Future in Java - Asynchronous Computation

These days I am doing a bit of Java programming. One of the new Features that I learned recently is Future.

A Future is a representation of the result of an asynchronous computation. A Future has methods to check if the computation has completed and retrieve the result of the task. You can also cancel a computation if you choose to, unless the computation has already completed.

A couple of notable methods in Future

•  isDone() returns a boolean representing if the task is done. This method is used for polling to check if the task is done.
• Get() returns the final result of the Future,it is a blocking method which waits until the computation is   done. This method take a timeout argument in milliseconds. Which specifies the maximum amount of   time we wait for the computation to complete. If the task is not done in the specified time, a  timeout exception will be thrown.
•  Cancel()  will abort the computation.

Let’s see the context in which we can see the usage of Futures.Think of a hypothetical scenario in which we have to fire a http request asynchronously and do some random task while waiting for the response to come.

 The steps involved look like the following    

• Fire http request and get the Future representing the outcome(i.e no waiting for the request to finish).
• Do some random task
• Check if the response is ready. Here we use the get() method to block and retrieve the response.

To demonstrate this with a code I will use an AsyncHttpClient from NING. This Client allows us to send out http requests and receive the response asynchronously. The execute method of this client returns a Future. 

 If you are using maven for your dependency management you can add NING to your project by adding this dependency.

<dependency>
<groupId>com.ning</groupId>
<artifactId>async-http-client</artifactId>
<version>1.8.11</version>
</dependency>

Then your code will look like the following code snipnet . I am using the RequestBuilder utility from the same  library that allow me set the url, headers and other properties corresponding to http request.

RequestBuilder requestBuilder = new com.ning.http.client.RequestBuilder();

requestBuilder.setUrl("http://www.example.com");

AsyncHttpClient client = new AsyncHttpClient();

Future response = asyncHttpClient.executeRequest(requestBuilder.build());

//do some task here while waiting for the response

//check if the task is done.If not done , do some other task

If(!response.isDone()) {

// Do some other task

}

//at this point , block and wait for the response

try {

NettyResponse  value =(NettyResponse) response.get();

}

Catch(TimeoutException ex) {

//Log the exception or bubble it

}

Catch (ExecutionException ex) {

//Log the exception  or bubble it

}

Catch (Exception ex) {

//Log the exception  or bubble it

}