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Frederico Benevides

Joined: Jan 15, 2006
Posts: 25
Hi, I was testing one thing with serializable and I'm having one problem that I don't understand why this happened.

I would like to know why in this code, class A not implementing Serializable, and not having the no args constructor get problem.

If I descomment the no args of the class A it will work, even the constructor of B is using "super(10)"

Or, if I put the class A implemeting Serializable(without descometting the no args of class A), the program will run without problem.

So, I don't understand why happen this problem with the constructor.

Thank you.

Purushoth Thambu
Ranch Hand

Joined: May 24, 2003
Posts: 425
Java specification for Serialization says:

A Serializable class must do the following:

- Implement the interface

- Identify the fields that should be serializable
(Use the serialPersistentFields member to explicitly declare them serializable or use the transient keyword to denote nonserializable fields.)

- Have access to the no-arg constructor of its first nonserializable superclass

When A is not serializable the Class information or metadata is not stored in the serialized stream. While deserializing the object B it need to construct back object A and I believe java must be using reflection to construct A back. If you look at java.lang.Class api you will see that newInstance() creates new instance by calling no-arg constructor. [Java by default defined no-arg constructors if you haven't defined argumented constructors.]

Apart from this I couldn't think why it requires no-arg constructor to be defined in non-serialized super class. If there is other reasons for no-arg constructor requirement in non-serialized supper class it will be great to know.
Joshua Smith
Ranch Hand

Joined: Aug 22, 2005
Posts: 193

Purushothaman is right. When objects are deserialized, their constructors are not run. The exceptions to that rule are if something is a non-serializable object referenced as an instance variable in a serializable class, or if a serializable class has a non-serializable class in its class hierachy. The latter is what we have in your example. B is serializable, but A is not. B's constructor will not be run, but we need a constructor for A.

Since B is serializable, its constructor never gets called and so there isn't an opportunity to pass an argument to A's constructor. To handle instantiating the "A part of B", the JVM needs a no-arg constructor. Since a constructor taking an int is defined, the compiler doesn't provide the no-arg constructor for free so compilation fails. If you change the value passed to A in super (10) so that it doesn't match the default value in A (also 10) and add a no-arg constructor to A, you'll see that B will lose its x value that it inherited from A (it will be returned to its default) if you serialize and deserialize B.

It is possible to serialize and deserialize B and keep the x value inherited from A even if A is not serializable, but you have to override writeObject() (see Serializable) using the defaultWriteObject() and writeInt() methods of ObjectOutputStream and override writeObject() (see Serializable) using the defaultReadObject() and readInt() methods of ObjectInputStream followed by a call to A's single int constructor.

For a good discussion and example of this see Chapter 6 in Kathy and Bert's latest SCJP book.

[ February 11, 2006: Message edited by: Joshua Smith ]

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Frederico Benevides

Joined: Jan 15, 2006
Posts: 25
thank you.
I agree. Here's the link:
subject: serializable