单列模式（single）

一、饿汉模式

public class SingleOne {

    private final static SingleOne INSTANCE = new SingleOne();

    private SingleOne(){}

    public SingleOne getInstance(){
        return INSTANCE;
    }

}

 

二、懒汉模式

public class SingleTwo {

    private static volatile SingleTwo instance = null;

    private SingleTwo(){}

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

 

三、静态内部类（推荐使用）

public class SingleThree {

    private SingleThree(){}

    private static class SingleInstance{
        private final static SingleThree INSTANCE = new SingleThree();
    }

    public SingleThree getInstance(){
        return SingleInstance.INSTANCE;
    }
}

 

四、枚举类模式（推荐使用）

public class SingleFour {

    private SingleFour(){}
    
    private enum SingleInstance{
        INSTANCE;
        
        private SingleFour instance;
        
        //枚举类的特性，此处只会调用一次
        SingleInstance(){
            instance = new SingleFour();
        }
        
        public SingleFour getSingleFour(){
            return instance;
        }
    }
    
    public static SingleFour getInstance(){
        return SingleInstance.INSTANCE.getSingleFour();
    }
}