The Server Code

The first thing is to add a hashCode() method to the Customer class:

src/main/java/com/restfully/shop/domain/Customer.java

@XmlRootElement(name = "customer")

public class Customer

{

...

   @Override

   public int hashCode()

   {

      int result = id;

      result = 31 * result + (firstName != null

                                  ? firstName.hashCode() : 0);

      result = 31 * result + (lastName != null

                                  ? lastName.hashCode() : 0);

      result = 31 * result + (street != null

                                  ? street.hashCode() : 0);

      result = 31 * result + (city != null ? city.hashCode() : 0);

      result = 31 * result + (state != null ? state.hashCode() : 0);

      result = 31 * result + (zip != null ? zip.hashCode() : 0);

      result = 31 * result + (country != null

                                    ? country.hashCode() : 0);

      return result;

   }

}

This method is used in the CustomerResource class to generate semi-unique ETag header values. While a hash code calculated in this manner isn’t guaranteed to be unique, there is a high probability that it will be. A database application might use an incremented version column to calculate the ETag value.

The CustomerResource class is expanded to support conditional GETs and PUTs. Let’s take a look at the relevant pieces of code:

src/main/java/com/restfully/shop/services/CustomerResource.java

@Path("/customers")

public class CustomerResource

{

...

   @GET

   @Path("{id}")

   @Produces("application/xml")

   public Response getCustomer(@PathParam("id") int id,

                                @Context Request request) {

      Customer cust = customerDB.get(id);

      if (cust == null)

      {

         throw new WebApplicationException(Response.Status.NOT_FOUND);

      }

      if (sent == null) System.out.println("No ETag sent by client");

      EntityTag tag = new EntityTag(Integer.toString(cust.hashCode()));

      CacheControl cc = new CacheControl();

      cc.setMaxAge(5);

The getCustomer() method first starts out by retrieving the current Customer object identified by the id parameter. A current ETag value is created from the hash code of the Customer object. A new Cache-Control header is instantiated as well.

      Response.ResponseBuilder builder =

                   request.evaluatePreconditions(tag);

      if (builder != null) {

         System.out.println(

                   "** revalidation on the server was successful");

         builder.cacheControl(cc);

         return builder.build();

      }

Next, Request.evaluatePreconditions() is called to perform a conditional GET. If the client has sent an If-None-Match header that matches the calculated current ETag, the method returns immediately with an empty response body. In this case, a new Cache-Control header is sent back to refresh the max-age the client will use.

      // Preconditions not met!

      cust.setLastViewed(new Date().toString());

      builder = Response.ok(cust, "application/xml");

      builder.cacheControl(cc);

      builder.tag(tag);

      return builder.build();

   }

}

If no If-None-Match header was sent or the preconditions were not met, the Customer is sent back to the client with an updated Cache-Control header.

   @Path("{id}")

   @PUT

   @Consumes("application/xml")

   public Response updateCustomer(@PathParam("id") int id,

                                   @Context Request request,

                                    Customer update ) {

      Customer cust = customerDB.get(id);

      if (cust == null)

          throw new WebApplicationException(Response.Status.NOT_FOUND);

      EntityTag tag = new EntityTag(Integer.toString(cust.hashCode()));

The updateCustomer() method is responsible for updating a customer. It first starts off by finding the current Customer with the given id. From this queried customer, it generates the up-to-date value of the ETag header.

      Response.ResponseBuilder builder =

                        request.evaluatePreconditions(tag);

      if (builder != null) {

         // Preconditions not met!

         return builder.build();

      }

The current ETag header is compared against any If-Match header sent by the client. If it does match, the update can be performed:

      // Preconditions met, perform update

      cust.setFirstName(update.getFirstName());

      cust.setLastName(update.getLastName());

      cust.setStreet(update.getStreet());

      cust.setState(update.getState());

      cust.setZip(update.getZip());

      cust.setCountry(update.getCountry());

      builder = Response.noContent();

      return builder.build();

   }

}

Finally, the update is performed.

public class CustomerResourceTest

{

   @Test

   public void testCustomerResource() throws Exception

   {

       WebTarget customerTarget =

             client.target("http://localhost:8080/services/customers/1");

       Response response = customerTarget.request().get();

       Assert.assertEquals(200, response.getStatus());

       Customer cust = response.readEntity(Customer.class);

       EntityTag etag = response.getEntityTag();

       response.close();

      System.out.println("Doing a conditional GET with ETag: "

                                                + etag.toString());

      response = customerTarget.request()

                               .header("If-None-Match", etag).get();

      Assert.assertEquals(304, response.getStatus());

      response.close();

      // Update and send a bad etag with conditional PUT

      cust.setCity("Bedford");

      response = customerTarget.request()

              .header("If-Match", "JUNK")

              .put(Entity.xml(cust));

      Assert.assertEquals(412, response.getStatus());

      response.close();

   }

}