What is the serialVersionUID for?

serialVersionUID is for tracking the compatibility of serialized versions of classes.

This is because if you serialize an instance of a X class and save it to a file, and some time later change the X class and deserialize that instance, the deserialized data may not be compatible with the new version of the class once it has undergone a change.

And how do you define which version of the class is? With serialVersionUID . That's what it's for.

The idea is that if you are going to create a A serializable class, you declare something like private static final long serialVersionUID = 1L; in it to define that this is the first version of the class. Then, if you make any structural changes incompatible with previous versions, you will change the serialVersionUID to 2L . In another change, it will change to 3L , and so on. Obviously, if the change is small and no incompatibility is introduced, you should not change serialVersionUID .

If you make an incompatible structural change in the class and do not change the serialVersionUID , the deserialization of an incompatible instance will fail.

If you want to be able to read a different version of the class to be deserialized, you can implement the readObject() and there, read% required%. The serialVersionUID method can also be useful.

Finally, if you do not set any readObjectNoData() , then one will automatically be generated for you from a hash taking into account the class name, superclasses, implemented interfaces, and class members (fields, methods, and builders). This means that it is very easy to happen from automatically generated% change% even if there are no significant changes in the class. For this reason, it is recommended that you always explicitly set serialVersionUID .

Additional links for more information:

• Java Object Serialization Specification - from Oracle.

• Understanding the serialVersionUID - by Caelum.

• SOen Question