Building a simple RESTful API with Spark

Disclaimer: This post is about the Java micro web framework named Spark and not about the data processing engine Apache Spark.

In this blog post we will see how Spark can be used to build a simple web service. As mentioned in the disclaimer, Spark is a micro web framework for Java inspired by the Ruby framework Sinatra. Spark aims for simplicity and provides only a minimal set of features. However, it provides everything needed to build a web application in a few lines of Java code.

Getting started with Spark

Let's assume we have a simple domain class with a few properties and a service that provides some basic CRUD functionality:

public class User {

  private String id;
  private String name;
  private String email;
  
  // getter/setter
}

public class UserService {

  // returns a list of all users
  public List<User> getAllUsers() { .. }
  
  // returns a single user by id
  public User getUser(String id) { .. }

  // creates a new user
  public User createUser(String name, String email) { .. }

  // updates an existing user
  public User updateUser(String id, String name, String email) { .. }
}

We now want to expose the functionality of UserService as a RESTful API (For simplicity we will skip the hypermedia part of REST ;-)). For accessing, creating and updating user objects we want to use following URL patterns:

The returned data should be in JSON format.

To get started with Spark we need the following Maven dependencies:

<dependency>
  <groupId>com.sparkjava</groupId>
  <artifactId>spark-core</artifactId>
  <version>2.0.0</version>
</dependency>
<dependency>
  <groupId>org.slf4j</groupId>
  <artifactId>slf4j-simple</artifactId>
  <version>1.7.7</version>
</dependency>

Spark uses SLF4J for logging, so we need to a SLF4J binder to see log and error messages. In this example we use the slf4j-simple dependency for this purpose. However, you can also use Log4j or any other binder you like. Having slf4j-simple in the classpath is enough to see log output in the console. We will also use GSON for generating JSON output and JUnit to write a simple integration tests. You can find these dependencies in the complete pom.xml.

Returning all users

Now it is time to create a class that is responsible for handling incoming requests. We start by implementing the GET /users request that should return a list of all users.

import static spark.Spark.*;

public class UserController {

  public UserController(final UserService userService) {
    
    get("/users", new Route() {
      @Override
      public Object handle(Request request, Response response) {
        // process request
        return userService.getAllUsers();
      }
    });
    
    // more routes
  }
}

Note the static import of spark.Spark.* in the first line. This gives us access to various static methods including get(), post(), put() and more. Within the constructor the get() method is used to register a Route that listens for GET requests on /users. A Route is responsible for processing requests. Whenever a GET /users request is made, the handle() method will be called. Inside handle() we return an object that should be sent to the client (in this case a list of all users).

Spark highly benefits from Java 8 Lambda expressions. Route is a functional interface (it contains only one method), so we can implement it using a Java 8 Lambda expression. Using a Lambda expression the Route definition from above looks like this:

get("/users", (req, res) -> userService.getAllUsers());

To start the application we have to create a simple main() method. Inside main() we create an instance of our service and pass it to our newly created UserController:

public class Main {
  public static void main(String[] args) {
    new UserController(new UserService());
  }
}

If we now run main(), Spark will start an embedded Jetty server that listens on Port 4567. We can test our first route by initiating a GET http://localhost:4567/users request. 
In case the service returns a list with two user objects the response body might look like this:

[com.mscharhag.sparkdemo.User@449c23fd, com.mscharhag.sparkdemo.User@437b26fe]

Obviously this is not the response we want.

Spark uses an interface called ResponseTransformer to convert objects returned by routes to an actual HTTP response. ReponseTransformer looks like this:

public interface ResponseTransformer {
  String render(Object model) throws Exception;
}

ResponseTransformer has a single method that takes an object and returns a String representation of this object. The default implementation of ResponseTransformer simply calls toString() on the passed object (which creates output like shown above).

Since we want to return JSON we have to create a ResponseTransformer that converts the passed objects to JSON. We use a small JsonUtil class with two static methods for this:

public class JsonUtil {

  public static String toJson(Object object) {
    return new Gson().toJson(object);
  }

  public static ResponseTransformer json() {
    return JsonUtil::toJson;
  }
}

toJson() is an universal method that converts an object to JSON using GSON. The second method makes use of Java 8 method references to return a ResponseTransformer instance. ResponseTransformer is again a functional interface, so it can be satisfied by providing an appropriate method implementation (toJson()). So whenever we call json() we get a new ResponseTransformer that makes use of our toJson() method.

In our UserController we can pass a ResponseTransformer as a third argument to Spark's get() method:

import static com.mscharhag.sparkdemo.JsonUtil.*;

public class UserController {
  
  public UserController(final UserService userService) {
    
    get("/users", (req, res) -> userService.getAllUsers(), json());
    
    ...
  }
}

Note again the static import of JsonUtil.* in the first line. This gives us the option to create a new ResponseTransformer by simply calling json().
Our response looks now like this:

[{
  "id": "1866d959-4a52-4409-afc8-4f09896f38b2",
  "name": "john",
  "email": "john@foobar.com"
},{
  "id": "90d965ad-5bdf-455d-9808-c38b72a5181a",
  "name": "anna",
  "email": "anna@foobar.com"
}]

We still have a small problem. The response is returned with the wrong Content-Type. To fix this, we can register a Filter that sets the JSON Content-Type:

after((req, res) -> {
  res.type("application/json");
});

Filter is again a functional interface and can therefore be implemented by a short Lambda expression. After a request is handled by our Route, the filter changes the Content-Type of every response to application/json. We can also use before() instead of after() to register a filter. Then, the Filter would be called before the request is processed by the Route.

The GET /users request should be working now :-)

Returning a specific user

To return a specific user we simply create a new route in our UserController:

get("/users/:id", (req, res) -> {
  String id = req.params(":id");
  User user = userService.getUser(id);
  if (user != null) {
    return user;
  }
  res.status(400);
  return new ResponseError("No user with id '%s' found", id);
}, json());

With req.params(":id") we can obtain the :id path parameter from the URL. We pass this parameter to our service to get the corresponding user object. We assume the service returns null if no user with the passed id is found. In this case, we change the HTTP status code to 400 (Bad Request) and return an error object.

ResponseError is a small helper class we use to convert error messages and exceptions to JSON. It looks like this:

public class ResponseError {
  private String message;

  public ResponseError(String message, String... args) {
    this.message = String.format(message, args);
  }

  public ResponseError(Exception e) {
    this.message = e.getMessage();
  }

  public String getMessage() {
    return this.message;
  }
}

We are now able to query for a single user with a request like this:
GET /users/5f45a4ff-35a7-47e8-b731-4339c84962be
If an user with this id exists we will get a response that looks somehow like this:

{
  "id": "5f45a4ff-35a7-47e8-b731-4339c84962be",
  "name": "john",
  "email": "john@foobar.com"
}

If we use an invalid user id, a ResponseError object will be created and converted to JSON. In this case the response looks like this:

{
  "message": "No user with id 'foo' found"
}

Creating and updating user with Spark

Creating and updating users is again very easy. Like returning the list of all users it is done using a single service call:

post("/users", (req, res) -> userService.createUser(
    req.queryParams("name"),
    req.queryParams("email")
), json());

put("/users/:id", (req, res) -> userService.updateUser(
    req.params(":id"),
    req.queryParams("name"),
    req.queryParams("email")
), json());

To register a route for HTTP POST or PUT requests we simply use the static post() and put() methods of Spark. Inside a Route we can access HTTP POST parameters using req.queryParams().
For simplicity reasons (and to show another Spark feature) we do not do any validation inside the routes. Instead we assume that the service will throw an IllegalArgumentException if we pass in invalid values.

Spark gives us the option to register ExceptionHandlers. An ExceptionHandler will be called if an Exception is thrown while processing a route. ExceptionHandler is another single method interface we can implement using a Java 8 Lambda expression:

exception(IllegalArgumentException.class, (e, req, res) -> {
  res.status(400);
  res.body(toJson(new ResponseError(e)));
});

Here we create an ExceptionHandler that is called if an IllegalArgumentException is thrown. The caught Exception object is passed as the first parameter. We set the response code to 400 and add an error message to the response body.

If the service throws an IllegalArgumentException when the email parameter is empty, we might get a response like this:

{
  "message": "Parameter 'email' cannot be empty"
}

The complete source the controller can be found here.

Testing with Spark

Because of Spark's simple nature it is very easy to write integration tests for our sample application.
Let's start with this basic JUnit test setup:

public class UserControllerIntegrationTest {

  @BeforeClass
  public static void beforeClass() {
    Main.main(null);
  }

  @AfterClass
  public static void afterClass() {
    Spark.stop();
  }
  
  ...
}

In beforeClass() we start our application by simply running the main() method. After all tests finished we call Spark.stop(). This stops the embedded server that runs our application.

After that we can send HTTP requests within test methods and validate that our application returns the correct response. A simple test that sends a request to create a new user can look like this:

@Test
public void aNewUserShouldBeCreated() {
  TestResponse res = request("POST", "/users?name=john&email=john@foobar.com");
  Map<String, String> json = res.json();
  assertEquals(200, res.status);
  assertEquals("john", json.get("name"));
  assertEquals("john@foobar.com", json.get("email"));
  assertNotNull(json.get("id"));
}

request() and TestResponse are two small self made test utilities. request() sends a HTTP request to the passed URL and returns a TestResponse instance. TestResponse is just a small wrapper around some HTTP response data. The source of request() and TestResponse is included in the complete test class found on GitHub.

Conclusion

Compared to other web frameworks Spark provides only a small amount of features. However, it is so simple you can build small web applications within a few minutes (even if you have not used Spark before). If you want to look into Spark you should clearly use Java 8, which reduces the amount of code you have to write a lot.

You can find the complete source of the sample project on GitHub.