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A guide on how to consume a REST service in Java: https://openliberty.io/guides/rest-client-java.html
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README.adoc

Consuming a RESTful web service

Note
This repository contains the guide documentation source. To view the guide in published form, view it on the Open Liberty website.

Explore how to access a simple RESTful web service and consume its resources in Java using JSON-B and JSON-P.

What you’ll learn

artists.json

link:finish/src/resources/artists.json[]

You will learn how to access a REST service, serialize a Java object that contains a list of artists and their albums, and use two different approaches to deserialize the returned JSON resources. The first approach consists of using the Java API for JSON Binding (JSON-B) to directly convert JSON messages into Java objects. The second approach consists of using the Java API for JSON Processing (JSON-P) to process the JSON.

The REST service that provides the artists and albums resources has already been written for you and is accessible at the following link when the server is running http://localhost:9080/artists

Which responds with the artists.json

You will implement the following two endpoints using the two deserialization approaches:

  • …​/artists/total to return the total number of artists in the JSON

  • …​/artists/total/<artist> to return the total number of albums in the JSON for the particular artist

If you are interested in learning more about REST services and how you can write them, read Creating a RESTful web service.

Starting the service

This guide is already setup with a general application. As you progress through the guide you will make updates to the code directly, and then push updates to the server so you can see the results.

To start the REST service, run the Maven install and liberty:start-server goals from the start directory:

mvn install liberty:start-server

When the server is running, you can find your service at http://localhost:9080/artists

Creating POJOs

Artist.java

link:finish/src/main/java/io/openliberty/guides/consumingrest/model/Artist.java[]

Album.java

link:finish/src/main/java/io/openliberty/guides/consumingrest/model/Album.java[]

To deserialize a JSON message, start with creating Plain Old Java Objects (POJOs) that represent what is in the JSON and whose instance members map to the keys in the JSON.

For the purpose of this guide, you are given two POJOs. The Artist object has two instance members name and albums, which map to the artist name and the collection of the albums they have written. The Album object represents a single object within the album collection, and contains three instance members title, artistName, and totalTracks, which map to the album title, the artist who wrote the album, and the number of tracks the album contains.

Introducing JSON-B and JSON-P

JSON-B is a feature introduced with Java EE 8 and strengthens Java support for JSON. With JSON-B you directly serialize and deserialize POJOs. This API gives you a variety of options for working with JSON resources.

In contrast, you need to use helper methods with JSON-P to process a JSON response. This tactic is more straightforward, but it can be cumbersome with more complex classes.

JSON-B is built on top of the existing JSON-P API. JSON-B can do everything that JSON-P can do and allows for more customization for serializing and deserializing.

Using JSON-B

Artist.java

link:finish/src/main/java/io/openliberty/guides/consumingrest/model/Artist.java[]

JSON-B requires a POJO to have a public default no-argument constructor for deserialization and binding to work properly.

The JSON-B engine includes a set of default mapping rules, which can be run without any customization annotations or custom configuration. In some instances, you might find it useful to deserialize a JSON message with only certain fields, specific field names, or classes with custom constructors. In these cases, annotations are necessary and recommended:

  • The @JsonbProperty annotation to map JSON keys to class instance members and vice versa. Without the use of this annotation, JSON-B will attempt to do POJO mapping, matching the keys in the JSON to the class instance members by name. JSON-B will attempt to match the JSON key with a Java field or method annotated with @JsonbProperty where the value in the annotation exactly matches the JSON key. If no annotation exists with the given JSON key, JSON-B will attempt to find a matching field with the same name. If no match is found, JSON-B attempts to find a matching getter method for serialization or a matching setter method for de-serialization. A match occurs when the property name of the method matches the JSON key. If no matching getter or setter method is found, serialization or de-serialization, respectively, fails with an exception. The Artist POJO does not require this annotation because all instance members match the JSON keys by name.

  • The @JsonbCreator and @JsonbProperty annotations to annotate a custom constructor. These annotations are required for proper parameter substitution when a custom constructor is used.

  • The @JsonbTransient annotation to define an object property that does not map to a JSON property. While the use of this annotation is good practice, it is only necessary for serialization.

For more information on customization with JSON-B, see the official JSON-B site.

Consuming the REST resource

Artist.java

link:finish/src/main/java/io/openliberty/guides/consumingrest/model/Artist.java[]

Album.java

link:finish/src/main/java/io/openliberty/guides/consumingrest/model/Album.java[]

Consumer.java

link:finish/src/main/java/io/openliberty/guides/consumingrest/Consumer.java[]

The Artist and Album POJOs are ready for deserialization. Next, we’ll learn to consume the JSON response from your REST service.

Create the Consumer class.
src/main/java/io/openliberty/guides/consumingrest/Consumer.java

Processing JSON using JSON-B

Consumer.java

link:finish/src/main/java/io/openliberty/guides/consumingrest/Consumer.java[]

pom.xml

link:finish/pom.xml[]

JSON-B is a Java API that is used to serialize Java objects to JSON messages and vice versa.

Open Liberty’s JSON-B feature on Maven Central includes the JSON-B provider through transitive dependencies. The JSON-B provider has been added as a dependency in your pom.xml

The consumeWithJsonb() method in the Consumer class makes a GET request to the running artist service and retrieves the JSON. To bind the JSON into an Artist array, use the Artist[] entity type in the readEntity call.

Processing JSON using JSON-P

Consumer.java

link:finish/src/main/java/io/openliberty/guides/consumingrest/Consumer.java[]

The consumeWithJsonp() method in the Consumer class makes a GET request to the running artist service and retrieves the JSON. This method then uses the collectArtists and collectAlbums helper methods. These helper methods will parse the JSON and collect its objects into individual POJOs. Notice that you can use the custom constructors to create instances of Artist and Album.

Creating additional REST resources

Consumer.java

link:finish/src/main/java/io/openliberty/guides/consumingrest/Consumer.java[]

ArtistResource.java

link:finish/src/main/java/io/openliberty/guides/consumingrest/service/ArtistResource.java[]

Now that you can consume a JSON resource you can put that data to use.

Replace the ArtistResource class.
src/main/java/io/openliberty/guides/consumingrest/service/ArtistResource.java
  • The getArtists() method provides the raw JSON data service that you accessed at the beginning of this guide.

  • The getJsonString() method uses JSON-B to return the JSON as a string that will be used later for testing.

  • The getTotalAlbums() method uses JSON-B to return the total number of albums present in the JSON for a particular artist. The method returns -1 if this artist does not exist.

  • The getTotalArtists() method uses JSON-P to return the total number of artists present in the JSON.

The methods that you wrote in the Consumer class could be written directly in the ArtistResource class. However, if you are consuming a REST resource from a third party service, you should separate your GET/POST requests from your data consumption.

When the server is running, you can find your service at http://localhost:9080/artists

Testing deserialization

ConsumingRestTest.java

link:finish/src/test/java/it/io/openliberty/guides/consumingrest/ConsumingRestTest.java[]
Create the ConsumingRestTest class.
src/test/java/it/io/openliberty/guides/consumingrest/ConsumingRestTest.java

Maven finds and executes all tests under it/ and each test method must be marked with the @Test annotation.

You can use the @BeforeClass and @AfterClass annotations to perform any one time setup and teardown tasks before and after all of your tests execute, as well as the @Before and @After annotations to do the same but for each individual test case.

Testing the binding process

ConsumingRestTest.java

link:finish/src/test/java/it/io/openliberty/guides/consumingrest/ConsumingRestTest.java[]

pom.xml

link:finish/pom.xml[]

For your test classes to have access to JSON-B, the yasson dependency has been added in your pom.xml.

The testArtistDeserialization test case checks that Artist instances created from the REST data and those that are hardcoded perform the same.

The assertResponse helper method ensures that the response code you receive is valid (200).

Processing with JSON-B test

ConsumingRestTest.java

link:finish/src/test/java/it/io/openliberty/guides/consumingrest/ConsumingRestTest.java[]

The testJsonBAlbumCount and testJsonBAlbumCountForUnknownArtist tests both use the total/{artist} endpoint which invokes JSON-B.

The testJsonBAlbumCount test case checks that deserialization with JSON-B was done correctly and that the correct number of albums is returned for each artist in the JSON.

The testJsonBAlbumCountForUnknownArtist test case is similar to testJsonBAlbumCount but instead checks an artist that does not exist in the JSON and ensures that a value of '-1' is returned.

Processing with JSON-P test

ConsumingRestTest.java

link:finish/src/test/java/it/io/openliberty/guides/consumingrest/ConsumingRestTest.java[]

The testJsonPArtistCount test uses the total endpoint which invokes JSON-P. This test checks that deserialization with JSON-P was done correctly and that the correct number of artists is returned.

-------------------------------------------------------
 T E S T S
-------------------------------------------------------
Running it.io.openliberty.guides.consumingrest.ConsumingRestTest
Tests run: 4, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 1.59 sec - in it.io.openliberty.guides.consumingrest.ConsumingRestTest

Results :

Tests run: 4, Failures: 0, Errors: 0, Skipped: 0

Great work! You’re done!

You have just accessed a simple RESTful web service and consumed its resources using JSON-B and JSON-P in Open Liberty.

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