Patched Java 9 Module Test Code Working and Thinking

From Java 13 java commands these are the two options you are trying to use:

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

but:

• --add-opensBecause it opens up a module, other modules don't help you. You only have one module.
• --patch-moduleThe directory (or in jarfiles) to patch into the module must be specified. I noticed that there is one you ;don't :like to use. It seems to me that you have told java to patch files from the same directory where your module is there :ModuleInfoExample.

You just need to add the file from ModuleInfoTest/to your module. I generated the structure from your question and ran it:

Compile:

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

Run main from module - no added class:

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

prints:
Hello world - I'm private

Running AnyClass from module - no class added - 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

Run AnyClass from a module - add AnyClass to 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

General 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\Calculator\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\Calculator\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);
    }
  }
}