从volatile看Java内存可见性

现象：

public class TestVolatile {
	public static void main(String[] args) {
		ThreadDemo td = new ThreadDemo();
		new Thread(td).start();
		while(true){
			if(td.isFlag()){
				System.out.println("------------------");
				break;
			}
		}
	}
}
class ThreadDemo implements Runnable {
	private  boolean flag = false;
	@Override
	public void run() {
		try {
			Thread.sleep(200);
		} catch (InterruptedException e) {
		}
		flag = true;
		System.out.println("flag=" + isFlag());
	}
	public boolean isFlag() {
		return flag;
	}
	public void setFlag(boolean flag) {
		this.flag = flag;
	}
}

原因：内存可见性

解决方法1：使用synchronized关键字

public class TestVolatile {
	public static void main(String[] args) throws Exception{
		ThreadDemo td = new ThreadDemo();
		new Thread(td).start();
		while(true){
		    synchronized(td){
                if(td.isFlag()){
                    System.out.println("------------------");
                    break;
                }
            }
		}
	}
}
class ThreadDemo implements Runnable {
	private  boolean flag = false;
	@Override
	public void run() {
		try {
			Thread.sleep(200);
		} catch (InterruptedException e) {
		}
		flag = true;
		System.out.println("flag=" + isFlag());
	}
	public boolean isFlag() {
		return flag;
	}
	public void setFlag(boolean flag) {
		this.flag = flag;
	}
}

2：使用volatile关键字

        volatile 关键字：当多个线程进行操作共享数据时，可以保证内存中的数据可见。相较于 synchronized 是一种较为轻量级的同步策略。

public class TestVolatile {
	public static void main(String[] args) throws Exception{
		ThreadDemo td = new ThreadDemo();
		new Thread(td).start();
		while(true){
			if(td.isFlag()){
				System.out.println("------------------");
				break;
			}
		}
	}
}
class ThreadDemo implements Runnable {
	private volatile boolean flag = false;
	@Override
	public void run() {
		try {
			Thread.sleep(200);
		} catch (InterruptedException e) {
		}
		flag = true;
		System.out.println("flag=" + isFlag());
	}
	public boolean isFlag() {
		return flag;
	}
	public void setFlag(boolean flag) {
		this.flag = flag;
	}
}