The Client API

The Client API is a Java based API used to access Web resources. It is not restricted to resources implemented using this API. It provides a higher-level abstraction compared to a plain HTTP communication API as well as integration with extension providers, in order to enable concise and efficient implementation of reusable client-side solutions that leverage existing and well established client-side implementations of HTTP-based communication.

The Client API encapsulates the Uniform Interface Constraint – a key constraint of the REST architectural style – and associated data elements as client-side Java artifacts and supports a pluggable architecture by defining multiple extension points.

Client API Bootstrapping and Configuration

The main entry point to the API is a ClientBuilder that is used to bootstrap Client instances - configurable, heavy-weight objects that manage the underlying communication infrastructure and serve as the root objects for accessing any Web resource. The following example illustrates the bootstrapping and configuration of a Client instance:

   Client client = ClientBuilder.newClient();

   client.property("MyProperty", "MyValue")
         .register(MyProvider.class)
         .register(MyFeature.class);
 

Accessing Web Resources

A Web resource can be accessed using a fluent API in which method invocations are chained to configure and ultimately submit an HTTP request. The following example gets a text/plain representation of the resource identified by "http://example.org/hello":

   Client client = ClientBuilder.newClient();
   Response res = client.target("http://example.org/hello").request("text/plain").get();
 

Conceptually, the steps required to submit a request are the following:

1. obtain an Client instance
2. create a WebTarget pointing at a Web resource
3. build a request
4. submit a request to directly retrieve a response or get a prepared Invocation for later submission

As illustrated above, individual Web resources are in the Client API represented as resource targets. Each WebTarget instance is bound to a concrete URI, e.g. "http://example.org/messages/123", or a URI template, e.g. "http://example.org/messages/{id}". That way a single target can either point at a particular resource or represent a larger group of resources (that e.g. share a common configuration) from which concrete resources can be later derived:

   // Parent target for all messages
   WebTarget messages = client.target("http://example.org/messages/{id}");

   // New target for http://example.org/messages/123
   WebTarget msg123 = messages.resolveTemplate("id", 123);

   // New target for http://example.org/messages/456
   WebTarget msg456 = messages.resolveTemplate("id", 456);
 

Generic Invocations

An Invocation is a request that has been prepared and is ready for execution. Invocations provide a generic interface that enables a separation of concerns between the creator and the submitter. In particular, the submitter does not need to know how the invocation was prepared, but only whether it should be executed synchronously or asynchronously.

   Invocation inv1 = client.target("http://example.org/atm/balance")
       .queryParam("card", "111122223333").queryParam("pin", "9876")
       .request("text/plain").buildGet();
   Invocation inv2 = client.target("http://example.org/atm/withdrawal")
       .queryParam("card", "111122223333").queryParam("pin", "9876")
       .request().buildPost(text("50.0")));

   Collection invs = Arrays.asList(inv1, inv2);
   // Executed by the submitter
   Collection ress = Collections.transform(invs, new F() {
      public Response apply(Invocation inv) {return inv.invoke(); }
   });