四种单例模式的实现

1、懒汉式：

public class Singleton01{
    
    private static final INSTANCE = new Singleton01();

    private Singleton01(){};

    public static Singleton getInstance(){
        return INSTANCE;    
    }    

    
}

2、线程安全的饿汉式

public class Singleton02{
    private static volatile instance;

    private Singleton02(){};

    public static Singleton02 getInstance(){
        if(insatnce == null){
            synchronized(this){
                if(instance == null)            
                    instance = new Singleton02(); 
            }
        }
        return instance;
    }


}

3、枚举类建立单例模式,可以反序列化

public enum Singleton03{
    
    INSTANCE;

}

4、静态内部类的方式,属于懒汉式的一种也是线程安全的

public class Singleton04{
    
    private Singleton04(){};

    private static class singleton04Holder{
        private static final INSTANCE = new Singleton04();
        
        public static Singleton04 getInstance(){
            return INSTANCE;
        }    
        
    }

}