JAX-RS - Writing a Context Provider by implementing ContextResolver

Context providers supply a context object to resource classes or to other providers.

A context provider class must implement the ContextResolver<T>interface.

The implementation should be annotated with @Provider for automatic discovery or we can programmatically register it too.

Context provider implementations may restrict the media types they support using the @Produces annotation. The absence of this annotation implies that any media type is supported.

On the usage side (a resource class/method or another provider), we have to inject Providers with @Context annotation to access our context.

Let's see how to write and use a context provider.

Example

In this very simple example, we are focusing on how to use context provider feature rather than implementing a meaningful context object.

Writing a Context Provider

public interface MyContext<T> {
  T get(String key);
}

Now we are going to implement ContextProvider interface which requires only one method to implement: getContext(Class<?> type):

@Provider
public class MyContextResolver implements
        ContextResolver<MyContext> {
    private MyContext context = new MyContextImpl();

    @Override
    public MyContext getContext(Class<?> type) {
        if (type == String.class) {
            return context;
        }
        return null;
    }

    private static class MyContextImpl implements MyContext<String> {
        @Override
        public String get(String key) {
            return "a context value for key=" + key;
        }
    }
}

Writing a Resource

@Path("/")
public class MyResource {

    @GET
    @Path("{path}")
    public String create(@PathParam("path") String path,
                         @Context Providers providers) {
        ContextResolver<MyContext> cr = providers
                .getContextResolver(MyContext.class, MediaType.WILDCARD_TYPE);
        MyContext<String> c = cr.getContext(String.class);
        String r = c.get(path);
        return "response:  " + r;
    }
}

mvn tomcat7:run-war

Writing a JAX-RS client

public class JaxRsClient {

  public static void main(String[] args) {
      Client client = ClientBuilder.newClient();
      WebTarget baseTarget = client.target("http://localhost:8080/example");
      String response = baseTarget.request()
                                  .get(String.class);
      System.out.println(response);
  }
}

Output

response:  a test context value for key=example

In the next example, we will see a more practical example. We will also learn how to access a ContextResolver in another provider (a class annotated with @Provider).

Example Project

Dependencies and Technologies Used:

• jersey-server 2.25.1: Jersey core server implementation.
• jersey-container-servlet 2.25.1: Jersey core Servlet 3.x implementation.
• JDK 1.8
• Maven 3.3.9