What is @StaticMetamodel and SingularAttribute<Obj,Obj>?

Question

I am trying to figure out this code for about two hours now, for example in below class what are these fields representing?

import java.util.Date; import javax.persistence.metamodel.ListAttribute; import javax.persistence.metamodel.SingularAttribute; import javax.persistence.metamodel.StaticMetamodel; import java.util.UUID;  @StaticMetamodel(Address.class) public class Address_ extends  {     public static volatile SingularAttribute<Address, Long> id;     public static volatile SingularAttribute<Address, UUID> personId;     public static volatile SingularAttribute<Address, Person> person; } 

The Address.class is a java class which has the following definition:

@Entity @Table(name = "address", schema = "public") public class Address{     private Long id;     private Person person;     private UUID personId;     //.... } 

Can you, please, explain what are the @StaticMetamodel and @SingularAttribute annotations used for? It might be simple but I can't understand.

Answer 1

As per documentation:

A static metamodel is a series of classes that "mirror" the entities and embeddables in the domain model and provide static access to the metadata about the mirrored class's attributes.

The static metamodel has the following properties:

• For each managed class X in package p, a metamodel class X_ in package p is created.
• For every persistent non-collection-valued attribute y declared by class X, where the type of y is Y, the metamodel class must contain a declaration as follows:

SingularAttribute example:

public static volatile SingularAttribute<X, Y> y; 

The static metamodel is useful for creating type-safe queries with the JPA's Criteria API.

For example, let's have the following two entities, Order and Item:

@Entity public class Order {     @Id      @GeneratedValue     Integer id;      @ManyToOne      Customer customer;      @OneToMany      Set<Item> items;      BigDecimal totalCost;      // accessors } 

and the Item entity:

@Entity   public class Item {      @Id     @GeneratedValue     Integer id;      int quantity;      @ManyToOne     Order order;      // accessors } 

Here's a typesafe criteria query, build with the Criteria API:

CriteriaBuilder cb = entityManager.getCriteriaBuilder(); CriteriaQuery<Order> cq = cb.createQuery(Order.class); SetJoin<Order, Item> itemNode = cq.from(Order.class).join(Order_.items); cq.where(cb.equal(itemNode.get(Item_.id), 5)).distinct(true); 

Note the usage of Item_.id and Order_.item. Those access statically the static meta-model properties (which mirror the entity properties) and this way it's ensured that the query is build properly.