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5. Working with Json

§Handling and serving JSON

In Java, Play uses the Jackson JSON library to convert objects to and from JSON. Play’s actions work with the JsonNode type and the framework provides utility methods for conversion in the play.libs.Json API.

§Mapping Java objects to JSON

Jackson allows you to easily convert Java objects to JSON by looking at field names, getters and setters. As an example we’ll use the following simple Java object:

// Note: can use getters/setters as well; here we just use public fields directly.
// if using getters/setters, you can keep the fields `protected` or `private`
public static class Person {
    public String firstName;
    public String lastName;
    public int age;
}

We can parse the JSON representation of the object and create a new Person:

Person person = new Person();
person.firstName = "Foo";
person.lastName = "Bar";
person.age = 30;
JsonNode personJson = Json.toJson(person); // {"firstName": "Foo", "lastName": "Bar", "age": 30}

Similarly, we can write the Person object to a JsonNode:

// parse the JSON as a JsonNode
JsonNode json = Json.parse("{\"firstName\":\"Foo\", \"lastName\":\"Bar\", \"age\":13}");
// read the JsonNode as a Person
Person person = Json.fromJson(json, Person.class);

§Handling a JSON request

A JSON request is an HTTP request using a valid JSON payload as request body. Its Content-Type header must specify the text/json or application/json MIME type.

By default an action uses an any content body parser, which you can use to retrieve the body as JSON (actually as a Jackson JsonNode):

public Result sayHello() {
    JsonNode json = request().body().asJson();
    if(json == null) {
        return badRequest("Expecting Json data");
    } else {
        String name = json.findPath("name").textValue();
        if(name == null) {
            return badRequest("Missing parameter [name]");
        } else {
            return ok("Hello " + name);
        }
    }
}

public Result sayHello() {
    Optional<Person> person = request().body().parseJson(Person.class);
    return person
      .map(p -> ok("Hello, " + p.firstName))
      .orElse(badRequest("Expecting Json data"));
}

Of course it’s way better (and simpler) to specify our own BodyParser to ask Play to parse the content body directly as JSON:

@BodyParser.Of(BodyParser.Json.class)
public Result sayHello() {
    JsonNode json = request().body().asJson();
    String name = json.findPath("name").textValue();
    if (name == null) {
        return badRequest("Missing parameter [name]");
    } else {
        return ok("Hello " + name);
    }
}

Note: This way, a 400 HTTP response will be automatically returned for non JSON requests with Content-type set to application/json.

You can test it with curl from a command line:

curl
  --header "Content-type: application/json"
  --request POST
  --data '{"name": "Guillaume"}'
  http://localhost:9000/sayHello

It replies with:

HTTP/1.1 200 OK
Content-Type: text/plain; charset=utf-8
Content-Length: 15

Hello Guillaume

§Serving a JSON response

In our previous example we handled a JSON request, but replied with a text/plain response. Let’s change that to send back a valid JSON HTTP response:

public Result sayHello() {
    ObjectNode result = Json.newObject();
    result.put("exampleField1", "foobar");
    result.put("exampleField2", "Hello world!");
    return ok(result);
}

Now it replies with:

HTTP/1.1 200 OK
Content-Type: application/json; charset=utf-8

{"exampleField1":"foobar","exampleField2":"Hello world!"}

You can also return a Java object and have it automatically serialized to JSON by the Jackson library:

public Result getPeople() {
    List<Person> people = personDao.findAll();
    return ok(Json.toJson(people));
}

§Advanced usage

Because Play uses Jackson, you can use your own ObjectMapper to create JsonNodes. The documentation for jackson-databind explains how you can further customize JSON conversion process.

If you would like to use Play’s Json APIs (toJson/fromJson) with a customized ObjectMapper, you first need to disable the default ObjectMapper in your application.conf:

play.modules.disabled += "play.core.ObjectMapperModule"

Then you can create a custom ObjectMapper:

public class JavaJsonCustomObjectMapper {

    JavaJsonCustomObjectMapper() {
        ObjectMapper mapper = Json.newDefaultMapper()
                // enable features and customize the object mapper here ...
                .enable(DeserializationFeature.USE_BIG_DECIMAL_FOR_FLOATS)
                .disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);
        // etc.
        Json.setObjectMapper(mapper);
    }

}

and bind it via Guice:

public class JavaJsonCustomObjectMapperModule extends AbstractModule{

    @Override
    protected void configure() {
        bind(JavaJsonCustomObjectMapper.class).asEagerSingleton();
    }

}

Afterwards enable the Module:

play.modules.enabled += "path.to.JavaJsonCustomObjectMapperModule"

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