Loading older versions of an object from a file

Question

What happens if I try to load an older version of an object from a file?

For example if I created an object, then saved this object to a file using a FileOutputStream. Then if I added a field, and attempted to reload this object would Java not recognize the object at all or would it set the fields the field to the default or...?

Thanks :)

Answer 1

You should receive an InvalidClassException when you try to deserialize your class with a different class version. From the javadocs:

The serialization runtime associates with each serializable class a version number, called a serialVersionUID, which is used during deserialization to verify that the sender and receiver of a serialized object have loaded classes for that object that are compatible with respect to serialization. If the receiver has loaded a class for the object that has a different serialVersionUID than that of the corresponding sender's class, then deserialization will result in an InvalidClassException. A serializable class can declare its own serialVersionUID explicitly by declaring a field named "serialVersionUID" that must be static, final, and of type long:

ANY-ACCESS-MODIFIER static final long serialVersionUID = 42L; If a serializable class does not explicitly declare a serialVersionUID, then the serialization runtime will calculate a default serialVersionUID value for that class based on various aspects of the class, as described in the Java(TM) Object Serialization Specification. However, it is strongly recommended that all serializable classes explicitly declare serialVersionUID values, since the default serialVersionUID computation is highly sensitive to class details that may vary depending on compiler implementations, and can thus result in unexpected InvalidClassExceptions during deserialization. Therefore, to guarantee a consistent serialVersionUID value across different java compiler implementations, a serializable class must declare an explicit serialVersionUID value. It is also strongly advised that explicit serialVersionUID declarations use the private modifier where possible, since such declarations apply only to the immediately declaring class--serialVersionUID fields are not useful as inherited members.

Answer 2

You'd most likely get an exception because the serialVersionUID stored in the file does not match the one on the current version of the class.

By default, Java just errors out.

If you want to provide a way to read "old versions", you have to implement the readObject method.

Classes that require special handling during the serialization and deserialization process must implement special methods with these exact signatures:

private void writeObject(java.io.ObjectOutputStream out)
 throws IOException
private void readObject(java.io.ObjectInputStream in)
 throws IOException, ClassNotFoundException;
private void readObjectNoData()
 throws ObjectStreamException;