恢复一个Externalizable对象时遇到的问题

在恢复一个Externalizable对象时，为什么对象的构造器不是public的访问权限就造成异常呢？

import java.io.*;

class Blip1 implements Externalizable {

    public Blip1() {

        System.out.println("Blip1 Constructor");

    }

public void writeExternal(ObjectOutput out) throws IOException {
    System.out.println("Blip1.writeExternal");
}

public void readExternal(ObjectInput in) throws IOException,ClassNotFoundException {
    System.out.println("Blip1.readExternal");
}


}

class Blip2 implements Externalizable {

    Blip2() {

        System.out.println("Blip2 Constructor");

    }

public void writeExternal(ObjectOutput out) throws IOException {
    System.out.println("Blip2.writeExternal");
}

public void readExternal(ObjectInput in) throws IOException,ClassNotFoundException {
    System.out.println("Blip2.readExternal");
}


}

public class Blips {

public static void main(String[] args) throws IOException, ClassNotFoundException{
        System.out.println("Constucting objects:");
        Blip1 b1=new Blip1();
        Blip2 b2=new Blip2();
        ObjectOutputStream o=new ObjectOutputStream(new FileOutputStream("Blips.out"));
        System.out.println("saving objects;");
        o.writeObject(b1);
        o.writeObject(b2);
        o.close();
        ObjectInputStream in=new ObjectInputStream(new FileInputStream("Blips.out"));
        System.out.println("Recovoring b1:");
        b1=(Blip1)in.readObject();
        //System.out.println("Recovoring b2:");
        //b2=(Blip2)in.readObject();
}


}

被注释掉的两句话就会导致java.io.InvalidClassException异常，大家帮看看是什么原因？

这是relfection的原理。要通过reflection构建一个class的instance,必须有public的无参构造器存在，才可以创建。