单例模式要点：

1：构造函数私多余的对象

2：只能拥有一个对象

饿汉式（最简单的一种写法，类加载直接new出一个对象）

class Singleton{

    private static final Singleton singleton = new Singleton();

    public static Singleton getInstance(){

        return singleton;

    }

}

懒汉式

1.线程不安全写法，多线程可能会同时new对象

class Singleton{

    private Singleton(){};

    private static final Singleton singleton = null;

    public static Singleton getInstance(){

    if(singleton == null){

        singleton = new Singleton();

    }

        return singleton;

    }

}

2.线程安全写法，但效率太低，一般不用 加synchronized

class Singleton{

    private Singleton(){};

    private static final Singleton singleton = null;

    public static synchronized Singleton getInstance(){

    if(singleton == null){

        singleton = new Singleton();

    }

        return singleton;

    }

}

3.双重校验锁，较常用，解决线程问题和效率问题

class Singleton{

    private Singleton(){};

    private static final Singleton singleton = null;

    public static Singleton getInstance(){

        if(singleton == null){

            synchronized (Singleton.class){

                if(singleton == null){

                    singleton = new Singleton();

                }

            }

        }

        return singleton;

    }

}

4.静态内部类

class Singleton{

    private Singleton(){};

    private static class InstanceHolder(){

        private static final Singleton SINGLETON= new Singleton();(定义常量都大写)

    }

    public static Singleton getInstance(){

        return InstanceHolder.SINGLETON;

    }

}