Chapter 46. Creating Resources

RESTful Web services implemented using JAX-RS APIs provide responses as representations of a resource implemented by Java classes. A resource class is a class that uses JAX-RS annotations to implement a resource. For most RESTful Web services, there is a collection of resources that need to be accessed. The resource class' annotations provide information such as the URI of the resources and which HTTP verb each operation handles.

The JAX-RS APIs allow you to create two basic types of resources:

Example 46.1, “Simple resource class” shows a simple resource class.

package demo.jaxrs.server;

import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.PathParam;

@Path("/customerservice")
public class CustomerService
{
  public CustomerService()
  {
  }

  @GET
  public Customer getCustomer(@QueryParam("id") String id)
  {
    ...
  }

  ...
}

Two items make the class defined in Example 46.1, “Simple resource class” a resource class:

The @Path annotation specifies the base URI for the resource.

The @GET annotation specifies that the method implements the HTTP GET method for the resource.

The most basic pieces of information required by a RESTful Web service implementation are:

JAX-RS defines a set of annotations that provide this basic information. All resource classes must have at least one of these annotations.

The @Path annotation specifies the URI of a resource. The annotation is defined by the javax.ws.rs.Path interface and it can be used to decorate either a resource class or a resource method. It takes a string value as its only parameter. The string value is a URI template that specifies the location of an implemented resource.

The URI template specifies a relative location for the resource. As shown in Example 46.2, “URI template syntax”, the template can contain the following:

@Path("resourceName/{param1}/../{paramN}")

For example, the URI template widgets/{color}/{number} would map to widgets/blue/12. The value of the color parameter is assigned to blue. The value of the number parameter is assigned 12.

How the URI template is mapped to a complete URI depends on what the @Path annotation is decorating. If it is placed on a root resource class, the URI template is the root URI of all resources in the tree and it is appended directly to the URI at which the service is published. If the annotation decorates a sub-resource, it is relative to the root resource URI.

JAX-RS uses five annotations for specifying the HTTP verb that will be used for a method:

When you map your methods to HTTP verbs, you must ensure that the mapping makes sense. For example, if you map a method that is intended to submit a purchase order, you would map it to a PUT or a POST. Mapping it to a GET or a DELETE would result in unpredictable behavior.

A root resource class is the entry point into a JAX-RS implemented RESTful Web service. It is decorated with a @Path that specifies the root URI of the resources implemented by the service. Its methods either directly implement operations on the resource or provide access to sub-resources.

In order for a class to be a root resource class it must meet the following criteria:

Example 46.3, “Root resource class” shows a root resource class that provides access to a sub-resource.

package demo.jaxrs.server;

import javax.ws.rs.DELETE;
import javax.ws.rs.GET;
import javax.ws.rs.POST;
import javax.ws.rs.PUT;
import javax.ws.rs.Path;
import javax.ws.rs.PathParam;
import javax.ws.rs.QueryParam;
import javax.ws.rs.core.Response;

@Path("/customerservice/")
public class CustomerService
{
  public CustomerService()
  {
    ...
  }

  @GET
  public Customer getCustomer(@QueryParam("id") String id)
  {
    ...
  }

  @DELETE
  public Response deleteCustomer(@QueryParam("id") String id)
  {
    ...
  }

  @PUT
  public Response updateCustomer(Customer customer)
  {
    ...
  }

  @POST
  public Response addCustomer(Customer customer)
  {
    ...
  }

  @Path("/orders/{orderId}/")
  public Order getOrder(@PathParam("orderId") String orderId)
  {
    ...
  }

}

The class in Example 46.3, “Root resource class” meets all of the requirements for a root resource class.

The class is decorated with the @Path annotation. The root URI for the resources exposed by the service is customerservice.

The class has a public constructor. In this case the no argument constructor is used for simplicity.

The class implements each of the four HTTP verbs for the resource.

The class also provides access to a sub-resource through the getOrder() method. The URI for the sub-resource, as specified using the the @Path annotation, is customerservice/order/id. The sub-resource is implemented by the Order class.

For more information on implementing sub-resources see Section 46.5, “Working with sub-resources”.

Resource methods are annotated using JAX-RS annotations. They have one of the HTTP method annotation specifying the types of requests that the method processes. JAX-RS places several constraints on resource methods.

All resource methods must meet the following conditions:

Resource method parameters take two forms:

Example 46.4, “Resource method with a valid parameter list” shows a resource method with a valid parameter list.

@POST
@Path("disaster/monster/giant/{id}")
public void addDaikaiju(Kaiju kaiju,
                        @PathParam("id") String id)
{
  ...
}

Example 46.5, “Resource method with an invalid parameter list” shows a resource method with an invalid parameter list. It has two parameters that are not annotated.

@POST
@Path("disaster/monster/giant/")
public void addDaikaiju(Kaiju kaiju,
                        String id)
{
  ...
}

Resource methods can return one of the following:

All resource methods return an HTTP status code to the requester. When the return type of the method is void or the value being returned is null, the resource method sets the HTTP status code to 204. When the resource method returns any value other than null, it sets the HTTP status code to 200.

It is likely that a service will need to be handled by more than one resource. For example, in an order processing service best-practices suggests that each customer would be handled as a unique resource. Each order would also be handled as a unique resource.

Using the JAX-RS APIs, you would implement the customer resources and the order resources as sub-resources. A sub-resource is a resource that is accessed through a root resource class. They are defined by adding a @Path annotation to a resource class' method. Sub-resources can be implemented in one of two ways:

Sub-resources are specified by decorating a method with the @Path annotation. The URI of the sub-resource is constructed as follows:

For example the URI of the sub-resource shown in Example 46.6, “Order sub-resource” could be baseURI/customerservice/order/12.

...
@Path("/customerservice/")
public class CustomerService
{
  ...
  @Path("/orders/{orderId}/")
  @GET
  public Order getOrder(@PathParam("orderId") String orderId)
  {
    ...
  }
}

A sub-resource method is decorated with both a @Path annotation and one of the HTTP verb annotations. The sub-resource method is directly responsible for handling a request made on the resource using the specified HTTP verb.

Example 46.7, “Sub-resource methods” shows a resource class with three sub-resource methods:

...
@Path("/customerservice/")
public class CustomerService
{
  ...
  @Path("/orders/{orderId}/")
  @GET
  public Order getOrder(@PathParam("orderId") String orderId)
  {
    ...
  }

  @Path("/orders/{orderId}/")
  @PUT
  public Order updateOrder(@PathParam("orderId") String orderId,
                           Order order)
  {
    ...
  }

  @Path("/orders/")
  @POST
  public Order newOrder(Order order)
  {
    ...
  }
}

Sub-resource locators are not decorated with one of the HTTP verb annotations and do not directly handle are request on the sub-resource. Instead, a sub-resource locator returns an instance of a resource class that can handle the request.

In addition to not having an HTTP verb annotation, sub-resource locators also cannot have any entity parameters. All of the parameters used by a sub-resource locator method must use one of the annotations described in Chapter 47, Passing Information into Resource Classes and Methods.

As shown in Example 46.8, “Sub-resource locator returning a specific class”, sub-resource locator allows you to encapsulate a resource as a reusable class instead of putting all of the methods into one super class. The processOrder() method is a sub-resource locator. When a request is made on a URI matching the URI template /orders/{orderId}/ it returns an instance of the Order class. The Order class has methods that are decorated with HTTP verb annotations. A PUT request is handled by the updateOrder() method.

...
@Path("/customerservice/")
public class CustomerService
{
  ...
  @Path("/orders/{orderId}/")
  public Order processOrder(@PathParam("orderId") String orderId)
  {
    ...
  }

  ...
}

public class Order
{
  ...
  @GET
  public Order getOrder(@PathParam("orderId") String orderId)
  {
    ...
  }

  @PUT
  public Order updateOrder(@PathParam("orderId") String orderId,
                           Order order)
  {
    ...
  }

}

Sub-resource locators are processed at runtime so that they can support polymorphism. The return value of a sub-resource locator can be a generic Object, an abstract class, or the top of a class hierarchy. For example, if your service needed to process both PayPal orders and credit card orders, the processOrder() method’s signature from Example 46.8, “Sub-resource locator returning a specific class” could remain unchanged. You would simply need to implement two classes, ppOrder and ccOder, that extended the Order class. The implementation of processOrder() would instantiate the desired implementation of the sub-resource based on what ever logic is required.

It is possible for a given URI to map to one or more resource methods. For example the URI customerservice/12/ma could match the templates @Path("customerservice/{id}") or @Path("customerservice/{id}/{state}"). JAX-RS specifies a detailed algorithm for matching a resource method to a request. The algorithm compares the normalized URI, the HTTP verb, and the media types of the request and response entities to the annotations on the resource classes.

The JAX-RS selection algorithm is broken down into three stages:

The first two stages of the selection algorithm determine the resource that will handle the request. In some cases the resource is implemented by a resource class. In other cases, it is implemented by one or more sub-resources that use the same URI template. When there are multiple resources that match a request URI, resource classes are preferred over sub-resources.

If more than one resource still matches the request URI after sorting between resource classes and sub-resources, the following criteria are used to select a single resource:

In many cases, selecting a resource that matches the request URI results in a single resource method that can process the request. The method is determined by matching the HTTP verb specified in the request with a resource method’s HTTP verb annotation. In addition to having the appropriate HTTP verb annotation, the selected method must also be able to handle the request entity included in the request and be able to produce the proper type of response specified in the request’s metadata.

When selecting a resource produces multiple methods that can handle a request the following criteria is used to select the resource method that will handle the request:

In some cases, developers have reported the algorithm being somewhat restrictive in the way multiple resource classes are selected. For example, if a given resource class has been matched and if this class has no matching resource method, then the algorithm stops executing. It never checks the remaining matching resource classes.

Apache CXF provides the org.apache.cxf.jaxrs.ext.ResourceComparator interface which can be used to customize how the runtime handles multiple matching resource classes. The ResourceComparator interface, shown in Example 46.9, “Interface for customizing resource selection”, has to methods that need to be implemented. One compares two resource classes and the other compares two resource methods.

package org.apache.cxf.jaxrs.ext;

import org.apache.cxf.jaxrs.model.ClassResourceInfo;
import org.apache.cxf.jaxrs.model.OperationResourceInfo;
import org.apache.cxf.message.Message;

public interface ResourceComparator
{
  int compare(ClassResourceInfo cri1,
              ClassResourceInfo cri2,
              Message message);

  int compare(OperationResourceInfo oper1,
              OperationResourceInfo oper2,
              Message message);
}

Custom implementations select between the two resources as follows:

If 0 is returned then the runtime will proceed with the default selection algorithm

You register a custom ResourceComparator implementation by adding a resourceComparator child to the service’s jaxrs:server element.