How do you create an asynchronous HTTP request in JAVA?

Question

I'm fairly new to Java, so this may seem obvious to some. I've worked a lot with ActionScript, which is very much event based and I love that. I recently tried to write a small bit of Java code that does a POST request, but I've been faced with the problem that it's a synchronous request, so the code execution waits for the request to complete, time out or present an error.

How can I create an asynchronous request, where the code continues the execution and a callback is invoked when the HTTP request is complete? I've glanced at threads, but I'm thinking it's overkill.

Answer 1

If you are in a JEE7 environment, you must have a decent implementation of JAXRS hanging around, which would allow you to easily make asynchronous HTTP request using its client API.

This would looks like this:

public class Main {      public static Future<Response> getAsyncHttp(final String url) {         return ClientBuilder.newClient().target(url).request().async().get();     }      public static void main(String ...args) throws InterruptedException, ExecutionException {         Future<Response> response = getAsyncHttp("http://www.nofrag.com");         while (!response.isDone()) {             System.out.println("Still waiting...");             Thread.sleep(10);         }         System.out.println(response.get().readEntity(String.class));     } } 

Of course, this is just using futures. If you are OK with using some more libraries, you could take a look at RxJava, the code would then look like:

public static void main(String... args) {     final String url = "http://www.nofrag.com";     rx.Observable.from(ClientBuilder.newClient().target(url).request().async().get(String.class), Schedulers             .newThread())             .subscribe(                     next -> System.out.println(next),                     error -> System.err.println(error),                     () -> System.out.println("Stream ended.")             );     System.out.println("Async proof"); } 

And last but not least, if you want to reuse your async call, you might want to take a look at Hystrix, which - in addition to a bazillion super cool other stuff - would allow you to write something like this:

For example:

public class AsyncGetCommand extends HystrixCommand<String> {      private final String url;      public AsyncGetCommand(final String url) {         super(Setter.withGroupKey(HystrixCommandGroupKey.Factory.asKey("HTTP"))                 .andCommandPropertiesDefaults(HystrixCommandProperties.Setter()                         .withExecutionIsolationThreadTimeoutInMilliseconds(5000)));         this.url = url;     }      @Override     protected String run() throws Exception {         return ClientBuilder.newClient().target(url).request().get(String.class);     }   } 

Calling this command would look like:

public static void main(String ...args) {     new AsyncGetCommand("http://www.nofrag.com").observe().subscribe(             next -> System.out.println(next),             error -> System.err.println(error),             () -> System.out.println("Stream ended.")     );     System.out.println("Async proof"); } 

PS: I know the thread is old, but it felt wrong that no ones mentions the Rx/Hystrix way in the up-voted answers.

Answer 2

You may also want to look at Async Http Client.