add demo for volatile keyword and Class loader

src/main/java/jvm/VolatileTest.java

@@ -0,0 +1,42 @@
+package jvm;
+
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.concurrent.locks.ReentrantLock;
+
+public class VolatileTest {
+
+    public  static  volatile int race = 0;
+    private static final Object lock = new Object();
+    private static final ReentrantLock lock1 = new ReentrantLock();
+    public static AtomicInteger race1 = new AtomicInteger(0);
+
+    public static void increase() {
+//       synchronized (lock) {
+//           race++;
+//       }
+        race1.getAndAdd(1);
+        lock1.lock();
+        race++;
+        lock1.unlock();
+    }
+
+    private static final int THREADS_COUNT = 20;
+
+    static void main() {
+        Thread[] threads = new Thread[THREADS_COUNT];
+        for (int i = 0; i < THREADS_COUNT; i++) {
+            threads[i] = new Thread(() -> {
+                for (int j = 0; j < 1000; j++) {
+                    increase();
+                }
+            });
+            threads[i].start();
+        }
+        // for idea only. otherwise should be 1
+        while (Thread.activeCount() > 2) {
+            Thread.yield();
+        }
+        System.out.println(race);
+        System.out.println(race1.get());
+    }
+}

src/main/java/loader/CustomLoader.java

@@ -0,0 +1,56 @@
+
+package loader;
+
+import java.io.*;
+
+public class CustomLoader extends ClassLoader{
+
+    private String classPath;
+
+    public CustomLoader(String classPath) {
+        this.classPath = classPath;
+    }
+
+    private byte[] loadClassBytes(String className) {
+        String path = STR."\{classPath}\{File.separatorChar}\{className.replace('.', File.separatorChar)}.class".trim();
+//        File file = new File("C:\\Users\\wjluk\\projects\\java_multithread\\src\\main\\java\\loader\\LoaderTest.class");
+        File file = new File(path);
+        System.out.println(file.exists());
+        try (InputStream is = new FileInputStream(path);
+             ByteArrayOutputStream baos = new ByteArrayOutputStream()) {
+            byte[] buffer = new byte[4096];
+            int bytesRead;
+            while ((bytesRead = is.read(buffer)) != -1) {
+                baos.write(buffer, 0, bytesRead);
+            }
+            return baos.toByteArray();
+        } catch (IOException e) {
+            return null;
+        }
+    }
+
+
+    @Override
+    protected Class<?> findClass(String name) throws ClassNotFoundException {
+        byte[] classBytes = loadClassBytes(name);
+        if (classBytes == null) {
+            throw new ClassNotFoundException();
+        }
+        return defineClass(name, classBytes, 0, classBytes.length);
+    }
+
+    static void main(String[] args) throws Exception {
+        CustomLoader loader = new CustomLoader(" D:\\");
+        Class<?> clazz = loader.loadClass("LoaderTest");
+        Object obj = clazz.newInstance();
+        clazz.getMethod("greeting").invoke(obj);
+        System.out.println(clazz.getClassLoader());
+
+        System.out.println("---------------------for parent loader-------------------");
+        ClassLoader classLoader = loader.getClass().getClassLoader();
+        while (classLoader != null) {
+            System.out.println( "\t" + classLoader);
+            classLoader = classLoader.getParent();
+        }
+    }
+}