Double-Checked-Locking模式

package com.ymjin.doubleMoshi;
/**
 * 这就是Double-Checked-Locking模式
 * -
 * @author xiaoyu
 *
 */
public class USTax {
	private static USTax instance = null;

	private USTax() {
	}

	private synchronized static void doSync() {
		if (instance == null) {
			System.out.println("实例不存在");
			instance = new USTax();
			System.out.println("实例创建成功");
		} else {
			System.out.println("实例已经被创建");
		}

	}

	@SuppressWarnings("unused")
	private static USTax getInstance() {
		if (instance == null) {
			System.out.println("实例不存在");
			doSync();
		} else {
			System.out.println("实例已经存在");
		}
		return instance;

	}
}