[J17] Add intermodule class loading example description

Author: temikfart

img/Java 17. Manual Loading (loading and loadable).jpg

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+## Java 17. Manual Loading (loading and loadable)
+---
+**Task:** load any application class using a custom class loader (`CustomClassLoader`) from another module (the modules are NOT linked through `module-info.java`).
+
+### Solution
+---
+Modules (declared via `module-info.java`):
+- `manual.fewmodules.separately.nodeps.loading`:
+    - [`Main`](loading/src/ru/ispras/j17/manual/fewmodules/separately/withdeps/loading/Main.java) - 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;
+    - [`CustomClassLoader`](loading/src/ru/ispras/j17/manual/fewmodules/separately/withdeps/loading/CustomClassLoader.java) - a class that is an implementation of a custom class loader;
+- `manual.fewmodules.separately.nodeps.loadable`:
+    - [`Cat`](loadable/src/ru/ispras/j17/manual/fewmodules/separately/withdeps/loadable/Cat.java) - loadable class with the `talk` method, which prints the string *"Meow"* to `stdout`.
+
+### Run
+---
+Using `modulepath`:
+
+```shell
+java17 -p loading-1.0.jar -m manual.fewmodules.separately.nodeps.loading
+```
+
+Output:
+
+```shell
+[loading] Main Class Module is module manual.fewmodules.separately.nodeps.loading
+[loading] Main Class ClassLoader is jdk.internal.loader.ClassLoaders$AppClassLoader@4554617c
+ClassLoader for [loadable] module is jdk.internal.loader.Loader@4a574795
+[loadable] Main Class ClassLoader is jdk.internal.loader.Loader@4a574795
+[loadable] Cat Class ClassLoader is jdk.internal.loader.Loader@4a574795
+[loadable] Main Class Module is module manual.fewmodules.separately.nodeps.loadable
+[loadable] Cat Class Module is module manual.fewmodules.separately.nodeps.loadable
+Meow
+```
+
+### Explanation
+---
+![Java 17. Manual Loading (loading and loadable).jpg](../../../../../img/Java%2017.%20Manual%20Loading%20(loading%20and%20loadable).jpg)
+
+The class modules are as we wanted, but the `Cat` class loader turned out to be not `CustomClassLoader` as we expected, but some `Loader`. The fact is that in the `defineModuleWithOneLoader` method, a new `Loader` loader is created (the same one that I mentioned in the **Java 8: Auto Loading** example), the parent of which becomes the passed `CustomClassLoader`.
+
+```java
+private static ModuleLayer createLayer(Path modulePath, String moduleName) {
+    /* ... */
+    return bootLayer.defineModulesWithOneLoader(config, new CustomClassLoader());
+}
+```
+
+Indeed, in [documentation](https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/lang/ModuleLayer.html#defineModulesWithOneLoader(java.lang.module.Configuration,java.util.List,java.lang.ClassLoader):~:text=of%20parent%20layers%20in%20search%20order-,parentLoader,-%2D%20The%20parent%20class%20loader%20for%20the) so and it is written:
+
+```java
+public ModuleLayer defineModulesWithOneLoader(Configuration cf, ClassLoader parentLoader)
+```
+
+> `parentLoader` - The parent class loader for the class loader created by this method; may be `null` for the 
+> bootstrap class loader.
+
+Thus, we do not have the opportunity to somehow pass our own class loader to the layer; we can only indicate the parent for the newly created one. Moreover, according to the same documentation, class loading will be delegated to the parent only if `Loader` does not find the package in which the class is located, here you can come up with different hacks based on this delegation model.
+
+### Notes
+---
+- You can rewrite the code a little and add a hack that will force `Loader` to delegate class loading to a custom class loader: [Java 17. Manual Loading (loading and loadable) - hack](https://github.com/temikfart/java-classloading/tree/main/java-17/manual/few-modules/separately/no-deps-hack)