Core Java

Ahoy There Callbacks!

Because it’s my bag, I like JavaScript. In fact, I’ve grown to love JavaScritp’s asynchronous callback oriented style of programming. Consequently, when I find myself in a non-JavaScript environment, say, like Java, I tend to miss using callbacks.callback

The good news is that you can emulate asynchronous callbacks in Java. In fact, I did just that recently with a library I’ve dubbed Ahoy!, which is an asynchronous SQS adapter for AWS’s Java SQS library.

For the uninitiated, SQS is a cloud based messaging platform – with SQS you can create queues and put messages onto those queues, which can then be read – later or immediately by some other process or the same exact process. All of this leverages Amazon’s massively redundant architecture to offer extremely high availability in the face of concurrent access.

Asynchronous callbacks in Java can be achieved with two features: anonymous classes (containing one method) and Java’s java.util.concurrent package.

Because Java doesn’t allow you to pass functions (or methods) easily as a parameter, to simulate a callback, you can create an interface that contains one method, which basically mimics a function. In the case of Ahoy, there are two interfaces: MessageSendCallback and MessageReceivedCallback – both have one method: onSend and onReceive respectively. Accordingly, Ahoy!”s primary class, dubbed SQSAdapter exposes two simple methods: send and receive and both take their related callback interface.

The most straightforward callback to understand is the receive method. As you can imagine, receive is intended to handle behavior when a message is received off of a particular queue. Thus, the receive method is defined as follows:

SQSAdapter’s receive method

public void receive(final MessageReceivedCallback callback) {}

The MessageReceivedCallback interface looks like this:

The MessageReceivedCallback interface

public interface MessageReceivedCallback {
    public void onReceive(String messageId, String message);

Note, the onReceive method takes a message id (which is particular to SQS) and the message itself – which in the case of SQS is always a String (that String can hold anything you want, keep in mind: JSON, XML, byte sequence, etc).

Thus, clients of Ahoy! provide the intended behavior for a message when it is received. This behavior could be to write something to a database, generate another message and send it on another queue, you name it.

Now the interesting part is the implementation of Ahoy!’s receive method. To achieve asynchronocity, I employed Java’s java.util.concurrent package, which sadly, seems to be under appreciated.

The receive method’s implementation with callback being invoked

private void receive(final AmazonSQS sqs, final String queueURL, final MessageReceivedCallback callback) {
  pool.execute(new Runnable() {
    public void run() {
      final List<Message> messages = sqs.receiveMessage(
              new ReceiveMessageRequest(queueURL).withMaxNumberOfMessages(10).withWaitTimeSeconds(20)).getMessages();
      if (messages.size() > 0) {
          for (final Message message : messages) {
            callback.onReceive(message.getMessageId(), message.getBody());
            sqs.deleteMessage(new DeleteMessageRequest(queueURL, message.getReceiptHandle()));

With a fixed Thread pool, a thread is created, which waits for messages to arrive on a particular queue; when one shows up, the passed in MessageReceivedCalledback is invoked for each message.

For an example of how this works for clients of Ahoy!, here’s a test case that verifies the execution of the callback:

The receive method implemented

final boolean[] wasReceived = {false};
ahoy.receive(new MessageReceivedCallback() {
  public void onReceive(String messageId, String message) {
    wasReceived[0] = true;
    assertNotNull("message id was null", messageId);
    assertEquals("message wasn't " + origMessage, origMessage, message);

Likewise, sending a message is similar – a new Runnable instance is created, which sends a particular message and invokes the passed in MessageSentCallback’s onSend method, passing in the newly sent messages’s id.

The send method is also asynchronous

private void send(final AmazonSQS sqs, final String queueURL, final String message, final MessageSentCallback callback) {
  pool.execute(new Runnable() {
    public void run() {
      SendMessageResult res = sqs.sendMessage(new SendMessageRequest(queueURL, message));
      if (callback != null) {

Incidentally, the AWS Java SDK does provide an asynchronous client; however, this client’s implementation leverages Java’s Futures. While Futures are a neat concept, Ahoy!’s implementation is more convenient (at least for me and in the patterns of how I employ SQS) than Futures as there isn’t any polling involved once a message is sent or received.

While callbacks aren’t necessarily supported natively in Java, you can emulate them quite nicely and achieve the same level of code conciseness as what’s common in JavaScript. And if you need a handy way to interface with AWS SQS, then give Ahoy! a try! Can you dig it, man?

Reference: Ahoy There Callbacks! from our JCG partner Andrew Glover at the The Disco Blog blog.
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