Serial (Java SE 22 & JDK 22)
Description: declaration: module: java.base, package: java.io, annotation type: Serial
Keywords: java.io.Serial class

Module java.base
Package java.io

# Annotation Interface Serial

@Target({METHOD,FIELD}) @Retention(SOURCE) public @interface 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 `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`. The five serialization-related methods are:

- `private void writeObject(java.io.ObjectOutputStream stream) throws IOException`
- `private void readObject(java.io.ObjectInputStream stream) throws IOException, ClassNotFoundException`
- `private void readObjectNoData() throws ObjectStreamException`
- ANY-ACCESS-MODIFIER `Object writeReplace() throws ObjectStreamException`
- ANY-ACCESS-MODIFIER `Object readResolve() throws ObjectStreamException`

The two serialization-related fields are:

- `private static final ObjectStreamField[] serialPersistentFields`
- `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:

- fields or methods in a class that is not `Serializable`
- fields or methods of the proper structural declaration, but in a 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.
- in a class that is `Externalizable`:
  - method declarations of `writeObject`, `readObject`, and `readObjectNoData`
  - 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 serialization mechanism accesses its designated fields and methods reflectively and those fields and methods may appear otherwise unused in a `Serializable` class.

Since:
  14
External Specifications
  - Java Object Serialization Specification
See Also:
  - `Serializable`
  - `Externalizable`