单例设计模式上初篇

单例模式：保证一个类只有一个实例，并且提供一个访问该实例的全局访问点

• 反射与反序列化的使用

package learn.java.com.java.pattern;

import java.io.*;
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;

public class SingletonDemo01 {
   public static void main(String[] args) throws ClassNotFoundException, NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException, IOException {
       SingletonDemo s1=SingletonDemo.getInstance();
       SingletonDemo s2=SingletonDemo.getInstance();
       System.out.println(s1);
       System.out.println(s2);
       //通过反射的方式直接调用
       Class<SingletonDemo> clazz =(Class<SingletonDemo>) Class.forName("learn.java.com.java.pattern.SingletonDemo");
       Constructor<SingletonDemo> c=clazz.getDeclaredConstructor(null);
       c.setAccessible(true);
//       (可以访问私有变量)
       SingletonDemo  s3=c.newInstance();
       SingletonDemo s4=c.newInstance();
       System.out.println(s3);
       System.out.println(s4);
       /**没有防止反射时的返回
        * learn.java.com.java.pattern.SingletonDemo@61443d8f
        * learn.java.com.java.pattern.SingletonDemo@61443d8f
        * learn.java.com.java.pattern.SingletonDemo@445b84c0
        * learn.java.com.java.pattern.SingletonDemo@61a52fbd
        */

        //序例化
       FileOutputStream fos =new FileOutputStream("./a.txt");
       ObjectOutputStream oos =new ObjectOutputStream(fos);
       oos.writeObject(s1);
       oos.close();
       oos.close();
       ObjectInputStream ois=new ObjectInputStream(new FileInputStream("./a.txt"));
       SingletonDemo snew=(SingletonDemo) ois.readObject();
       System.out.println(snew);

//没有添加readResolve 时输出对象
       /**
        *learn.java.com.java.pattern.SingletonDemo@61443d8f
        * learn.java.com.java.pattern.SingletonDemo@61443d8f
        * learn.java.com.java.pattern.SingletonDemo@327471b5
        */


//添加readResolve 时输出对象
       /**
        learn.java.com.java.pattern.SingletonDemo@591f989e
        learn.java.com.java.pattern.SingletonDemo@591f989e
        learn.java.com.java.pattern.SingletonDemo@591f989e
        */

   }
}
 class SingletonDemo implements Serializable {
    private static SingletonDemo instance=new SingletonDemo();
    //加载类时，天然是线程安全的
    //类初始化时立即加载（不延时）
//    private SingletonDemo(){}
    private SingletonDemo(){
        if (instance!=null){
           throw new RuntimeException();
        }//用来防止反射
    }
    //方法没有同步，调用效率高
    public static SingletonDemo getInstance(){
        return instance;
    }
//用来防止反序列化，反序列化时如果定义了readsolve()则直接返回指定的对象，而不重建对象
    public Object readResolve() {
        return instance;
    }
}
 class SingletonDemo02{
    private static SingletonDemo02 s;
    private SingletonDemo02(){}
    private static synchronized SingletonDemo02 getInstance(){
        if(s==null)
            s=new SingletonDemo02();
        return s;
    }

}

class SingletonDemo0{
    private static class SingletonDemo04{
        private static SingletonDemo04 s=new SingletonDemo04();
        private static SingletonDemo04 getInstance(){
            return s;
        }
    }
}


 enum Sing{
    instance;
}