public class Singleton {	private static volatile Singleton instance;	public static Singleton getInstance() {		if (null == instance) {			synchronized (Singleton.class) {				if (null == instance) {					instance = new Singleton();				}			}		}		return instance;	}}

// 利用私有静态内部类public class Singleton2 {	private Singleton2() {	}	private static class Singleton2Holder {		private static Singleton2 instance = new Singleton2();	}	public static Singleton2 getInstance() {		return Singleton2Holder.instance;	}}

// 测试import java.util.concurrent.CyclicBarrier;import java.util.concurrent.BrokenBarrierException;public class Singleton1Test {	private static int SIZE = 10;	private static CyclicBarrier cb;	public static void main(String[] args) {		cb = new CyclicBarrier(SIZE);		for (int i = 0; i < SIZE; i++) {			new InnerThread().start();		}	}	static class InnerThread extends Thread {		public void run() {			try {				cb.await();				Singleton1 instance = Singleton1.getInstance();				System.out.println(instance + "\t" + this.getName() + "");			} catch (BrokenBarrierException e) {				e.printStackTrace();			} catch (InterruptedException e) {				e.printStackTrace();			}		}	}}