package Test;public class ThreadAndSingleton {	private ThreadAndSingleton() {	}	private static ThreadAndSingleton threadAndSingleton = null;	/**	 * 试试不加synchronized关键字，会出现两个实例的	 * @param sleepTime	 * @return	 */	public static synchronized ThreadAndSingleton getIntstance(int sleepTime) {		if (threadAndSingleton == null) {			try {				Thread.sleep(sleepTime);			} catch (InterruptedException e) {				e.printStackTrace();			}			threadAndSingleton = new ThreadAndSingleton();		}		return threadAndSingleton;			}	public static void main(String[] args) {		/**		 * 下面这就属于模型驱动模式		 */		Thread th1 = new Thread(new Runnable() {			@Override			public void run() {				System.out.println("th1:" + ThreadAndSingleton.getIntstance(1000));			}		});		Thread th2 = new Thread(new Runnable() {			@Override			public void run() {				System.out.println("th2:" + ThreadAndSingleton.getIntstance(0));			}		});		th1.start();		th2.start();	}}

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getIntstance方法被同步的说明：如果不同步，同事访问这个方法可能造成进入后等待，然后另一次访问开始了，结果造成两个实例，就不是单例模式了。