H2 Labz

Use-case:

Book.java:

BookResource.java:

In the above REST method, we have just returned Java object itself to the client as response. If you notice the @Produces, we are returning the JSON contents back to the client.

Stumbled! The magic happens at the marshalling layer. Now we will see how the marshalling code will look like.

BookJsonMarshaller.java:

@Provider
@Produces(MediaType.APPLICATION_JSON)
public class BookJsonMarshaller implements MessageBodyWriter<Book> {

@Override
public long getSize(Book book, Class<?> clazz, Type type, Annotation[] annotations, MediaType mediaType) {
return -1;
}

@Override
public boolean isWriteable(Class<?> clazz, Type type, Annotation[] annotations, MediaType mediaType) {
return clazz == Book.class;
}

@Override
public void writeTo(Book book, Class<?> clazz, Type type, Annotation[] annotations, MediaType mediaType,
MultivaluedMap<String, Object> valueMap, OutputStream stream) throws IOException, WebApplicationException {

JsonObject jsonObject = Json.createObjectBuilder()
.add("title", book.getTitle())
.add("author", book.getAuthor()).build();

DataOutputStream outputStream = new DataOutputStream(stream);
outputStream.writeBytes(jsonObject.toString());
}
}

We have to look 'writeTo' method just to understand the process. The first arguments is coming from the injection layer so the current Java object will be available to this processor. Now we have process the Java object and should generate JSON object. In this example I have used the 'javax.json.jar' for generating the JSON content. Finally the JSON content should be written into output stream.

Finally we have register the provider (BookJsonMarshaller) into our application like other resources...

Before we conclude this article, you may have one question. Why we need to write the custom marshaller for processing the Java object. Is it not possible by default? You question is valid. This is still possible. But if you want to have a full control over the generated JSON content, we have to write our own @Providers...

I hope you have enjoyed reading this article. If you have any questions or comments please reply to this thread... We will meet again with another discussion.

For code reference, don't forget to visit Github

Advance Christmas wishes to my readers!

Background:

Introduction:

Possible ways of REST client:

• CURL
• Advanced REST client (browser plugin)
• Post Man (browser plugin)

http://localhost:8080/restfullab/api

Client code:

• Base resources and sub-resources can be annotated with @Path.
• @Path represents the actual end-point for REST resources.
• Only public methods can be exposed as resource end-points.

• GET
• POST
• PUT
• DELETE
• HEAD and OPTIONS (special cases)

• PathParam

• Extracts the values from the resource path. For example when a resource path is mentioned like, @Path("users/{id}"), the value of id will be passed along with the path. This will be accessed using @PathParam inside the method.

• QueryParam

• Extracts the values passed along with the query string. 

• Example: http://<host_name>:<port>/users?response=json. Here "response" will be treated as a query parameter and will be accessed like @QueryParam inside the method

• FormParam

• Kind of special parameters. When a html form is submitted to the resource, all the form values will be available through @FormParam

• MatrixParam

• Just a key-value pair of parameters available through the resource end-point. This will extracts the path parameters

• HeaderParam

• The values passed through the HTTP headers will be available through @HeaderParam

• Context

• This indicates about the values available from client to server. For example UriInfo and HttpHeaders and some of the servlet information can be accessible through @Context annotations

• CookieParam

• Client cookie related variables can be accessed through @CookieParam annotations

Mentioned by the client what kind of data the server will accept (Exact match of the request content-type will be decided at the run-time based on the priority. We will see this in detail in the coming series)

Based on the @produces value, the server will send the output packets to the client.

• What is RESTful web services?
• How to write RESTful web service using Java

@Path("/hello")
public class HelloWorldResource {
@GET
@Produces(MediaType.TEXT_PLAIN_TYPE)
public String sayHello() {
return "Welcome RESTful Java";
}
}

User user = new User();
user.setId("123");
user.setName("John");
Gson gson = new Gson();
return gson.toJson(user);

{
"id": "123",
"name": "John"
}