"C:\Program Files\java\jdk-b148\jdk-9\bin\java" --module-path classes --add-modules m2 Test
output:
file:/D:/intellij/getresource/classes/m2/p2/./abc.txt
-------------------------------------
Ideally it should give null here because p2 is not opened by m2 to unnamed module (Test)
-----------------------------------------------------
Lets say if the resource is parent package:
getresource\classes\m2\abc.txt

and we change the "Test.java" to "../abc.txt" (instead of ./abc.txt)

compile:
"C:\Program Files\java\jdk-b148\jdk-9\bin\javac" Test.java

execute:
"C:\Program Files\java\jdk-b148\jdk-9\bin\java" --module-path classes --add-modules m2 Test
output:
file:/D:/intellij/getresource/classes/m2/p2/../abc.txt