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@Projection

As explained in Java Persistence with Spring Data and Hibernate epub

@Projection is a Spring Data REST annotation that declares a named projection for selected entity properties and expressions.

@Projection declares a named Spring Data REST view for selected properties of an entity, allowing an exported resource to expose a tailored representation instead of the default domain model. section 16.6 section 16.7

Named user summary

A projection can target User entities and expose the name through getName(). It can also compute an address representation with @Value("#{target.address.toString()}") and getAddress(). Naming it summary enables /users/1?projection=summary. section 16.6

Listing 16.12 The UserProjection interface
Path: Ch16/spring-data-rest-projections/src/main/java/com/manning
 /javapersistence/ch16/model/UserProjection.java
 
@Projection(name = "summary", types = User.class)    #1
public interface UserProjection {
 
   String getName();                                 #2
 
   @Value("#{target.address.toString()}")            #3
   String getAddress();                              #3
}
#1 The projection is named <kbd>summary</kbd>, and it applies to the <kbd>User</kbd> entities.
#2 As the field is called <kbd>name</kbd>, we need to write a <kbd>getName</kbd> method to export it, following the getter names convention.
#3 Export the <kbd>address</kbd> according to how it is displayed by the <kbd>toString</kbd> method. We use the <kbd>@Value</kbd> annotation, containing a SpEL expression. We also need to follow the getter names convention, so the method is called <kbd>getAddress</kbd>.

The complete UserProjection declaration shows the @Projection name and target, getter-based property export, and SpEL expression used for a computed value. section 16.6

Use excerptProjection = UserProjection.class on @RepositoryRestResource when the projection should become the default representation for an entire collection, such as /users/. section 16.6

Practical considerations:

  • Expose only the information required by a particular consumer rather than every entity field. section 16.6
  • Use getter names matching entity properties; use @Value and SpEL when the exported value is computed or formatted. section 16.6
  • Treat projections and excerpts as complementary: a request parameter selects a named view, while an excerpt configures a collection default. section 16.6

How Java Persistence with Spring Data and Hibernate epub covers it

How deep it goes
Defines annotation, getter, SpEL, URL, and repository configuration, but does not detail serialization or query internals.
Examples
Uses a User summary exposing name and a formatted Address, selected through a projection parameter or applied to a users collection.
What it assumes
Assumes Spring Boot, Spring Data REST, entities, repository resources, Java getters, and basic SpEL expressions.
What you can do after
Apply it via Spring Data REST to expose named or collection-default representations containing selected and computed entity values.
How it connects
Connects Spring Data REST with entity properties, Spring Expression Language, REST repositories, and excerpts.
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