readResolve()的使用

对于一些singleton class，如果你让其implements Serializable，会导致该class不再是singleton。使用ObjectInputStream.readObject()读取进来之后，如果是多次读取，就会创建多个object，下面的代码可以证明这一点，解决的办法之一就是override一个 method，readResolve()

package foo;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;

// Singleton Box:
class Box implements Serializable {
    private static Box instance = new Box("TEST");
    public static Box getInstance() {
        return Box.instance;
    }
    private Box(String name) {  
        this.name = name;
    }
    private String name;
    @Override
    public String toString() {
        return "Box " + name;
    }
    
    /* ********************************
    private Object readResolve(    ) {
        return Box.getInstance();
    }
    * ********************************/
}

public class Foo {
    public static void main(String[] args) {
        Box box = Box.getInstance();
        System.out.println(box.toString());   
        try {   
           ObjectOutputStream o = new ObjectOutputStream(   
                        new FileOutputStream("e:/box.out"));   
           o.writeObject(box);   
           o.close();   
        } catch(Exception e) {
            e.printStackTrace();
        }
        
        Box box1 = null, box2 = null;
        
        try {   
           ObjectInputStream in =new ObjectInputStream(   
                        new FileInputStream("e:/box.out"));
           box1 = (Box)in.readObject();   
           in.close();
        } catch(Exception e) {
            e.printStackTrace();
        }
        
        try {   
           ObjectInputStream in =new ObjectInputStream(   
                        new FileInputStream("e:/box.out"));
           box2 = (Box)in.readObject();   
           in.close();
        } catch(Exception e) {
            e.printStackTrace();
        }
        
        System.out.println("box1.equals(box2) : " + box1.equals(box2));
        System.out.println(box1);
        System.out.println(box2);  
    }
}