In this example we created an async servlet that adds contacts to the list, parse requests and process form
validation with the help of HttpDecoder. You can find full example sources on
Here we will consider only HttpDecoderExample class with AsyncServlet as it contains
Consider this example as a concise presentation of MVC pattern:
To model a Contact representation, we will create a plain java class with fields (name, age, address), constructor and accessors to the fields.
To simplify example, we will use an ArrayList to store the Contact objects. ContactDAO interface and its implementation are used for this purpose.
To build a view we will use a single html file, compiled with the help of the Mustache template engine.
An AsyncServlet will be used as a controller. We will also add RoutingServlet to determine respond to a particular endpoint.
HttpDecoder provides you with tools for parsing requests.
Creating HttpDecoderExample Class
Let’s create HttpDecoderExample class which extends HttpServerLauncher. HttpServerLauncher will take care
of the lifecycle of our server, will start and stop needed services. Next, provide two custom parsers which will be used
for validation - ADDRESS_DECODER and CONTACT_DECODER:
Also, we need to create applyTemplate(Mustache mustache, Map<String, Object> scopes) method, which will fill the provided
Mustache template with the given data:
And provide a ContactDAOImpl factory method:
Now we have everything needed to create AsyncServlet to handle requests:
Here we provide an AsyncServlet, which receives HttpRequest from clients, creates HttpResponse depending on route path and sends it.
Inside the RoutingServlet two route paths are defined. First one matches requests to the root route "/" -
it simply displays a contact list.
The second one, "/add" - is an HTTP POST method which adds or dismisses new users. We will process this request parsing with the
help of aforementioned HttpDecoder, using decode(request) method:
Either represents a value of two possible data types (Contact, DecodeErrors). Either is either Left or Right.
We can check if Either contain only Left(Contact) or Right(DecodeErrors) using isLeft and isRight methods.
Finally, write down the main() method which will launch our application:
Running the application
If you want to run the example, you need to clone DataKernel and
import it as a Maven project. Before running the example, build the project (Ctrl + F9 for IntelliJ IDEA).
Then open HttpDecoderExample class, which is located at datakernel -> examples -> tutorials -> decoder
and run its main() method. Open your favourite browser and go to localhost:8080.