Testing WebSockets in Play Framework
posted at August 7, 2014 with tags java, play framework, websocket

In a recent job interview, I was asked to implement a fully fledged (that is, including unit and integration tests) Lubang Menggali (Mancala) game, where multiple users are allowed to play in real-time. I went with Play Framework and used WebSockets to implement the real-time server-client communication. (You can find the complete sources here.) Play provides necessary leverage for the integration tests – start the server, connect two browsers to it, click on the buttons to make a move and check the board updates. However, the tricky bit was the implementation of unit tests for WebSocket handlers.


A typical WebSocket handler in Play has the following entry point for the request dispatcher.

public static WebSocket<JsonNode> join() {
    return new WebSocket<JsonNode>() {
        public void onReady(In<JsonNode> in, Out<JsonNode> out) {
            // ...

When a request hits to join(), the function returns a new WebSocket instance, where onReady() will be invoked upon connection establishment. After that, the book keeping of the input and output sockets are delegated to the programmer. Note that the client-server communication is performed in JSON messages in the above code snippet.


In order to mock a client-server WebSocket communication, I need mock WebSocket.In and WebSocket.Out class implementations. For that purpose, I first tried googling alternatives and checked what other people have done previously, but did not find much material. Hence, I came up with the following MockInputWebSocket.

class MockInputWebSocket {

    protected final List<F.Callback<JsonNode>> messageListeners =
            Collections.synchronizedList(new ArrayList<F.Callback<JsonNode>>());

    protected final List<F.Callback0> closeListeners =
            Collections.synchronizedList(new ArrayList<F.Callback0>());

    protected final WebSocket.In<JsonNode> inputSocket = new WebSocket.In<JsonNode>() {

        public void onMessage(F.Callback<JsonNode> callback) { messageListeners.add(callback); }

        public void onClose(F.Callback0 callback) { closeListeners.add(callback); }


    public void write(JsonNode data) throws Throwable {
        for (F.Callback<JsonNode> listener : messageListeners)

    public void close() throws Throwable {
        for (F.Callback0 listener : closeListeners)

    public WebSocket.In<JsonNode> getInputSocket() { return inputSocket; }


Here, I first implement a WebSocket.In object, where message and close event listeners can register themselves to messageListeners and closeListeners list, respectively. Next, in order to pass data to the socket listeners, I wrote my custom write() and close() methods.

I also implemented MockOutputWebSocket similarly:

class MockOutputWebSocket {

    private final static ObjectMapper objectMapper = new ObjectMapper();

    protected final BlockingQueue<JsonNode> messageQueue = new LinkedBlockingQueue<>();

    protected final WebSocket.Out<JsonNode> outputSocket = new WebSocket.Out<JsonNode>() {

        public void write(JsonNode frame) { messageQueue.add(frame); }

        public void close() {
            try { messageQueue.add(objectMapper.readTree("{\"closed\": true}")); }
            // This should not happen.
            catch (IOException e) { throw new RuntimeException(e); }

    public BlockingQueue<JsonNode> getMessageQueue() { return messageQueue; }

    public WebSocket.Out<JsonNode> getOutputSocket() { return outputSocket; }


If we would forget about the custom close message hack (nasty!) in close() method of the implemented WebSocket.Out class, things are self-explanative here as well. That is, when we receive a new message, we push it to messageQueue. The test user will be able to consume the messages written to the mock output socket by polling JsonData from the messageQueue.

Since I come this far, I also implemented an entire WebSocket mock (MockWebSocket) that employs the aforementioned MockInputWebSocket and MockOutputWebSocket as follows.

class MockWebSocket {

    protected final MockInputWebSocket mockInput = new MockInputWebSocket();
    protected final MockOutputWebSocket mockOutput = new MockOutputWebSocket();
    protected final WebSocket<JsonNode> socket;

    MockWebSocket(WebSocket<JsonNode> socket) {
        this.socket = socket;
        socket.onReady(mockInput.getInputSocket(), mockOutput.getOutputSocket());

    public JsonNode read() throws InterruptedException {
        return mockOutput.getMessageQueue().poll(1, TimeUnit.SECONDS);

    public void write(JsonNode data) throws Throwable { mockInput.write(data); }

    public void close() throws Throwable { mockInput.close(); }


MockWebSocket provides the caller an interface such that the two-way WebSocket communication can be intercepted through provided read(), write() and close() methods – much like a regular network socket.


Remember that I used JSON as the messaging medium. For that purpose, I created a couple of event classes (e.g., WaitingForOpponent, ReadyToStart, IllegalMove, etc.) and used Jackson for POJO-JSON (de)serialization. Next, I integrated my brand new MockWebSocket into the unit tests as follows.

private static <T> T readPojo(MockWebSocket socket, Class<T> clazz)
        throws InterruptedException, JsonProcessingException {
    JsonNode data = socket.read();
    try { return objectMapper.convertValue(data, clazz); }
    catch (IllegalArgumentException iae) {
        throw new IllegalArgumentException(
                "Invalid JSON: " + objectMapper.writeValueAsString(data), iae);

private static void writeMove(MockWebSocket socket, Object pit) throws Throwable {

public void testJoin() throws Throwable {
    // Introduce the first user and read the "WaitingForOpponent" message.
    MockWebSocket fstSocket = new MockWebSocket(Application.join());
    WaitingForOpponent fstWfo = readPojo(fstSocket, WaitingForOpponent.class);

    // Introduce the second user and read the."WaitingForOpponent" message.
    MockWebSocket sndSocket = new MockWebSocket(Application.join());
    WaitingForOpponent sndWfo = readPojo(sndSocket, WaitingForOpponent.class);

    // Validate "ReadyToStart" messages.
    ReadyToStart fstRts = readPojo(fstSocket, ReadyToStart.class);
    ReadyToStart sndRts = readPojo(sndSocket, ReadyToStart.class);

    // Let 2nd player make a move, while this is not his turn.
    writeMove(sndSocket, 0);
    IllegalMove im = readPojo(sndSocket, IllegalMove.class);
    assertThat(im.reason).isEqualTo("It is opponent's turn.");

    // Let 1st player make a move to an invalid pit index.
    writeMove(fstSocket, "n/a");
    im = readPojo(fstSocket, IllegalMove.class);
    assertThat(im.reason).matches("^Invalid pit index: .*");

    // Let 1st player make a move with a negative pit index.
    writeMove(fstSocket, -1);
    im = readPojo(fstSocket, IllegalMove.class);
    assertThat(im.reason).matches("^Invalid pit index: .*");

    // ...

I suppose I have got a nearly 99% code coverage of the game engine by using the mock WebSockets. I hope this scheme would help others trying to do a similar testing stuff.