NaturalId (Hibernate Javadocs)

```
@Target({METHOD,FIELD})
@Retention(RUNTIME)
public @interface NaturalId
```
Specifies that a field or property of an entity class is part of the natural id of the entity. This annotation is very useful when the primary key of an entity class is a surrogate key, that is, a system-generated synthetic identifier, with no domain-model semantics. There should always be some other field or combination of fields which uniquely identifies an instance of the entity from the point of view of the user of the system. This is the natural id of the entity.
A natural id may be a single field or property of the entity:
```
 @Entity
 @Cache @NaturalIdCache
 class Person {

     //synthetic id
     @GeneratedValue @Id
     Long id;

     @NotNull
     String name;

     //simple natural id
     @NotNull @NaturalId
     String ssn;

     ...
 }
 
```
or it may be a composite value:
```
 @Entity
 @Cache @NaturalIdCache
 class Vehicle {

     //synthetic id
     @GeneratedValue @Id
     Long id;

     //composite natural id

     @Enumerated
     @NotNull @NaturalId
     Region region;

     @NotNull @NaturalId
     String registration;

     ...
 }
 
```
The Session interface offers several methods that allow an entity instance to be retrieved by its simple or composite natural id value. If the entity is also marked for natural id caching, then these methods may be able to avoid a database round trip.
See Also:

NaturalIdCache

- Optional Element Summary
  
  Optional Elements
  Modifier and Type Optional Element Description
  
  boolean mutable
  Specifies whether the natural id is mutable or immutable.

- Element Detail
  - mutable
```
boolean mutable
```
    Specifies whether the natural id is mutable or immutable.
    
    Returns:
    
    false (the default) indicates it is immutable; true indicates it is mutable.
    
    Default:
    
    false