Minijax 0.5.7

Hello JSON

This example builds on Hello World and demonstrates:

  • Multiple HTTP verbs such as GET and POST
  • Compound paths
  • Using path parameters
  • Reading and writing JSON content

The source code is available at minijax-examples/minijax-example-json

Estimated reading time: 15 minutes

Lines of code:

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Language                     files          blank        comment           code
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Java                             2             36              0            107
Maven                            1              0              0             31
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SUM:                             3             36              0            138
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pom.xml

As with the Hello World example, let’s start with the Maven pom.xml.

In addition to “minijax-undertow”, we add one new dependency:

<dependency>
    <groupId>org.minijax</groupId>
    <artifactId>minijax-json</artifactId>
    <version>${project.version}</version>
</dependency>

The “minijax-json” dependency includes everything we need for reading and writing JSON content. Behind the scenes, it includes and configures Jackson.

HelloJson.java

HelloJson.java includes all of the Java code for our application.

Key points:

@Path("/widgets")
@Produces(APPLICATION_JSON)
public class HelloJson {

We define a Resource Class called HelloJson at the path “/widgets”.

The @Produces annotation declares that, by default, resource methods produce JSON content.

Next, let’s declare an entity type:

@XmlRootElement
public static class Widget {
    public String id;
    public String value;

    public Widget() {
    }

    public Widget(final String id, final String value) {
        this.id = id;
        this.value = value;
    }
}

The Widget class is a simple POJO (“plain old Java object”). It has two properties: id and value. Getters and setters were omitted for demonstration purposes, but they would normally be a good idea.

We also include a new annotation: @XmlRootElement. This is a JAXB annotation, which defines rules for serializing the Java class to XML or JSON.

While the name includes “XML”, JAXB is the de facto standard for specifying JSON conversion rules. It is supported by major JSON libraries such as Jackson and MOXy.

The @XmlRootElement annotation declares that the Widget class can be used as a root type when serializing and deserializing.

Next let’s declare a data store for widgets:

static final Map<String, Widget> WIDGETS = new HashMap<>();

As you probably discerned, this is a grossly oversimplified implementation. In most real world applications, you would use a database or some persistent storage. For this example, a hashtable will suffice.

Next let’s declare some resource methods:

@GET
public static Collection<Widget> read() {
    return WIDGETS.values();
}

@GET
@Path("/{id}")
public static Widget read(@PathParam("id") final String id) {
    return WIDGETS.get(id);
}

The first method is a simple “get everything” method. Note that we’re returning a java.util.Collection. Jackson has built-in support for Java collection types such as Collection, List, and Map.

The second method demonstrates a few new features.

  • There is a @Path annotation on the method, similar to the @Path annotation on the class. These annotations are combined so the effective path is “/widgets/{id}”.
  • The path includes a curly brace syntax “{id}” which specifies a path parameter.
  • The method takes an argument annotated with @PathParam which connects to the path variable by name “id”

Ok, we have now demonstrated how to “GET” content. How do we create content? One more resource method:

@POST
@Consumes(APPLICATION_JSON)
public static Response create(final Widget widget) {
    WIDGETS.put(widget.id, widget);
    return Response.created(URI.create("/widgets/" + widget.id)).build();
}

Several new features:

  • The @POST annotation declares that this method should be called on HTTP POST requests
  • The @Consumes annotation declares the HTTP Content-Type that the method consumes
  • The single method argument connects to the @Consumes annotation
  • The Response class contains rich functionality for all kinds of HTTP response capabilities

One last thing before testing: We need to register() the JSON feature:

new Minijax()
        .register(JsonFeature.class)
        .register(HelloJson.class)
        .start();

In addition to the setup from “Hello World”, we now include register(JsonFeature.class). That call does the following:

  • Initializes Jackson
  • Adds JSON-enabled MessageBodyReader and MessageBodyWriter
  • Adds a JSON-aware ExceptionMapper for REST endpoints

At this point, it’s time to run the application and start testing it out.

Let’s run the application using Maven:

mvn exec:java -Dexec.mainClass="com.example.HelloJson"

You can view the results in your web browser:

http://localhost:8080/widgets

You should see an empty collection: “[]”

For working with JSON and REST endpoints, the curl command line tool tends to be more useful, so let’s use that.

curl for the empty collection:

$ curl http://localhost:8080/widgets
[]

Create a new widget:

$ curl -d '{"id":"1","value":"Hello"}' http://localhost:8080/widgets

And then confirm that the widget was actually created:

$ curl http://localhost:8080/widgets
[{"id":"1","value":"Hello"}]

Let’s take a step back. When we created the widget, there was not any output. Let’s create another widget and look at the HTTP headers with the “-i” option:

$ curl -i -d '{"id":"2","value":"World"}' http://localhost:8080/widgets
HTTP/1.1 201 Created
Date: Sat, 18 Nov 2017 00:30:40 GMT
Location: /widgets/2
Content-Length: 0

Notice the status is “201 Created”, meaning that the widget was successfully created. It also includes a “Location” header with the URI of the new entity. You may recall that we returned these values with Response.created().

Let’s get the new entity directly:

$ curl http://localhost:8080/widgets/2
{"id":"2","value":"World"}

There is our new entity. Notice that this is a single entity (curly braces, not square braces).

And finally, let’s get the full collection again:

$ curl http://localhost:8080/widgets
[{"id":"1","value":"Hello"},{"id":"2","value":"World"}]

Sure enough, there are the two entities.

HelloJsonTest.java

The unit tests look quite similar to the Hello World unit tests. There are two new features that are worth noting.

First, let’s look at how to “get” a complex type:

final Collection<Widget> widgets = target("/widgets")
        .request()
        .get(new GenericType<Collection<Widget>>() {});

assertNotNull(widgets);
assertEquals(1, widgets.size());

What is this crazy GenericType?

In a normal simple case, if you want to read a non-generic class, you can use the simple syntax:

get(Widget.class)

If you want to read a generic complex type, you might try (and fail) to use this invalid syntax:

get(Collection<Widget>.class)

Unfortunately, due to limitations of the Java language, you cannot do that.

Instead, we use the clever syntax of GenericType<T>. Now the get() method returns a Collection<Widget> as expected.

Second, let’s look at how to “post” content:

final String json = "{\"id\":\"2\",\"value\":\"World\"}";

final Response response = target("/widgets")
        .request()
        .post(Entity.entity(json, APPLICATION_JSON_TYPE));

assertEquals(Status.CREATED.getStatusCode(), response.getStatus());
assertEquals("/widgets/2", response.getLocation().toString());

Declaring the JSON content in a string is pretty straightforward.

Posting the content requires wrapping the JSON in a Entity using Entity.entity(Object, MediaType).

This has poor discoverability, but it is worth committing to memory. As you write tests, you find yourself using the Entity creators quite frequently.

Next