单例设计模式

所谓单例设计模式，就是采取一定的方法保证在整个的软件系统中，对某个类只能存在一个对象实例

记忆技巧：懒汉式—太懒了，什么时候用，什么时候造

​ 饿汉式—太饿了，上来直接造好

package com.danli;

public class Singleton01 {
    public static void main(String[] args) {
        Bank bank = Bank.getBank();
        Bank bank1 = Bank.getBank();
        System.out.println(bank==bank1);
    }

}

class Bank{

    //1.私有化类的构造器(禁止其它类调用)
    private Bank() {
    }

    //2.内部创建类的对象(必须是静态的)
    private static Bank bank = new Bank();

    //3.提供公共的静态方法，返回类的对象
    public static Bank getBank() {
        return bank;
    }
}

下面这种方法的线程是不安全的

package com.danli;

public class Singleton02 {
    public static void main(String[] args) {
        Supermarket supermarket = Supermarket.getSupermarket();
        Supermarket supermarket1 = Supermarket.getSupermarket();
        System.out.println(supermarket==supermarket1);
    }

}

class Supermarket{

    //1.私有化类的构造器
    private Supermarket() {
    }

    //2.声明当前类对象，不初始化(静态的)
    private static Supermarket supermarket = null;

    //3.声明public，static的返回当前类对象的方法
    public static Supermarket getSupermarket() {
        if (supermarket==null){
            supermarket = new Supermarket();
        }
        return supermarket;
    }
}

以下，在getSupermarket方法上加上同步，此时线程是安全的

package com.danli;

public class Singleton02 {
    public static void main(String[] args) {
        Supermarket supermarket = Supermarket.getSupermarket();
        Supermarket supermarket1 = Supermarket.getSupermarket();
        System.out.println(supermarket==supermarket1);
    }

}

class Supermarket{

    //1.私有化类的构造器
    private Supermarket() {
    }

    //2.声明当前类对象，不初始化(静态的)
    private static Supermarket supermarket = null;

    //3.声明public，static的返回当前类对象的方法
    public static synchronized Supermarket getSupermarket() {
        if (supermarket==null){
            supermarket = new Supermarket();
        }
        return supermarket;
    }
}

饿汉式：对象加载时间过长，线程是安全的

懒汉式：延迟对象的创建，线程不安全(第一种写法)