Java 17. Manual Loading (Java 8 style)

Task: load any application class using a custom class loader (CustomClassLoader) in Java 8 style (without module-info.java).

Solution

Project with three classes:

Main - a class containing the main method, where an instance of the CustomClassLoader class is created, the Cat class is loaded using it, creating an instance of the class Cat and calling the Cat::talk method;
Cat - loadable class with the talk method, which prints the string "Meow" to stdout;
CustomClassLoader - a class that is an implementation of a custom class loader.

Run

Using classpath:

java17 -cp . ru.ispras.j17.manual.onemodule.j8style.Main

or

java17 -jar java8style-1.0.jar

Output:

Main Class Module is unnamed module @99e937b
Cat Class Module is unnamed module @6d6f6e28
Main Class ClassLoader is jdk.internal.loader.ClassLoaders$AppClassLoader@531d72ca
Cat Class ClassLoader is ru.ispras.j17.manual.onemodule.j8style.CustomClassLoader@15db974
Meow

Using modulepath:

java17 -p . -m java8style

Output:

Main Class Module is module java8style
Cat Class Module is unnamed module @3a71f4dd
Main Class ClassLoader is jdk.internal.loader.ClassLoaders$AppClassLoader@28d93b30
Cat Class ClassLoader is ru.ispras.j17.manual.onemodule.j8style.CustomClassLoader@5ca881b5
Meow

Explanation

When a CustomClassLoader is created, its parent becomes jdk.internal.loader.ClassLoaders$AppClassLoader;
All classes will be under unnamed module if launched using classpath. Experimentally, it was possible to find out that each class loader is related to two modules: firstly, as a class, it refers to the class loader module that loaded this class, and secondly, it has its own unnamed module (which can be obtained using the method getUnnamedModule), which will contain all the classes that it will load on its own. Therefore, in this example, two unnamed modules appear: the first is loaded by the system loader and the Main class is located there, and the second is loaded by the CustomClassLoader loader, where the Cat class is already located.
If we run the java8style-1.0.jar file, then using the -p and -m options (using modulepath), we will get an Automatic Module with the name java8style, which is implicitly obtained by the algorithm from documentation for ModuleFinder.

Notes

CustomClassLoader gets some system classes to be loaded first, for example classes from java.* packages. If you try to load a class from the java.* package, the JVM will throw java.lang.SecurityException: Prohibited package name: java.*;