Java 常用单例

一、线程不安全的单例模式

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

二、线程安全且高效的单例模式

1.双重校验锁

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

2.静态内部类

public class SingleTon{
    private SingleTon(){}
    
    private static class Holder{
        private static SingleTon INSTANCE = new SingleTon();
    } 
    
    public static SingleTon getInstance(){       
        return Holder.INSTANCE;
    }
}