This section introduces the client API and some features such as filters and how to use them with security features in the JDK. The Jersey client API is a high-level Java based API for interoperating with RESTful Web services. It makes it very easy to interoperate with RESTful Web services and enables a developer to concisely and efficiently implement a reusable client-side solution that leverages existing and well established client-side HTTP implementations.

The Jersey client API can be utilized to interoperate with any RESTful Web service, implemented using one of many frameworks, and is not restricted to services implemented using JAX-RS. However, developers familiar with JAX-RS should find the Jersey client API complementary to their services, especially if the client API is utilized by those services themselves, or to test those services.

The goals of the Jersey client API are threefold:

The Jersey Client API supports a pluggable architecture to enable the use of different underlying HTTP client implementations. Two such implementations are supported and leveraged: the Http(s)URLConnection classes supplied with the JDK; and the Apache HTTP client.

The uniform interface constraint bounds the architecture of RESTful Web services so that a client, such as a browser, can utilize the same interface to communicate with any service. This is a very powerful concept in software engineering that makes Web-based search engines and service mash-ups possible. It induces properties such as:

Further constraints are required:

The above process repeated over and again should be familiar to anyone who has used a browser to fill in HTML forms and follow links. That same process is applicable to non-browser based clients.

Many existing Java-based client APIs, such as the Apache HTTP client API or java.net.HttpURLConnection supplied with the JDK place too much focus on the Client-Server constraint for the exchanges of request and responses rather than a resource, identified by a URI, and the use of a fixed set of HTTP methods.

A resource in the Jersey client API is an instance of the Java class WebResource, and encapsulates a URI. The fixed set of HTTP methods are methods on WebResource or if using the builder pattern (more on this later) are the last methods to be called when invoking an HTTP method on a resource. The representations are Java types, instances of which, may contain links that new instances of WebResource may be created from.

Since a resource is represented as a Java type it makes it easy to configure, pass around and inject in ways that is not so intuitive or possible with other client-side APIs.

The Jersey Client API reuses many aspects of the JAX-RS and the Jersey implementation such as:

Some APIs, like the Apache HTTP client or java.net.HttpURLConnection, can be rather hard to use and/or require too much code to do something relatively simple.

This is why the Jersey Client API provides support for wrapping HttpURLConnection and the Apache HTTP client. Thus it is possible to get the benefits of the established implementations and features while getting the ease of use benefit.

It is not intuitive to send a POST request with form parameters and receive a response as a JAXB object with such an API. For example with the Jersey API this is very easy:

  1 Form f = new Form();
  2 f.add("x", "foo");
  3 f.add("y", "bar");
  4 
  5 Client c = Client.create();
  6 WebResource r = c.resource("http://localhost:8080/form");
  7 
  8 JAXBBean bean = r.
  9     type(MediaType.APPLICATION_FORM_URLENCODED_TYPE)
 10     .accept(MediaType.APPLICATION_JSON_TYPE)
 11     .post(JAXBBean.class, f);

In the above code a Form is created with two parameters, a new WebResource instance is created from a Client then the Form instance is POSTed to the resource, identified with the form media type, and the response is requested as an instance of a JAXB bean with an acceptable media type identifying the Java Script Object Notation (JSON) format. The Jersey client API manages the serialization of the Form instance to produce the request and de-serialization of the response to consume as an instance of a JAXB bean.

If the code above was written using HttpURLConnection then the developer would have to write code to serialize the form sent in the POST request and de-serialize the response to the JAXB bean. In addition further code would have to be written to make it easy to reuse the same resource “http://localhost:8080/form” that is encapsulated in the WebResource type.

Refer to the dependencies chapter, and specifically the Core client section, for details on the dependencies when using the Jersey client with Maven and Ant.

Refer to the Java API documentation for details on the Jersey client API packages and classes.

Refer to the Java API Apache HTTP client documentation for details on how to use the Jersey client API with the Apache HTTP client.

To utilize the client API it is first necessary to create an instance of a Client, for example:

    Client c = Client.create();

    c.getProperties().put(
        ClientConfig.PROPERTY_FOLLOW_REDIRECTS, true);

    c.setFollowRedirects(true);

    ClientConfig cc = new DefaultClientConfig();
    cc.getProperties().put(
        ClientConfig.PROPERTY_FOLLOW_REDIRECTS, true);
    Client c = Client.create(cc);

    WebResource r = c.resource("http://localhost:8080/xyz");

    String response = r.accept(
        MediaType.APPLICATION_JSON_TYPE,
        MediaType.APPLICATION_XML_TYPE).
        header("X-FOO", "BAR").
        get(String.class);

    String request = "content";
    String response = r.accept(
        MediaType.APPLICATION_JSON_TYPE,
        MediaType.APPLICATION_XML_TYPE).
        header("X-FOO", "BAR").
        post(String.class, request);

    String response = r.accept(
         MediaType.APPLICATION_JSON_TYPE,
         MediaType.APPLICATION_XML_TYPE).
         header("X-FOO", "BAR").
         type(MediaType.TEXT_PLAIN_TYPE).
         post(String.class, request);

    String response = r.accept(
         MediaType.APPLICATION_JSON_TYPE,
         MediaType.APPLICATION_XML_TYPE).
         header("X-FOO", "BAR").
         entity(request, MediaType.TEXT_PLAIN_TYPE).
         post(String.class);

    ClientResponse response = r.get(ClientResponse.class);
    EntityTag e = response.getEntityTag();
    String entity = response.getEntity(String.class);

    try {
        String entity = r.get(String.class);
    } catch (UniformInterfaceException ue) {
        ClientResponse response = ue.getResponse();
    }

    WebResource r = c.resource("http://localhost:8080/xyz");

    MultivaluedMap<String, String> params = new MultivaluedMapImpl();
    params.add("foo", "x");
    params.add("bar", "y");
    
    String response = r.path("abc").
        queryParams(params).
        get(String.class);

    InputStream in = r.get(InputStream.class);
    // Read from the stream
    in.close();

    File f = ...
    String response = r.post(String.class, f);

The support for new application-defined representations as Java types requires the implementation of the same provider-based interfaces as for the server side JAX-RS API, namely MessageBodyReader and MessageBodyWriter, respectively, for request and response entities (or inbound and outbound representations). Refer to the entity provider sample for such implementations utilized on the server side.

Classes or implementations of the provider-based interfaces need to be registered with a ClientConfig and passed to the Client for creation. The following registers a provider class MyReader which will be instantiated by Jersey:

    ClientConfig cc = new DefaultClientConfig();
    cc.getClasses().add(MyReader.class);
    Client c = Client.create(cc);

The following registers an instance or singleton of MyReader:

    ClientConfig cc = new DefaultClientConfig();
    MyReader reader = ...
    cc.getSingletons().add(reader);
    Client c = Client.create(cc);

Filtering requests and responses can provide useful functionality that is hidden from the application layer of building and sending requests, and processing responses. Filters can read/modify the request URI, headers and entity or read/modify the response status, headers and entity.

The Client and WebResource classes extend from Filterable and that enables the addition of ClientFilter instances. A WebResource will inherit filters from its creator, which can be a Client or another WebResource. Additional filters can be added to a WebResource after it has been created. For requests, filters are applied in reverse order, starting with the WebResource filters and then moving to the inherited filters. For responses, filters are applied in order, starting with inherited filters and followed by the filters added to the WebResource. All filters are applied in the order in which they were added. For instance, in the following example the Client has two filters added, filter1 and filter2, in that order, and the WebResource has one filter added, filter3:

    ClientFilter filter1 = ...
    ClientFilter filter2 = ...
    Client c = Client.create();
    c.addFilter(filter1);
    c.addFilter(filter2);
    
    ClientFilter filter3 = ...
    WebResource r = c.resource(...);
    r.addFilter(filter3);

After a request has been built the request is filtered by filter3, filter2 and filter1 in that order. After the response has been received the response is filtered by filter1, filter2 and filter3 in that order, before the response is returned.

Filters are implemented using the “russian doll” stack-based pattern where a filter is responsible for calling the next filter in the ordered list of filters (or the next filter in the “chain” of filters). The basic template for a filter is as follows:

    class AppClientFilter extends ClientFilter {
        public ClientResponse handle(ClientRequest cr) {
            // Modify the request
            ClientRequest mcr = modifyRequest(cr);
            // Call the next filter
            ClientResponse resp = getNext().handle(mcr);
            // Modify the response
            return modifyResponse(resp);
        }
    }

The filter modifies the request (if required) by creating a new ClientRequest or modifying the state of the passed ClientRequest before calling the next filter. The call to the next request will return the response, a ClientResponse. The filter modifies the response (if required) by creating a new ClientResponse or modifying the state of the returned ClientResponse. Then the filter returns the modified response. Filters are re-entrant and may be called by multiple threads performing requests and processing responses.

The Jersey client API was originally developed to aid the testing of the Jersey server-side, primarily to make it easier to write functional tests in conjunction with the JUnit framework for execution and reporting. It is used extensively and there are currently over 1000 tests.

Embedded servers, Grizzly and a special in-memory server, are utilized to deploy the test-based services. Many of the Jersey samples contain tests that utilize the client API to server both for testing and examples of how to use the API. The samples utilize Grizzly or embedded Glassfish to deploy the services.

The following code snippets are presented from the single unit test HelloWorldWebAppTest of the helloworld-webapp sample. The setUp method, called before a test is executed, creates an instance of the Glassfish server, deploys the application, and a WebResource instance that references the base resource:

    @Override
    protected void setUp() throws Exception {
        super.setUp();

        // Start Glassfish
        glassfish = new GlassFish(BASE_URI.getPort());
        
        // Deploy Glassfish referencing the web.xml
        ScatteredWar war = new ScatteredWar(
            BASE_URI.getRawPath(), new File("src/main/webapp"),
            new File("src/main/webapp/WEB-INF/web.xml"),
            Collections.singleton(
                new File("target/classes").
                    toURI().toURL()));
        glassfish.deploy(war);
        
        Client c = Client.create();
        r = c.resource(BASE_URI);
    }

The tearDown method, called after a test is executed, stops the Glassfish server.

    @Override
   protected void tearDown() throws Exception {
        super.tearDown();
        glassfish.stop();
   }

The testHelloWorld method tests that the response to a GET request to the Web resource returns “Hello World”:

    public void testHelloWorld() throws Exception {
       String responseMsg = r.path("helloworld").
           get(String.class);
       assertEquals("Hello World", responseMsg);
   }

Note the use of the path method on the WebResource to build from the base WebResource.