What is the serialVersionUID in Java classes?

Here is a description of serialVersionUID from Java API DOC. 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;

The serialization mechanism will notice this value and use it instead of computing its own. 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. 

Do not change the value of this field in future versions, unless you are knowingly making changes to the class which will render it incompatible with old serialized objects. Change your Serializable classes may or may not be able to read old serialized objects;  it depends upon the nature of the change (5.6 Type Changes Affecting Serialization in Java Object Serialization Specification).


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