单例模式之饿汉模式实现

单例模式之饿汉模式实现
不要问为什么，问就是爱过，上代码：

package singleton;

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;

/**
 * 
 * @ClassName:  HungrySingleton   
 * @Description:TODO(这里用一句话描述这个类的作用)   
 * @author: 长子科技 
 * @date:   2021年7月7日 下午9:07:35   
 *     
 * @Copyright: 2021 www.tydic.com Inc. All rights reserved. 
 * 注意：本内容仅限于长子科技股份有限公司内部传阅，禁止外泄以及用于其他的商业目
 */
public class HungrySingleton implements Cloneable,Serializable{
    
    
	/**   
	 * @Fields serialVersionUID : TODO(用一句话描述这个变量表示什么)   
	 */  
	private static final long serialVersionUID = 1L;
	private volatile static HungrySingleton instance=new HungrySingleton();
	
	//避免反射产生新的单例
	private HungrySingleton(){
    
    
		if(instance!=null){
    
    
			throw new RuntimeException("单例已经实例化了，不能再实例化对象了");
		}
	}
	public static HungrySingleton getInstance() {
    
    
		return instance;
	}
	
	//避免克隆产生多个实例。
	@Override
	public HungrySingleton clone(){
    
    
		return instance;
	}
	//避免反序列化导致产生多个实例
	private Object readResolve(){
    
    
        return instance;
    }
	
	public static void main(String[] args) throws NoSuchMethodException, SecurityException, InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException, IOException, ClassNotFoundException, CloneNotSupportedException {
    
    
		
		HungrySingleton instance1=HungrySingleton.getInstance();
		System.out.println("instance1:"+instance1.hashCode());
		
		//通过反序列话获取单例
		ByteArrayOutputStream bos=new ByteArrayOutputStream();
		ObjectOutputStream oos=new ObjectOutputStream(bos);
		oos.writeObject(instance1);
		ByteArrayInputStream bis=new ByteArrayInputStream(bos.toByteArray());
		ObjectInputStream ois=new ObjectInputStream(bis);
		HungrySingleton instance3=(HungrySingleton)ois.readObject();
		System.out.println("instance3:"+instance3.hashCode());
		//通过克隆获取对象
		HungrySingleton instance4=(HungrySingleton) instance1.clone();
		System.out.println("instance4:"+instance4.hashCode());
		
		//反射方式获取对象
		Constructor<HungrySingleton> declaredconstructor=HungrySingleton.class.getDeclaredConstructor();
		declaredconstructor.setAccessible(true);
		HungrySingleton instance2=declaredconstructor.newInstance();
		System.out.println("instance2:"+instance2.hashCode());
	}
}

结果：
instance1:366712642
instance3:366712642
instance4:366712642
Exception in thread “main” java.lang.reflect.InvocationTargetException
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:408)
at singleton.HungrySingleton.main(HungrySingleton.java:69)
Caused by: java.lang.RuntimeException: 单例已经实例化了，不能再实例化对象了
at singleton.HungrySingleton.(HungrySingleton.java:32)
… 5 more

经过上一节（单例模式之懒汉模式实现）的演示，本章直接上最终代码，给大家，看一个安全的饿汉单例模式的实现。