单例模式

保证一个类仅有一个实例，并提供一个访问它的全局访问点。
核心思路： 有就返回，没有就创建。

1、饿汉

public class Singleton{
    private static Singleton = new Singlton();
    private Sigleton(){}
    private static Singleton getInstance(){
        return instance;
    }
}

2、懒汉

2.1 普通懒汉 线程不安全

public class Singleton {
    private static Singleton instance;
    private Singletion(){
    }
    public static Singletion getInstance(){
        if (instance == null){
            instance = new Singleton();
        }
        return instance;
    }
}

2.2 线程安全第一版

public class Singleton{
    private static Singleton instance;
    private Singleton(){}
    public static synchronized Singleton(){
        if( instance == null){
            instance = new Singleton();
        }
        return instance;
    }
}

2.3 DCL 双重校验锁

public class Singleton{
    private static volatile Singleton instance;
    private Singleton(){}
    private static Singleton(){
        if( instance == null){
            synchronized(Singleton.class){
                if( instance == null){
                    instance = new Singleton();
                }
            }
        }
    return instance;
    }
}

2.4 枚举