Java中多线程与并发_volatile关键字的深入理解

class MyData {
 volatile int num = 0;

 public void addT060() {
 this.num = 60;
 }
}

 //volatile可以保证可见性，及时通知其它线程，主内存的值已经被修改
 @Test
 public void seeOkByVolatile() {
 MyData myData = new MyData();//资源类

 new Thread(() -> {
  System.out.println(Thread.currentThread().getName() + "\t come in");
  //暂停一会线程
  try{
  TimeUnit.SECONDS.sleep(3);
  }catch (InterruptedException e) {
  e.printStackTrace();
  }
  myData.addT060();
  System.out.println(Thread.currentThread().getName() + "\t update num value: " + myData.num);
 },"AAA").start();

 //第2个线程是我们的main线程
 while (myData.num == 0) {
  //main线程就一直在这里等待循环，直到num值不再等于0.
 }
 System.out.println(Thread.currentThread().getName() + "\t mission is over,main get num value: " + myData.num );
 }

class MyData {//MyData.java ===> MyData.class ===> JVM字节码
 int num = 0;

 public void addT060() {
 this.num = 60;
 }

 //请注意，此时num前面是加了关键字修饰的,volatile不保证原子性
 public void addPlusPlus() {
 num++;
 }
}

public static void main(String[] args) {
 MyData myData = new MyData();

 for (int i = 1; i <= 20; i++ ) {

  new Thread(() -> {
  for (int j = 1; j <= 2000; j++) {
   myData.addPlusPlus();
  }

  },String.valueOf(i)).start();
 }

 //需要等待上面20个线程都全部计算完成后，再用main线程取得最终的结果值看是多少？
 while(Thread.activeCount() > 2) {
  Thread.yield();
 }

 System.out.println(Thread.currentThread().getName() + "\t finally num value:" + myData.num);
 }

class com.slx.juc.MyData {
 volatile int num;

 com.slx.juc.MyData();
 Code:
 0: aload_0
 1: invokespecial #1   // Method java/lang/Object."<init>":()V
 4: aload_0
 5: iconst_0
 6: putfield #2   // Field num:I
 9: return

 public void addT060();
 Code:
 0: aload_0
 1: bipush 60
 3: putfield #2   // Field num:I
 6: return

 public void addPlusPlus();
 Code:
 0: aload_0
 1: dup
 2: getfield #2   // Field num:I
 5: iconst_1
 6: iadd
 7: putfield #2   // Field num:I
 10: return
}

 AtomicInteger atomicInteger = new AtomicInteger();
 public void addMyAtomic() {
 atomicInteger.getAndIncrement();
 }

 public static void main(String[] args) {
 MyData myData = new MyData();

 for (int i = 1; i <= 20; i++ ) {

  new Thread(() -> {
  for (int j = 1; j <= 2000; j++) {
   myData.addMyAtomic();
  }

  },String.valueOf(i)).start();
 }

 //需要等待上面20个线程都全部计算完成后，再用main线程取得最终的结果值看是多少？
 while(Thread.activeCount() > 2) {
  Thread.yield();
 }

 System.out.println(Thread.currentThread().getName() + "\t AtomicInteger type ,finally num value:" + myData.atomicInteger);
 }