线程安全之单例模式懒汉式

public class BankTest {
    public static void main(String[] args) {
        Bank bank1 = Bank.getInstance();
    }
}

class Bank{
    private Bank(){}

    private static Bank instance = null;

    public static Bank getInstance(){//或者用同步方法，直接对这个方法添加synchronized属性
/*        //方式一：效率较差
        synchronized (Bank.class) {
            if(instance == null){
                instance = new Bank();
                return instance;
            }
            return instance;
        }*/

        //方式二：效率更高
        if(instance == null){

            synchronized (Bank.class) {
                if (instance == null) {
                    instance = new Bank();
                }
            }

        }
        return instance;
    }



}