JAVA 并发实现六(Volatile的使用)

package com.subject01;

public class VolatileDemo extends Object implements Runnable {
	//value变量没有被标记为volatile
	private int value;  
	//missedIt变量被标记为volatile
	private volatile boolean missedIt;
	//creationTime不需要声明为volatile，因为代码执行中它没有发生变化
	private long creationTime; 

	public VolatileDemo() {
		value = 10;
		missedIt = false;
		//获取当前时间，亦即调用Volatile构造函数时的时间
		creationTime = System.currentTimeMillis();
	}

	public void run() {
		print("entering run()");

		//循环检查value的值是否不同
		while ( value < 20 ) {
			//如果missedIt的值被修改为true，则通过break退出循环
			if  ( missedIt ) {
				//进入同步代码块前，将value的值赋给currValue
				int currValue = value;
				//在一个任意对象上执行同步语句，目的是为了让该线程在进入和离开同步代码块时，
				//将该线程中的所有变量的私有拷贝与共享内存中的原始值进行比较，
				//从而发现没有用volatile标记的变量所发生的变化
				Object lock = new Object();
				synchronized ( lock ) {
					//不做任何事
					try {
						Thread.sleep(100);
					} catch (InterruptedException e) {
						// TODO Auto-generated catch block
						e.printStackTrace();
					}
				}
				//离开同步代码块后，将此时value的值赋给valueAfterSync
				int valueAfterSync = value;
				print("in run() - see value=" + currValue +", but rumor has it that it changed!");
				print("in run() - valueAfterSync=" + valueAfterSync);
				break; 
			}
		}
		print("leaving run()");
	}

	public void workMethod() throws InterruptedException {
		print("entering workMethod()");
		print("in workMethod() - about to sleep for 2 seconds");
		Thread.sleep(2000);
		//仅在此改变missedIt的值
		missedIt = true;
		print("in workMethod() - just set missedIt=" + missedIt);
		print("in workMethod() - about to sleep for 3 seconds");
	//	Thread.sleep(5000);
		//仅在此改变value的值
		value = 50;
		print("in workMethod() - just set value=" + value);
		print("in workMethod() - about to sleep for 5 seconds");
		
		Thread.sleep(3000);
		print("leaving workMethod()");
	}

/*
*该方法的功能是在要打印的msg信息前打印出程序执行到此所化去的时间，以及打印msg的代码所在的线程
*/
	private void print(String msg) {
		//使用java.text包的功能，可以简化这个方法，但是这里没有利用这一点
		long interval = System.currentTimeMillis() - creationTime;
		String tmpStr = "    " + ( interval / 1000.0 ) + "000";		
		int pos = tmpStr.indexOf(".");
		String secStr = tmpStr.substring(pos - 2, pos + 4);
		String nameStr = "        " + Thread.currentThread().getName();
		nameStr = nameStr.substring(nameStr.length() - 8, nameStr.length());	
		System.out.println(secStr + " " + nameStr + ": " + msg);
	}

	public static void main(String[] args) {
		try {
			//通过该构造函数可以获取实时时钟的当前时间
			VolatileDemo vol = new VolatileDemo();

			//稍停100ms，以让实时时钟稍稍超前获取时间，使print（）中创建的消息打印的时间值大于0
			Thread.sleep(100);  

			Thread t = new Thread(vol);
			t.start();

			//休眠100ms，让刚刚启动的线程有时间运行
			Thread.sleep(100);  
			//workMethod方法在main线程中运行
			vol.workMethod();
		} catch ( InterruptedException x ) {
			System.err.println("one of the sleeps was interrupted");
		}
	}
}