ObjectInputStream and default constructor in Tiger

[Leonardo Crespo]

Hi all. Can anyone give me a hand on this issue?


In page 338 of Complete Java 2 Certification, Study Guide, 5th edition (Tiger), it says:
-----------
Notice that the readObject() mechanism of ObjectInputStream relies on the existence of
a no-args constructor for any class that implements Externalizable. If an externalizable class
has no no-args constructor, the readObject() method will throw
java.io.InvalidClassException.
-----------

I checked in the API and it says:

 public class InvalidClassException
extends ObjectStreamException
 
Thrown when the Serialization runtime detects one of the following problems with a Class.
 
    * The serial version of the class does not match that of the class descriptor read from the stream
    * The class contains unknown datatypes
  (THIS ONE!)  * The class does not have an accessible no-arg constructor  

However, this is not true.

This is the class i'm serializing and i'm not providing a default constructor. Since i'm providing a constructor, the compiler does not create the default one.

(ps: just to clarify things, Fusca equals Beagle, it's a type of car)

 
 
public class Fusca extends Car {
public String year = "1997";

public Fusca (String ano){
this.year = year;
}
 
.. in serialization class
 
Fusca c = new Fusca("1949");
FileOutputStream fos = new FileOutputStream("c:\\myobj.ser");ObjectOutputStream oos = new ObjectOutputStream(fos);
oos.writeObject(c);
oos.close();
fos.close();

This code compiles just fine and i dont get errors in deserialization either.

FileInputStream file_input_stream = new FileInputStream("c:\\myobj.ser");
ObjectInputStream object_input_stream = new ObjectInputStream(file_input_stream);
d = (Fusca) object_input_stream.readObject();
out.println(d.ano);

Why is that?

Thank you very much. Leonardo.

[Joyce Lee]

Hi Leonardo,

Notice that the readObject() mechanism of ObjectInputStream relies on the existence of a no-args constructor for any class that implements Externalizable. If an externalizable class has no no-args constructor, the readObject() method will throw java.io.InvalidClassException.

Here's an example which demostrates an externalizable class without a no-arg constructor:

import java.io.*;
 
class Car implements Externalizable {
    private String year;
 
    // comment the no-arg constructor
    // public Car() { }  
 
    public Car(String year) {
        this.year = year;
    }
    
    public String getYear() {
        return year;
    }
    
    public void writeExternal(ObjectOutput out) throws IOException {
    out.writeObject(year);
    }
    
    public void readExternal(ObjectInput in) throws IOException, 
            ClassNotFoundException {
        year = (String)in.readObject();
    }
}
 
public class Test {
    public static void main(String[] args) throws Exception {
        
        Car myCar = new Car("1997");
        FileOutputStream fos = new FileOutputStream("car.ser");
        ObjectOutputStream oos = new ObjectOutputStream(fos);
        oos.writeObject(myCar);
        oos.close();
        
        FileInputStream fis = new FileInputStream("car.ser");
        ObjectInputStream ois = new ObjectInputStream(fis);
        myCar = (Car) ois.readObject();
        System.out.println(myCar.getYear());        
        ois.close();
    }
}
 
[B]Output:[/B]
Exception in thread "main" java.io.InvalidClassException: Car; no valid constructor
        at java.io.ObjectStreamClass.<init>(Unknown Source)
        at java.io.ObjectStreamClass.lookup(Unknown Source)
        at java.io.ObjectOutputStream.writeObject0(Unknown Source)
        at java.io.ObjectOutputStream.writeObject(Unknown Source)
        at Test.main(Test.java:31)

Without a no-arg constructor, it'll throw InvalidClassException. Now try uncomment the no-arg constructor and run again. It should run without problem.

More examples on Externalization here.

Joyce
[ July 26, 2005: Message edited by: Joyce Lee ]

[Tony Morris]

Note that a 'no-arg constructor' and a 'default constructor' are not the same thing.

JLS 8.8.7 for more information.