serialVersionUID

serialVersionUID is a static final long primitive data type in Java. It is used during the serialization and deserialization process. When an object is serialized, its serialVersionUID is written to the serialization stream. When the object is deserialized, the JVM compares the serialVersionUID of the deserialized object with the serialVersionUID of the class definition in the current JVM. If the two serialVersionUID values do not match, a java.io.InvalidClassException is thrown.

The primary purpose of serialVersionUID is to ensure version compatibility between serialized data and the class that reads it. If a class is modified after objects of that class have been serialized, and the serialVersionUID is not explicitly defined, a new serialVersionUID will be generated by the compiler. This new ID will likely differ from the original, leading to an InvalidClassException during deserialization. By explicitly defining a serialVersionUID, developers can maintain backward and forward compatibility. For example, if a class has been modified, and the developer wishes to maintain compatibility with previously serialized objects, they can keep the old serialVersionUID. Conversely, if they want to ensure that only objects serialized with the current version of the class can be deserialized, they can generate a new serialVersionUID.

It is a best practice to explicitly declare a serialVersionUID for all serializable classes. This can be done by adding a private static final long field named serialVersionUID to the class. If not explicitly declared, the serialization mechanism will generate one based on the class name, interface names, methods, and fields. However, this generated ID can change if the class definition is altered in ways that do not affect the serialization compatibility, leading to potential deserialization failures.