如何判断ObjectOutputStream已经结束？

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;public class ObjectOutputStreamu { public static void main(String[] args) { try {
FileOutputStream fos = new FileOutputStream("a.txt");
ObjectOutputStream oos = new ObjectOutputStream(fos);
Employee e1 = new Employee(30, "jack", 100);
Employee e2 = new Employee(30, "jack", 99.999999);
Employee e3 = new Employee(30, "jack", 99.999998);
oos.writeObject(e1);
oos.writeObject(e2);
oos.writeObject(e3);
oos.close();  
FileInputStream fis = new FileInputStream("a.txt");
ObjectInputStream ois = new ObjectInputStream(fis);
Object o;
while((o = ois.readObject()) != null) {  // 这个判断怎么没有起到作用？最后还是跳出异常 if (o instanceof Employee)
System.out.println(((Employee)o).name + ":" + ((Employee)o).age + ":" + ((Employee)o).score);   }

} catch (FileNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (ClassNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}}
class Employee implements Serializable
{
int age;
String name;
double score;

public Employee(int age, String name, double score)
{
this.age = age;
this.name = name;
this.score = score;
}

public Employee() {
}
}