Spring Data Fedora Commons Repository (v. 4.x)

Description

Spring Data Fedora is a proof-of-concept implementation of Spring Data for Fedora. It provides Object Content Mapping (OCM) between annotated Java beans and Fedora resources. It also allows to query the external triplestore using Spring Data Repository API.

Acknowledgements

This project is based heavily on the code from the following projects (including configuration files: Maven, Travis CI, etc.)

• fcrepo4-client (deprecated)

• fcrepo-java-client

• spring-data-solr

FedoraTemplate

FedoraTemplate is the main object responsible for executing CRUD and query operations against Fedora backend.

This is an example of configuring FedoraTemplate using Spring Java configuration. It just requires a hostname and a port of Fedora installation.

@Bean
public FedoraTemplate fedoraTemplate() throws FedoraException {
	return new FedoraTemplate(/* config omitted */);
}

Once the template is initialized one can use it to store and retrieve a bean from the Fedora repository. For example, if a bean is declared as following

@FedoraObject(namespace = "vehicle")
public class Vehicle {

    @Path
    private long id;

    @Property
    private String make;

    @Property
    private String color;

    @Property
    private int miles;

    @Property
    private float consumption;

	// constructors/getters/setters are omitted
}

This is how using FedoraTemplate for basic CRUD operations may look like.

Vehicle vehicle = new Vehicle(1L, "Ford", "Green", 15000, 6.5f);
fedoraTemplate.save(vehicle);

Vehicle anotherVehicle = fedoraTemplate.load(1L, Vehicle.class);

Assuming the Fedora instance is running under http://localhost:8080/fcrepo/rest, a set of corresponding resources will be created at the backend with the corresponding RDF properties (shown here as decoded RDF graph for clarity).

<http://localhost:8080/fcrepo/rest/vehicle/1>
	<!-- default resource properties are omitted -->
	<info:fedora/test/color>        "Green"^^<http://www.w3.org/2001/XMLSchema#string> ;
	<info:fedora/test/consumption>  "6.5"^^<http://www.w3.org/2001/XMLSchema#float> ;
	<info:fedora/test/make>         "Ford"^^<http://www.w3.org/2001/XMLSchema#string> ;
	<info:fedora/test/miles>        "15000"^^<http://www.w3.org/2001/XMLSchema#int> .

Spring Data Repository

This module implements Spring Data repository abstraction, in order to facilitate queries against the triplestore. This is how to create a Spring Data enabled repository.

// declare the interface

public interface VehicleCrudRepository extends FedoraCrudRepository<Vehicle, Long> {

    List<Vehicle> findByMake(String make);

    List<Vehicle> findByMilesGreaterThan(int miles);

    Page<Vehicle> findByMilesGreaterThan(int miles, Pageable pageable);

    // other useful queries
}

// use EnableFedoraRepositories annotation to scan the package for FedoraRepository interfaces

@Configuration
@PropertySource("classpath:fcrepo4.properties")
@EnableFedoraRepositories
public static class AppConfig {
	@Bean
    public FedoraTemplate fedoraTemplate() throws FedoraException {
    	return new FedoraTemplate(/* config omitted */);
    }
}

// use the the Autowired instance of VehicleCrudRepository like this

List<Vehicle> vehiclesWithLargeMileage = vehicleRepo.findByMilesGreaterThan(15000);

The module will then issue a SPARQL query (approximately) as follows

SELECT  ?ch_unil_fcrepo4_spring_data_repository_Vehicle
WHERE
  { ?ch_unil_fcrepo4_spring_data_repository_Vehicle <info:fedora/test/miles> ?ch_unil_fcrepo4_spring_data_repository_Vehicle_miles
	FILTER ( ?ch_unil_fcrepo4_spring_data_repository_Vehicle_miles > "1000"^^<http://www.w3.org/2001/XMLSchema#int> )
  }

Datastreams

A relationship between object and a (binary) datastream can be declared as follows

class VehiclePicture {

    @Binary(mimetype="image/jpg")
    private InputStream picture;
}

@FedoraObject
class Vehicle {

    @Datastream
    private VehiclePicture picture;

}

A datastream property must be annotated with Datastream annotation. The java.io.InputStream property providing the actual binary content of the datastream must be annotated with Binary annotation. The datastreams will be persisted as direct children of their parent FedoraObject bean, using the name specified with Datastream annotation or the name of the datastream property (by default) as a the path suffix.

All datastreams are lazy-loaded as dynamic proxies generated using Byte Buddy, so datastream content is only requested only when the client code actually tries to access either content or properties of the datastream.

Resource properties

Some of the default resource properties automatically generated and updated by the Fedora repository can be accessed by specifying bean properties with the corresponding annotations. For example, if one wants to access http://fedora.info/definitions/v4/repository#created properties of the persisted resource, then the corresponding attribute of the bean should be annotated as following:

@FedoraObject
class Vehicle {
    @Created
    private ZonedDateTime createdDate;
}

The module will automatically perform some useful conversions, i.e. to ZonedDateTime from the created timestamp if needed.

Java to RDF type conversion

To serialize values of Java properties as RDF properties uses in SPARQL updates and queries this module uses TypeMapper provided by jena-core module and also used by Fedora repository. See ExtendedXsdDatatypeConverter for implementation details.