AWS Developer Tools Blog

Asynchronous Requests with the AWS SDK for Java

In addition to the standard, blocking/synchronous clients in the AWS SDK for Java that you’re probably already familiar with, the SDK also contains non-blocking/asynchronous clients that are just as easy to use, and often more convenient for certain types of applications.

When you call an operation with one of the standard, synchronous clients in the SDK, your code is blocked while the SDK sends your request, waits for the service to process it, and parses the response. This is an easy way to work with the SDK, but there are some situations where you just want to kick off the request, and let your code continue executing. The asynchronous clients in the SDK allow you to do exactly that. Kick off your requests, and check back later to see if they completed.

AmazonDynamoDBAsync dynamoDB = new AmazonDynamoDBAsyncClient(myCredentials);
dynamoDB.describeTableAsync(new DeleteTableRequest(myTableName));
// Your code immediately continues executing, while your request runs in the background

Now that you know how to kick off your asynchronous request, how do you handle the response when it arrives? All of the asynchronous operations return a Future object that you can poll to see if your request has completed processing and if a response object is available. But sitting around polling a Future defeats the purpose of freeing up your code to continue executing after you kick off the request.

Usually, what you really want to do is, when the request finishes, execute some code to process the response. The asynchronous operations allow you to pass in an AsyncHandler implementation, which the SDK automatically runs as soon as your request finishes processing.

For example, the following piece of code kicks off an asynchronous request to describe an Amazon DynamoDB table. It passes in an AsyncHandler implementation, and when the request completes, the SDK runs the onSuccess method, which updates a UI label with the table’s status. AsyncHandler also provides an onError method, that allows you to handle any errors that occur while processing your request.

AmazonDynamoDBAsync dynamoDB = new AmazonDynamoDBAsyncClient(myCredentials);
dynamoDB.describeTableAsync(new DescribeTableRequest().withTableName(myTableName), 
    new AsyncHandler<DescribeTableRequest, DescribeTableResult>() {
        public void onSuccess(DescribeTableRequest request, DescribeTableResult result) {
        public void onError(Exception exception) {
            System.out.println("Error describing table: " + exception.getMessage());
            // Callers can also test if exception is an instance of 
            // AmazonServiceException or AmazonClientException and cast 
            // it to get additional information

Using the asynchronous clients in the SDK is easy and convenient. There are a lot of applications where processing requests in the background makes sense. UI applications are a great fit for asynchronous clients, since you don’t want to lock up your main UI thread, and consequently, the entire UI, while the SDK processes a request. Network issues could result in longer processing times, and an unresponsive UI that results in unhappy customers.

Another great use for the asynchronous clients is when you want to kick off a large batch of requests. If the requests don’t need to be executed serially, then you can gain a lot of throughput in your application by using the asynchronous clients to kick off many requests, all from a single thread.

Have you tried the asynchronous clients in the AWS SDK for Java yet? What kinds of applications are you using them for? Let us know how they’re working for you in the comments below.

More information on asynchronous programming with the AWS SDK for Java