单例模式的5种实现方式

1.懒汉式

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

2.饿汉式

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

3.静态内部类

public class Singleton{
    private static class SingletonHolder{
        private static final Singleton INSTANCE =  new Singleton();
    }
    private Singleton(){}
    public static final Singleton getInstance(){
        return SingletonHolder.INSTANCE;
    }
}

4.双重校验

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

5.枚举

public enum Singleton{
    INSTANCE;
    public void WhateverMethod(){  
    }
}