[J17] Add manual class loading example (one module)

Java 8 style.

Author: temikfart

.idea/runConfigurations/java_17_manual_one_module_java_8_style___run_.xml

@@ -0,0 +1,31 @@
+plugins {
+    id 'java-library'
+    id 'application'
+}
+
+group = 'com.habr.j17.manual.onemodule.j8style'
+version = '1.0'
+
+compileJava {
+    java {
+        toolchain {
+            languageVersion = JavaLanguageVersion.of(17)
+        }
+    }
+}
+
+application {
+    mainClassName = 'com.habr.j17.manual.onemodule.j8style.Main'
+}
+
+sourceSets {
+    main.java.srcDirs = ['src']
+}
+
+jar {
+    manifest {
+        attributes(
+                "Main-Class": application.mainClass
+        )
+    }
+}

java-17/manual/one-module/java8style/build.gradle

@@ -0,0 +1,7 @@
+package com.habr.j17.manual.onemodule.j8style;
+
+public class Cat {
+    public void talk() {
+        System.out.println("Meow");
+    }
+}

java-17/manual/one-module/java8style/src/com/habr/j17/manual/onemodule/j8style/Cat.java

@@ -0,0 +1,50 @@
+package com.habr.j17.manual.onemodule.j8style;
+
+import java.io.ByteArrayOutputStream;
+import java.io.File;
+import java.io.IOException;
+import java.io.InputStream;
+
+public class CustomClassLoader extends ClassLoader {
+    @Override
+    public Class<?> loadClass(String name) throws ClassNotFoundException {
+        synchronized (getClassLoadingLock(name)) {
+            Class<?> c = findLoadedClass(name);
+            if (c != null)
+                return c;
+
+            String APP_GROUP = "com.habr";
+            if (name.startsWith(APP_GROUP)) {
+                System.out.println("CCL: Loading " + name);
+                c = loadClassFromFile(name);
+
+                if (c != null)
+                    return c;
+            }
+
+            System.out.println("CCL: Delegating " + name);
+            return super.loadClass(name);
+        }
+    }
+
+    private Class<?> loadClassFromFile(String name) {
+        String classFile = name.replace('.', File.separatorChar) + ".class";
+        try (InputStream inputStream = getClass().getClassLoader().getResourceAsStream(classFile)) {
+            if (inputStream == null)
+                throw new RuntimeException();
+
+            byte[] buffer;
+            ByteArrayOutputStream byteStream = new ByteArrayOutputStream();
+
+            int nextValue;
+            while ((nextValue = inputStream.read()) != -1) {
+                byteStream.write(nextValue);
+            }
+
+            buffer = byteStream.toByteArray();
+            return defineClass(name, buffer, 0, buffer.length);
+        } catch (RuntimeException | IOException e) {
+            throw new RuntimeException("Failed to read from input stream", e);
+        }
+    }
+}

java-17/manual/one-module/java8style/src/com/habr/j17/manual/onemodule/j8style/CustomClassLoader.java

@@ -0,0 +1,26 @@
+package com.habr.j17.manual.onemodule.j8style;
+
+import java.lang.reflect.InvocationTargetException;
+import java.lang.reflect.Method;
+
+public class Main {
+    public static void main(String[] args) {
+        ClassLoader customClassLoader = new CustomClassLoader();
+        try {
+            Class<?> catClass = customClassLoader.loadClass("com.habr.j17.manual.onemodule.j8style.Cat");
+            Object cat = catClass.getDeclaredConstructor().newInstance();
+
+            System.out.println("System ClassLoader is " + ClassLoader.getSystemClassLoader());
+            System.out.println("Cat Class ClassLoader is " + catClass.getClassLoader());
+
+            System.out.println("Main Class Module is " + Main.class.getModule());
+            System.out.println("Cat Class Module is " + catClass.getModule());
+
+            Method talkMethod = catClass.getMethod("talk");
+            talkMethod.invoke(cat);
+        } catch (ClassNotFoundException | NoSuchMethodException | InvocationTargetException
+                 | InstantiationException | IllegalAccessException e) {
+            throw new RuntimeException(e);
+        }
+    }
+}

java-17/manual/one-module/java8style/src/com/habr/j17/manual/onemodule/j8style/Main.java

@@ -1,5 +1,6 @@
 rootProject.name = 'java-classloading'
 
-include 'java-8'
 include 'java-8:manual'
 include 'java-8:auto'
+
+include 'java-17:manual:one-module:java8style'