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= Exposing JPA Entities Through RESTful Services =
 
= Exposing JPA Entities Through RESTful Services =
 
THIS PAGE IS IN PROGRESS. PLEASE SEE DISCUSSION.
 
  
 
<br> This page describes how to expose JPA entities through a standards-based (JAX-RS/JAXB/JPA) RESTful service. REST is an acronym for REpresentational State Transfer, which is a design idiom that embraces a stateless client-server architecture in which Web services are viewed as resources which can be identified by their URLs.  
 
<br> This page describes how to expose JPA entities through a standards-based (JAX-RS/JAXB/JPA) RESTful service. REST is an acronym for REpresentational State Transfer, which is a design idiom that embraces a stateless client-server architecture in which Web services are viewed as resources which can be identified by their URLs.  

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Elug api package icon.png Key API rest


Exposing JPA Entities Through RESTful Services


This page describes how to expose JPA entities through a standards-based (JAX-RS/JAXB/JPA) RESTful service. REST is an acronym for REpresentational State Transfer, which is a design idiom that embraces a stateless client-server architecture in which Web services are viewed as resources which can be identified by their URLs.

Implementing a JPA-Based JAX-RS Service

To implement a JAX-RS service, extend JPASingleKeyResource or JPACompositeKeyResource depending upon the key type. The following example extends JPASingleKeyResource:

import javax.ejb.LocalBean;
import javax.ejb.Stateless;
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;
import javax.persistence.PersistenceContextType;
import javax.ws.rs.Path;
 
import org.eclipse.persistence.jpa.rest.JPASingleKeyResource;
 
@Stateless
@LocalBean
@Path("/customers")
public class CustomerService extends JPASingleKeyResource<Customer, Long> {
 
    @PersistenceContext(unitName="CustomerService", type=PersistenceContextType.TRANSACTION)
    EntityManager entityManager;
 
    public CustomerService() {
        super(Customer.class);
    }
 
    @Override
    protected EntityManager entityManager() {
        return entityManager;
    }
 
}


How URIs Are Constructed

Data in RESTful services are referenced using URIs. The common parts of the example URIs in this section are:

  • http://www.example.com/customer-app/rest - The first part of the URI is based on how the application is deployed.
  • customers - This part of the path corresponds to the JAX-RS @Path annotation on the RESTful service.

Other aspects of URI construction are discussed in the following sections:

URIs for JPA Entities with Unary Keys

If the JPA entity has a single-part primary key, the primary key is represented as a path parameter in the corresponding URI. This is a common RESTful operation.

@Entity
public class Customer implements Serializable {
 
    @Id
    private long id;
 
}

The URI corresponding to the Customer entity with id == 1 is:

http://www.example.com/customer-app/rest/customers/1

URIs for JPA Entities with Composite Keys

To locate a resource with composite keys, the URI includes the property names from the JPA key class as matrix parameters. The advantage of using matrix parameters is that they may be cached. The same representation is also used if composite keys are represented using an embedded key class.

@Entity
@IdClass(CustomerID.class)
public class Customer implements Serializable {
 
    @Id
    private long id;
 
    @Id
    private String country;
 
}
 
public class CustomerID {
 
    private String country;
    private long id;
 
    public CustomerID() {
        super();
    }
 
    public CustomerID(String country, long id) {
        this.country = country;
        this.id = id;
    }
 
    public String getCountry() {
        return country;
    }
 
    public long getId() {
        return id;
    }
 
}

The URI corresponding to the instance of Customer with id == 1 and country == CA is:

http://www.example.com/customer-app/rest/customers;id=1;country=CA

URIs for Named Read Queries

A named read query call must be mapped to a URI. Below is an example named read query:

@NamedQuery(name = "findCustomerByName",
            query = "SELECT c " +
                    "FROM Customer c " +
                    "WHERE c.firstName = :firstName AND " +
                    "      c.lastName = :lastName")

An example of a URI for retrieving a single result is:

http://www.example.com/customer-app/rest/customers/singleResult/findCustomerByName;firstName=Jane;lastName=Doe

An example of a URI for retrieving a result list is:

http://www.example.com/customer-app/rest/customers/resultList/findCustomerByName;firstName=Jane;lastName=Doe?firstResult=1&maxResults=10

The components of the URI in the previous example are:

  • findCustomersByName - This corresponds to the name of the named query.
  • singleResult or resultList - This portion indicates whether one or many results are returned.
  • ;firstName=Jane;lastName=Doe - These are matrix parameters. The name of the parameter must match exactly the parameter name in the named query.
  • ?firstResult=1&maxResults=10 - These are optional query parameters to specify firstResult and maxResults.

The parameters are used to build the equivalent of the following:

Query query = entityManager.createNamedQuery("findCustomersByCity");
query.setParameter("firstName", "Jane");
query.setParameter("lastName", "Doe");
query.setFirstResult(1);
query.setMaxResults(10);
return query.getResultList();

URIs for Named Update and Delete Queries

A named update and delete query call must be mapped to a URI. Below is an example named update query:

@NamedQuery(name = "updateCustomersByCity",
            query = "UPDATE Customer c " +
                    "SET c.address.city = :newCity " +
                    "WHERE c.address.city = :oldCity")

The URL for executing that query is:

http://www.example.com/customer-app/rest/customers/execute/updateCustomersByCity;oldCity=Nepean;newCity=Ottawa

The components of the URI in the previous example are:

  • updateCustomersByCity - This corresponds to the name of the named query.
  • execute - This portion indicates the query will be executed.
  • ;oldCity=Nepean;newCity=Ottawa - These are matrix parameters; the name of the parameter must match exactly the parameter name in the named query.

The parameters are used to build the equivalent of the following:

Query query = entityManager.createNamedQuery("updateCustomersByCity");
query.setParameter("oldCity", "Nepean");
query.setParameter("newCity", "Ottawa");
query.executeUpdate();

Using Create, Read, Update, and Delete (CRUD) Operations on RESTful Services

The CRUD operations are described in the following sections:

The examples in these sections use the Jersey client APIs. Jersey is the open source JAX-RS (JSR 311) Reference Implementation for building RESTful Web services. (For more information about Jersey, see http://jersey.java.net/.)

POST - Create Operation

The following example shows a post operation called on a service, using the Jersery client APIs. The XML message is converted to the appropriate object type using JAXB.

Client c = Client.create();
WebResource resource = client.resource("http://www.example.com/customer-app/rest/customers");
ClientResponse response = resource.type("application/xml").post(ClientResponse.class, "<customer>...</customer>");
System.out.println(response);

This call will be received by the following:

@POST
@Consumes({"application/xml", "application/json"})
public Response create(@Context UriInfo uriInfo, EntityType entity) {
    entityManager().persist(entity);
    UriBuilder uriBuilder = pkUriBuilder(uriInfo.getAbsolutePathBuilder(), entity);
    return Response.created(uriBuilder.build()).build();
}

Examples of successful responses to this call are:

  • 200 OK
  • The URI (as described earlier) for the created entity is returned.

Examples of error responses are:

  •  ? (The spec only has this question mark.)

GET - Read Operation

Get is a read-only operation. It is used to query resources. The following is an example of how to invoke a GET call using the Jersey client APIs:

WebResource resource = client.resource("http://www.example.com/customer-app/rest/customers;id=1;country=CA");
ClientResponse response = resource.accept(mimeType).get(ClientResponse.class);

Single key cases that use path parameters and composite key cases that use matrix parameters are handled differently, as shown below:

Single Key - Path Parameter

@GET
@Path("{id}")
@Produces({"application/xml", "application/json"})
public EntityType read(@PathParam("id") KeyType id) {
    return entityManager().find(entityClass, id);
}

Composite Key - Matrix Parameters

@GET
@Produces({"application/xml", "application/json"})
public EntityType read(@Context UriInfo info) {
    return entityManager().find(entityClass, getPrimaryKey(info));
}

Examples of successful responses to this call are:

  • 200 OK - If a result is returned
  • 204 No Content - If no results are returned

Examples of error responses are:

  •  ? (The spec only has this question mark.)

PUT - Update Operation

The put operation updates the underlying resource. When using put the client knows the identity of the resource being updated. The following is an example of how to invoke a PUT call using the Jersey client APIs:

Client c = Client.create();
WebResource resource = client.resource("http://www.example.com/customer-app/rest/customers/1");
ClientResponse response = resource.type("application/xml").put(ClientResponse.class, "<customer>...</customer>");
System.out.println(response);

This call will be received by

@PUT
@Consumes({"application/xml", "application/json"})
public void update(EntityType entity) {
    entityManager().merge(entity);
}

An example of a successful response is:

  • 200 OK

An example of an error response is:

  • 409 Conflict - Locking related exception

DELETE - Delete Operation

The delete operation is used to remove resources. It is not an error to remove a non-existent resource. Below is an example using the Jersey client APIs:

WebResource resource = client.resource("http://www.example.com/customer-app/rest/customers/1");
ClientResponse response = resource.delete(ClientResponse.class);

Single key cases that use path parameters and composite key cases that use matrix parameters are handled differently, as shown below:

Single Key - Path Parameter

@DELETE
@Path("{id}")
public void delete(@PathParam("id") KeyType id) {
    super.delete(id);
}

Composite Key - Matrix Parameters

@DELETE
public void delete(@Context UriInfo info) {
    super.delete(getPrimaryKey(info));
}

An example of a successful response is:

  • 200 OK

An example of an error response is:

  •  ? (The spec only has this question mark.)

Matrix Parameters to Instance of ID Class

You can convert matrix parameters to an ID class, as shown below: (From spec: "(Note the code below is currently using query parameters and needs to be updated):")

private KeyType getPrimaryKey(UriInfo info) {
    try {
        KeyType pk = (KeyType) PrivilegedAccessHelper.newInstanceFromClass(keyClass);
        for(Entry<String, List<String>> entry : info.getQueryParameters().entrySet()) {
            Field pkField = PrivilegedAccessHelper.getField(keyClass, entry.getKey(), true);
            Object pkValue = ConversionManager.getDefaultManager().convertObject(entry.getValue().get(0), pkField.getType());
            PrivilegedAccessHelper.setValueInField(pkField, pk, pkValue);
        }
        return pk;
    } catch(Exception e) {
        throw new RuntimeException(e);
    }
}

Obtain the key class using the the JPA metamodel facility:

keyClass = (Class<KeyType>) entityManager().getMetamodel().entity(entityClass).getIdType().getJavaType();



Configuration Files

No extra configuration files are required for RESTful services. However, you are responsible for providing the required JAX-RS, JPA, and JAXB configuration files:

  • JAX-RS - You must create the JAX-RS deployment artifacts appropriate to your deployment platform.
  • JPA - You must create the necessary artifacts for JPA.
  • JAXB - You must create the necessary artifacts for JAXB:
    • jaxb.properties file to specify JAXB implementation
    • eclipselink-oxm.xml as an alternate metadata representation



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