java中volatile关键字的作用与实例代码

    class MyData{
​    volatile int number = 0;
​    Object object = new Object();

    public void addTo60(){
        this.number = 60;
    }
    
    public void addPlusPlus(){
        this.number++;
    }
    
    AtomicInteger atomicInteger = new AtomicInteger();
    
    public void addAtomic(){
        atomicInteger.getAndIncrement();
    }
}


public class VolatileDemo {
    public static void main(String[] args) {
        //seeByVolatile();
        atomic();
    }
    //验证原子性
    public static void atomic() {
        MyData myData = new MyData();
        for (int i = 1; i <= 20; i++) {
            new Thread(new Runnable() {
                @Override
                public void run() {
                    for (int j = 1; j <= 1000; j++) {
                        
                        myData.addPlusPlus();
                        myData.addAtomic();
                    }
                }
            }).start();
        }

        //等待上面20个线程全部计算结束
        while (Thread.activeCount() > 2){
            Thread.yield();
        }
        
        System.out.println(Thread.currentThread().getName() + "int finally number is " + myData.number);
        System.out.println(Thread.currentThread().getName() + "AtomicInteger finally number is " + myData.atomicInteger);
    }

    //验证可见性的方法
    public static void seeByVolatile() {
        MyData myData = new MyData();
        //第一个线程
        new Thread(){
            public void run(){
                System.out.println(Thread.currentThread().getName() + " come in");
                try {
                    sleep(3000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                myData.addTo60();
                System.out.println(Thread.currentThread().getName() + " update number to " + myData.number);
            }
        }.start();

        //第二个线程 main
        while (myData.number == 0){
        
        }
        System.out.println(Thread.currentThread().getName() + "mission is over");
    }
}

 单例DCL的代码

public class SingletonDemo {
    private static SingletonDemo instance = null;

    private SingletonDemo(){
        System.out.println(Thread.currentThread().getName() + "构造方法");
    }
    
    //DCL双端加锁机制
    public static SingletonDemo getInstance(){
        if (instance == null){
            synchronized (SingletonDemo.class){
                if (instance == null){
                    instance = new SingletonDemo();
                }
            }
        }
        return instance;
    }
}

public class SingletonDemo {
    private static volatile SingletonDemo instance = null;

    private SingletonDemo(){
        System.out.println(Thread.currentThread().getName() + "构造方法");
    }
    //DCL双端加锁机制
    public static SingletonDemo getInstance(){
        if (instance == null){
            synchronized (SingletonDemo.class){
                if (instance == null){
                    instance = new SingletonDemo();
                }
            }
        }
        return instance;
    }
}