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");

		}

	}

}