单例模式的实现

特点：

1、单例类只能有一个实例。

2、单例类必须自己创建自己的唯一实例。

3、单例类必须给所有其他对象提供这一实例。

一、饿汉式单例

//饿汉模式
public class Hungry {

    //可能会浪费资源
    private byte[] bytes1 = new byte[1024*1024];
    private byte[] bytes2 = new byte[1024*1024];
    private byte[] bytes3 = new byte[1024*1024];
    private byte[] bytes4 = new byte[1024*1024];

    private Hungry(){}

    private final static Hungry HUNGRY = new Hungry();

    public Hungry getInstance(){
        return  HUNGRY;
    }

}

二、懒汉式单例（双重检测锁 + 原子性操作）

public class Singleton{
        
        private static Boolean flag = false;
        private Singleton(){
                if(!flag){
                        flag = true;
                }else{
                        throw new RuntimeException("不要试图用反射破坏单例模式");
                }
        }
        
        private static volatile Singleton singleton = null;
        
        public static Singleton getInstance(){
                if(singleton==null){
                        synchronized(Singleton.class){
                                if(singleton==null){
                                        singleton = new Singleton();
                                }
                        }
                }
                return singleton;
        }
}

三、静态内部类（非线程安全）

package com.dana.single;

public class Holder {
    private Holder(){}

    public static Holder getInstance(){
        return InnerClass.HOLDER;
    }


    public static class InnerClass{
        private static final  Holder HOLDER = new Holder();
    }
}

四、枚举

public enum EnumSingle {
    INSTANE;

    public EnumSingle getInstane(){
        return INSTANE;
    }

    
    //test
    public static void main(String[] args) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {
        EnumSingle instane = EnumSingle.INSTANE;
        System.out.println(instane);

        Constructor enumSingleConstructor =
                EnumSingle.class.getDeclaredConstructor(String.class,int.class);
        enumSingleConstructor.setAccessible(true);
        EnumSingle enumSingle = enumSingleConstructor.newInstance();
        //异常：Cannot reflectively create enum objects
        System.out.println(enumSingle);

    }
}