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## Chapter 16. JAX-RS Resource Locators and Sub Resources

Resource classes are able to partially process a request and provide another "sub" resource object that can process the remainder of the request. For example:

    @Path("/")
    public class ShoppingStore {

       @Path("/customers/{id}")
       public Customer getCustomer(@PathParam("id") int id) {
          Customer cust = ...; // Find a customer object
          return cust;
       }
    }

    public class Customer {

        @GET
        public String get() {...}

        @Path("/address")
        public String getAddress() {...}

    }

Resource methods that have a @Path annotation, but no HTTP method are considered sub-resource locators. Their job is to provide an object that can process the request. In the above example ShoppingStore is a root resource because its class is annotated with @Path. The getCustomer() method is a sub-resource locator method.

If the client invoked:

    GET /customer/123

The ShoppingStore.getCustomer() method would be invoked first. This method provides a Customer object that can service the request. The http request will be dispatched to the Customer.get() method. Another example is:

    GET /customer/123/address

In this request, again, first the ShoppingStore.getCustomer() method is invoked. A customer object is returned, and the rest of the request is dispatched to the Customer.getAddress() method.

Another interesting feature of Sub-resource locators is that the locator method result is dynamically processed at runtime to figure out how to dispatch the request. So, the ShoppingStore.getCustomer() method does not have to declare any specific type.

    @Path("/")
    public class ShoppingStore {

       @Path("/customers/{id}")
       public java.lang.Object getCustomer(@PathParam("id") int id) {
          Customer cust = ...; // Find a customer object
          return cust;
       }
    }

    public class Customer {

        @GET
        public String get() {...}

        @Path("/address")
        public String getAddress() {...}

    }

In the above example, getCustomer() returns a java.lang.Object. Per request, at runtime, the JAX-RS server will figure out how to dispatch the request based on the object returned by getCustomer(). What are the uses of this? Well, maybe you have a class hierarchy for your customers. Customer is the abstract base, CorporateCustomer and IndividualCustomer are subclasses. Your getCustomer() method might be doing a Hibernate polymorphic query and doesn't know, or care, what concrete class is it querying for, or what it returns.

    @Path("/")
    public class ShoppingStore {

       @Path("/customers/{id}")
       public java.lang.Object getCustomer(@PathParam("id") int id) {
          Customer cust = entityManager.find(Customer.class, id);
          return cust;
       }
    }

    public class Customer {

        @GET
        public String get() {...}

        @Path("/address")
        public String getAddress() {...}

    }

    public class CorporateCustomer extendsCustomer {

        @Path("/businessAddress")
        public String getAddress() {...}

    }

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