java单例模式，考虑到线程安全写法有一下几种

/**
 * Created by jinyangyang on 2019/5/8.
 */

public class Singleton {
    private static Singleton singleton = null;
    //饿汉式单例模式
    //Singleton singleton = new Singleton();
    private Singleton(){

    }

    //饿汉式单例模式
    /*public static synchronized Singleton getInstance(){
        return singleton;
    }*/

    //懒汉式单例模式
    /*public static synchronized Singleton  getInstance(){
        if(singleton == null){
            singleton = new Singleton();
        }
        return singleton;
    }*/
    //线程安全单例模式
    /*public static Singleton getInstance(){
        synchronized (Singleton.class){
            if(singleton == null){
                singleton = new Singleton();
            }
        }
        return singleton;
    }*/
    //双重索检查，提升性能
    public static Singleton getInstance(){
        if(singleton == null){
            synchronized (Singleton.class){
                if(singleton == null){
                    singleton = new Singleton();
                }
            }
        }
        return singleton;
    }