线程安全的单例懒汉式

package com.day0206_2;
/**
 * 使用同步机制将单例模式中的懒汉式改写成线程安全的
 * 
 */
public class BankTest {
}
class Bank{
    private Bank(){
    }
    private static Bank instance =null;
    public static  Bank getInstance(){//synchronized
        //方式一：效率稍差
//        synchronized(Bank.class) {
//            if (instance == null) {
//                instance = new Bank();
//            }
//
//            return instance;
//        }
        //方式二：效率更高
        if (instance==null) {
            synchronized (Bank.class) {
                if (instance == null) {
                    instance = new Bank();
                }
            }
        }
        return instance;
    }
}