单例模式

单例模式：

• 单例类只能有一个实例。
• 单例类必须自己创建自己的唯一实例。
• 单例类必须给所有其他对象提供这一实例。

//懒汉
class Singleton1{
    private static Singleton1 instance = null;
    private Singleton1(){}
    public static Singleton1 getInstance(){
        if (instance != null){
            instance = new Singleton1();
        }
        return instance;
    }
}
//饿汉
class Singleton2{
    private static final Singleton2 instance = new Singleton2();
    private Singleton2(){}
    public static Singleton2 getInstance(){
        return instance;
    }
}
//枚举
class Singleton3{
    private Singleton3(){}
    private enum Singleton{
        INSTANCE;
        private final Singleton3 instance;
        private Singleton(){
            instance = new Singleton3();
        }
        private Singleton3 getInstance(){
            return instance;
        }
    }
    public Singleton3 getInstance(){
        return Singleton.INSTANCE.getInstance();
    }
}
//双重校验锁
class Singleton4{
    private static volatile Singleton4 instance;
    private Singleton4(){}
    public static Singleton4 getInstance(){
        if(instance == null){
            synchronized (instance){
                if (instance == null){
                    instance = new Singleton4();
                }
            }
        }
        return instance;
    }
}