Patch Java 9 module with test-code to work with reflections

How do I add my tests to my production code at test-runtime so that both are in the same Java 9 module and can access each other using reflections?

I have tried so far:

Remove the Java 9 modularity (actually the module-info.java) → it worked perfectly, but is not what I'm looking for.
Move my tests to a dedicated module (and therefore also package) → it worked perfectly, but is not what I'm looking for. (I want my Unit-Tests near my code.)
Use --patch-module to virtually add another folder (to the one specified using --module-path) → it worked with "normal" code but not with reflections, it doesn't find the classes specified by --module-path.
- Specify both my test and production code using --patch-module → it only finds classes in the folder I specify first.
- Explicitly add --add-opens mymodule/mypackge=mymodule or ...=ALL-UNNAMED to open it for reflection → doesn't look to have any effect.

So my full test line is:

java \
  --patch-module com.stackoverflow.examplemodule=ModuleInfoTest:ModuleInfoExample \
  --module-path ModuleInfoExample \
  --add-opens com.stackoverflow.examplemodule/com.stackoverflow.examplepackage=com.stackoverflow.examplemodule \
  --add-opens com.stackoverflow.examplemodule/com.stackoverflow.examplepackage=ALL-UNNAMED \
  --module com.stackoverflow.examplemodule/com.stackoverflow.examplepackage.Main

I'm in a directory containing the following subdirectories and files:

ModuleInfoExample/module-info.class
ModuleInfoExample/com/stackoverflow/examplepackage/Main.class
ModuleInfoTest/com/stackoverflow/examplepackage/AnyClass.class

I'm using:

openjdk version "13.0.2" 2020-01-14
OpenJDK Runtime Environment (build 13.0.2+8)
OpenJDK 64-Bit Server VM (build 13.0.2+8, mixed mode, sharing)

As I learned from the approved answer, my problem was not really "access other classes", as I phrased it. But more like find them (by scanning classpath/modulepath). However this part is already answered in this other StackOverflow question.

Solution

From Oracle's Java 13 java command these are the two options you tried to use:

--add-opens module/package=target-module(,target-module)*
--patch-module module=file(;file)*

but:

--add-opens doesn't help you because it opens up one module to other modules. You only have one module.
--patch-module must specify the directories (or jarfiles) you want to patch into the module. I noticed that there's a ; not a : like you used. It seems to me that you've been telling java to patch files from same the directory where your module is at with :ModuleInfoExample.

You only need to add the files from ModuleInfoTest/ into your module. I created the structure from your Question and ran it:

Compiling:

javac -d target/ModuleInfoExample src/ModuleInfoExample/*.java src/ModuleInfoExample/com/stackoverflow/examplepackage/*.java
javac -cp target/ModuleInfoExample -d target/ModuleInfoTest src/ModuleInfoTest/com/stackoverflow/examplepackage/*.java

Running Main from module - no added classes:

java --module-path target/ModuleInfoExample --module com.stackoverflow.examplemodule/com.stackoverflow.examplepackage.Main

prints:
Hello world - I'm private

Running AnyClass from module - no added classes - exception expected

java --module-path target/ModuleInfoExample --module com.stackoverflow.examplemodule/com.stackoverflow.examplepackage.AnyClass

Error: Could not find or load main class com.stackoverflow.examplepackage.AnyClass in module com.stackoverflow.examplemodule

Running AnyClass from module - adding AnyClass into the package:

java --module-path target/ModuleInfoExample --patch-module com.stackoverflow.examplemodule=target/ModuleInfoTest --module com.stackoverflow.examplemodule/com.stackoverflow.examplepackage.AnyClass

prints:
Inside AnyClass - calling Main: Hello world - I'm private

  field.get() = I'm private
  field.get() = I'm not private anymore

Total structure:

>tree /f
..snip..
C:.
+---src
¦   +---ModuleInfoExample
¦   ¦   ¦   module-info.java
¦   ¦   ¦
¦   ¦   +---com
¦   ¦       +---stackoverflow
¦   ¦           +---examplepackage
¦   ¦                   Main.java
¦   ¦
¦   +---ModuleInfoTest
¦       +---com
¦           +---stackoverflow
¦               +---examplepackage
¦                       AnyClass.java
¦
+---target
    +---ModuleInfoExample
    ¦   ¦   module-info.class
    ¦   ¦
    ¦   +---com
    ¦       +---stackoverflow
    ¦           +---examplepackage
    ¦                   Main.class
    ¦
    +---ModuleInfoTest
        +---com
            +---stackoverflow
                +---examplepackage
                        AnyClass.class

src\ModuleInfoExample\module-info.java:

module com.stackoverflow.examplemodule {
//  exports com.stackoverflow.examplepackage; // no need to export. Nothing is using this
}

src\ModuleInfoExample\com\stackoverflow\examplepackage\Main.java:

package com.stackoverflow.examplepackage;

public class Main {
  private String privateString = "I'm private";

  public static void main(String[] args) {
    new Main().hello();
  }
  public void hello(){
    System.out.println("Hello world - " + privateString);
  }
}

src\ModuleInfoTest\com\stackoverflow\examplepackage\AnyClass.java:

package com.stackoverflow.examplepackage;

import java.lang.reflect.Constructor;
import java.lang.reflect.Field;

public class AnyClass {
  public static void main(String[] args) {
    testhello();
    System.out.println();
    breakhello();
  }

  public static void testhello(){
    System.out.print("Inside AnyClass - calling Main: ");
    Main test = new Main();
    test.hello();
  }

  public static void breakhello(){
    try {
      // Not necessary - same package, but..
      Class<?> mainClass = Class.forName("com.stackoverflow.examplepackage.Main");
      Constructor<?> constructor = mainClass.getConstructor();
      Object main = constructor.newInstance();

      // Getting, printing and changing the field..
      Field field = mainClass.getDeclaredField("privateString");
      field.setAccessible(true);
      System.out.println("  field.get() = " + field.get(main));
      field.set(main,"I'm not private anymore");
      System.out.println("  field.get() = " + field.get(main));

    } catch (Exception e) {  // Sorry, all in one big bucket
       System.out.println("Error: " + e);
    }
  }
}