Quarkus - Writing JSON REST Services

JSON is now the lingua franca between microservices.

In this guide, we see how you can get your REST services to consume and produce JSON payloads.

Prerequisites

To complete this guide, you need:

  • less than 15 minutes

  • an IDE

  • JDK 1.8+ installed with JAVA_HOME configured appropriately

  • Apache Maven 3.5.3+

Architecture

The application built in this guide is quite simple: the user can add elements in a list using a form and the list is updated.

All the information between the browser and the server are formatted as JSON.

Solution

We recommend that you follow the instructions in the next sections and create the application step by step. However, you can go right to the completed example.

Clone the Git repository: git clone https://github.com/quarkusio/quarkus-quickstarts.git, or download an archive.

The solution is located in the rest-json directory.

Creating the Maven project

First, we need a new project. Create a new project with the following command:

mvn io.quarkus:quarkus-maven-plugin:0.12.0:create \
    -DprojectGroupId=org.acme \
    -DprojectArtifactId=rest-json \
    -DclassName="org.acme.rest.json.FruitResource" \
    -Dpath="/fruits" \
    -Dextensions="resteasy-jsonb"

This command generates a Maven structure importing the RESTEasy/JAX-RS and JSON-B extensions.

Creating your first JSON REST service

In this example, we will create an application to manage a list of fruits.

First, let’s create the Fruit bean as follows:

package org.acme.rest.json;

import java.util.Objects;

public class Fruit {

    private String name;

    private String description;

    public Fruit() {
    }

    public Fruit(String name, String description) {
        this.name = name;
        this.description = description;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getDescription() {
        return description;
    }

    public void setDescription(String description) {
        this.description = description;
    }

    @Override
    public boolean equals(Object obj) {
        if (!(obj instanceof Fruit)) {
            return false;
        }

        Fruit other = (Fruit) obj;

        return Objects.equals(other.name, this.name);
    }

    @Override
    public int hashCode() {
        return Objects.hash(this.name);
    }
}

Nothing fancy. One important thing to note is that having a default constructor is required by the JSON serialization layer.

Now, edit the org.acme.rest.json.FruitResource class as follows:

package org.acme.rest.json;

import java.util.Collections;
import java.util.LinkedHashMap;
import java.util.Set;

import javax.ws.rs.Consumes;
import javax.ws.rs.DELETE;
import javax.ws.rs.GET;
import javax.ws.rs.POST;
import javax.ws.rs.Path;
import javax.ws.rs.Produces;
import javax.ws.rs.core.MediaType;

@Path("/fruits")
@Produces(MediaType.APPLICATION_JSON)
@Consumes(MediaType.APPLICATION_JSON)
public class FruitResource {

    private Set<Fruit> fruits = Collections.newSetFromMap(Collections.synchronizedMap(new LinkedHashMap<>()));

    public FruitResource() {
        fruits.add(new Fruit("Apple", "Winter fruit"));
        fruits.add(new Fruit("Pineapple", "Tropical fruit"));
    }

    @GET
    public Set<Fruit> list() {
        return fruits;
    }

    @POST
    public Set<Fruit> add(Fruit fruit) {
        fruits.add(fruit);
        return fruits;
    }

    @DELETE
    public Set<Fruit> delete(Fruit fruit) {
        fruits.remove(fruit);
        return fruits;
    }
}

The implementation is pretty straightforward and you just need to define your endpoints using the JAX-RS annotations.

The Fruit objects will be automatically serialized/deserialized by JSON-B.

Note

While RESTEasy supports auto-negotiation, when using Quarkus, it is very important to define the @Produces and @Consumes annotations. They are analyzed at build time and Quarkus restricts the number of JAX-RS providers included in the native executable to the minimum required by the application. It allows to reduce the size of the native executable.

Creating a frontend

Now let’s add a simple web page to interact with our FruitResource. Quarkus automatically serves static resources located under the META-INF/resources directory. In the src/main/resources/META-INF/resources directory, add a fruits.html file with the content from this fruits.html file in it.

You can now interact with your REST service:

Building a native executable

You can build a native executable with the usual command ./mvnw package -Pnative.

Running it is as simple as executing ./target/rest-json-1.0-SNAPSHOT-runner.

You can then point your browser to http://localhost:8080/fruits.html and use your application.

About serialization

The library we use to serialize Java objects to JSON documents is JSON-B. It uses Java reflection to get the properties of an object and serialize them.

Using native executables with GraalVM, all classes that will be used with reflection need to be registered. The good news is that Quarkus does that work for you most of the time. So far, we haven’t registered any class, not even Fruit, for reflection usage and everything is working fine.

Quarkus performs some magic when it is capable of inferring the serialized types from the REST methods. When you have the following REST method, Quarkus determines that Fruit will be serialized:

@GET
@Produces("application/json")
public List<Fruit> list() {
    // ...
}

Quarkus does that for you automatically by analyzing the REST methods at build time and that’s why we didn’t need any reflection registration in the first part of this guide.

Another common pattern in the JAX-RS world is to use the Response object. Response comes with some nice perks:

  • you can return different entity types depending on what happens in your method (a Legume or an Error for instance);

  • you can set the attributes of the Response (the status comes to mind in the case of an error).

Your REST method then looks like this:

@GET
@Produces("application/json")
public Response list() {
    // ...
}

It is not possible for Quarkus to determine at build time the type included in the Response as the information is not available. In this case, Quarkus won’t be able to automatically register for reflection the required classes.

This leads us to our next section.

Using Response

Let’s create the Legume class which will be serialized as JSON, following the same model as for our Fruit class:

package org.acme.rest.json;

import java.util.Objects;

public class Legume {

    private String name;

    private String description;

    public Legume() {
    }

    public Legume(String name, String description) {
        this.name = name;
        this.description = description;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getDescription() {
        return description;
    }

    public void setDescription(String description) {
        this.description = description;
    }

    @Override
    public boolean equals(Object obj) {
        if (!(obj instanceof Legume)) {
            return false;
        }

        Legume other = (Legume) obj;

        return Objects.equals(other.name, this.name);
    }

    @Override
    public int hashCode() {
        return Objects.hash(this.name);
    }
}

Now let’s create a LegumeResource REST service with only one method which returns the list of legumes.

This method returns a Response and not a list of Legume.

package org.acme.rest.json;

import java.util.Collections;
import java.util.LinkedHashMap;
import java.util.Set;

import javax.ws.rs.Consumes;
import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.Produces;
import javax.ws.rs.core.MediaType;
import javax.ws.rs.core.Response;

@Path("/legumes")
@Produces(MediaType.APPLICATION_JSON)
@Consumes(MediaType.APPLICATION_JSON)
public class LegumeResource {

    private Set<Legume> legumes = Collections.newSetFromMap(Collections.synchronizedMap(new LinkedHashMap<>()));

    public LegumeResource() {
        legumes.add(new Legume("Carrot", "Root vegetable, usually orange"));
        legumes.add(new Legume("Zucchini", "Summer squash"));
    }

    @GET
    public Response list() {
        return Response.ok(legumes).build();
    }
}

Now let’s add a simple web page to display our list of legumes. In the src/main/resources/META-INF/resources directory, add a legumes.html file with the content from this legumes.html file in it.

Open a browser to http://localhost:8080/legumes.html and you will see our list of legumes.

The interesting part starts when running the application as a native executable:

  • create the native executable with ./mvnw package -Pnative.

  • execute it with ./target/rest-json-1.0-SNAPSHOT-runner

  • open a browser and go to http://localhost:8080/legumes.html

No legumes there.

As mentioned above, the issue is that Quarkus was not able to determine the Legume class will require some reflection by analyzing the REST endpoints. JSON-B tries to get the list of fields of Legume and gets an empty list so it does not serialize the fields' data.

Note

At the moment, when JSON-B tries to get the list of fields of a class, if the class is not registered for reflection, no exception will be thrown. GraalVM will simply return an empty list of fields.

Hopefully, this will change in the future and make the error more obvious.

We can register Legume for reflection manually by adding the @RegisterForReflection annotation on our Legume class:

@RegisterForReflection
public class Legume {
    // ...
}

Let’s do that and follow the same steps as before:

  • hit Ctrl+C to stop the application

  • create the native executable with ./mvnw package -Pnative.

  • execute it with ./target/rest-json-1.0-SNAPSHOT-runner

  • open a browser and go to http://localhost:8080/legumes.html

This time, you can see our list of legumes.

Conclusion

Creating JSON REST services with Quarkus is easy as it relies on proven and well known technologies.

As usual, Quarkus further simplifies things under the hood when running your application as a native executable.

There is only one thing to remember: if you use Response and Quarkus can’t determine the beans that are serialized, you need to annotate them with @RegisterForReflection.