add demo for synchronized keyword

add demo for VarHandler

src/main/java/jvm/VarHandleDemo.java

@@ -0,0 +1,16 @@
+package jvm;
+
+import java.util.Arrays;
+
+public class VarHandleDemo {
+
+    public int publicVar =1;
+    protected int protectedVar =2;
+    private int privateVar =3;
+    public int[] publicArrayData ={1,2,3};
+
+    @Override
+    public String toString() {
+        return STR."VarHandleDemo{publicVar=\{publicVar}, protectedVar=\{protectedVar}, privateVar=\{privateVar}, arrayData=\{Arrays.toString(publicArrayData)}}";
+    }
+}

src/main/java/threaddemo/dataracing/SyncDemo.java

@@ -0,0 +1,70 @@
+package threaddemo.dataracing;
+
+public class SyncDemo {
+
+    public synchronized static void methodA() {
+        System.out.println("enter static method methodA, sleep for 1s");
+        try {
+            Thread.sleep(1000);
+        } catch (InterruptedException e) {
+            e.printStackTrace();
+        }
+        System.out.println("leaving methodA");
+    }
+
+    public synchronized static void methodB() {
+        System.out.println("enter static method methodB, sleep for 1s");
+        try {
+            Thread.sleep(1000);
+        } catch (InterruptedException e) {
+            e.printStackTrace();
+        }
+        System.out.println("leaving methodB");
+    }
+
+    public synchronized void methodC() {
+        System.out.println("enter method methodC, sleep for 1s");
+        try {
+            Thread.sleep(1000);
+        } catch (InterruptedException e) {
+            e.printStackTrace();
+        }
+        System.out.println("leaving methodC");
+    }
+
+    public void methodD() {
+        System.out.println("enter method methodD, sleep for 1s");
+        synchronized (SyncDemo.class) {
+            System.out.println("start run methodD");
+            try {
+                Thread.sleep(1000);
+            } catch ( InterruptedException e) {
+                e.printStackTrace();
+            }
+        }
+        System.out.println("leaving methodD");
+    }
+
+    public static void main(String[] args) {
+        SyncDemo sd = new SyncDemo();
+        long start = System.currentTimeMillis();
+        Thread t1 = new Thread(SyncDemo::methodA);
+        Thread t2 = new Thread(SyncDemo::methodB);
+        Thread t3 = new Thread(sd::methodC);
+        Thread t4 = new Thread(sd::methodD);
+
+        t1.start();
+        t2.start();
+        t3.start();
+        t4.start();
+        try {
+            t1.join();
+            t2.join();
+            t3.join();
+            t4.join();
+            System.out.println(STR."cost: \{System.currentTimeMillis() - start}");
+        } catch (InterruptedException e) {
+            e.printStackTrace();
+        }
+    }
+}