Serial in Java

java.io.Serial

Indicates that an annotated field or method is part of the serialization mechanism defined by the Java Object Serialization Specification. This annotation type is intended to allow compile-time checking of serialization-related declarations, analogous to the checking enabled by the java.lang.Override annotation type to validate method overriding.

Serializable classes are encouraged to use @Serial annotations to help a compiler catch mis-declared serialization-related fields and methods,mis-declarations that may otherwise be difficult to detect.

Specifically, annotations of this type should be applied to serialization-related methods and fields in classes declared to be Serializable.

This annotation is present in the java.io package and in java.base module.

Declaration

@Target({ElementType.METHOD, ElementType.FIELD})
@Retention(RetentionPolicy.SOURCE)
public @interface Serial {}

The five serialization-related methods are:

1. private void writeObject(java.io.ObjectOutputStream stream) throws IOException

2. private void readObject(java.io.ObjectInputStream stream) throws IOException, ClassNotFoundException

3. private void readObjectNoData() throws ObjectStreamException.

4. ANY-ACCESS-MODIFIER Object writeReplace() throws ObjectStreamException

5. ANY-ACCESS-MODIFIER Object readResolve() throws  ObjectStreamException.

The two serialization-related fields are:

1. private static final ObjectStreamField[] serialPersistentFields

2. private static final long serialVersionUID.

Compilers are encouraged to validate that a method or field marked with a @Serial annotation is one of the defined serialization-related methods or fields declared in a meaningful context and issue a warning if that is not the case.

It is a semantic error to apply this  annotation to other fields or methods, including:

1. fields or methods in a class that is not Serializable.

2. fields or methods of the proper structural declaration, but in the type where they are ineffectual. For example, enum types are defined to have a serialVersionUID of 0L so a serialVersionUID field declared in an enum type is ignored. The five serialization-related methods identified above are likewise ignored for an enum type.

3. in a class that is  Externalizable:

a) method declarations of writeObject, readObject, and readObjectNoData.

b) a field declaration for serialPersistentFields.

While the Externalizable interface extends Serializable, the three methods and one field above are not used for externalizable classes.

Note that the serialization mechanism accesses its designated fields and methods reflectively and those fields and methods may appear otherwise unused in a Serializable class.