Asynchronous HTTP Client for Java

问题

As a relative newbie in the Java world, I am finding many things frustratingly obtuse to accomplish that are relatively trivial in many other frameworks. A primary example is a simple solution for asynchronous http requests. Seeing as one doesn't seem to already exist, what is the best approach? Creating my own threads using a blocking type lib like httpclient or the built-in java http stuff, or should I use the newer non-blocking io java stuff - it seems particularly complex for something which should be simple.

What I am looking for is something easy to use from a developer point of view - something similar to URLLoader in AS3 - where you simply create a URLRequest - attach a bunch of event handlers to handle the completion, errors, progress, etc, and call a method to fire it off.

If you are not familiar with URLLoader in AS3, its so super easy and looks something like this:

private void getURL(String url)
{
    URLLoader loader = new URLLoader();
    loader.addEventListener(Event.Complete, completeHandler);
    loader.addEventListener(HTTPStatusEvent.HTTP_STATUS, httpStatusHandler);
    loader.addEventListener(IOErrorEvent.IO_ERROR, ioErrorHandler);

    URLRequest request = new URLRequest(url);

    // fire it off - this is asynchronous so we handle
    // completion with event handlers
    loader.load(request);
}

private void completeHandler(Event event)
{
    URLLoader loader = (URLLoader)event.target;
    Object results = loader.data;

    // process results
}

private void httpStatusHandler(Event event)
{
    // check status code
}

private void ioErrorHandler(Event event)
{
    // handle errors
}

回答1:

Use the "Async Http Client" formerly called ning http client library. See http://code.ning.com/2010/03/introducing-nings-asynchronous-http-client-library/

Now Available at GitHub https://github.com/ning/async-http-client

回答2:

Version 4.0 of Apache Commons HttpClient (now in HttpComponents/HttpCore) also support Java's NIO (non-blocking IO). I think this is your best bet.

回答3:

The Jetty HTTP client is asynchronous.

回答4:

If you haven't looked at it already, check out the Java 5 java.util.concurrent -- it makes multi-threaded apps much easier to develop. You can set up a ThreadPoolExecutor that manages, say, four Threads. You then feed the pool any number of tasks to complete. Each task is a Runnable. The ThreadPoolExecutor will queue up the Runnable tasks and feed them to available Threads in parallel. The Pool's afterExecute() method is called when each Runnable task completes.

I vividly remember writing a fetch thread pool for a web browser written in Java back in 1999, and it was a bear to get right. Last month I wrote a load tester for a web server. The tester has a ThreadPoolExecutor that has n threads, and the Runnable tasks I feed it each fetch a page using Apache HTTP Client. It took just an hour or two to get it working reasonably well. I think you'll like java.util.concurrent coupled with Apache HTTP Client, though it sounds like you'll need to do some customization for progress indication.

(Note that Apache HTTP Client does its own thread pooling, and the default configuration limits you to 20 threads max, and only two to each web server.)

Update: Here's the link to Apache HTTP Client. Be sure to read up on MultiThreadedHttpConnectionManager, it's what handles the connection pool, and it's not shown in the most basic example.

回答5:

Looks like you want (a part of) NIO -- there's a good tutorial here, the asynchronous networking part starts at p. 30 and there are many useful links at the end.

回答6:

there are libraries and frameworks written on top of NIO/Netty - RxNetty and Vertx which can be helpful to write asynchronous HTTP client

Below is an example code using vertx

public class Client extends AbstractVerticle {

  @Override
  public void start() throws Exception {
    //lambda callback would be called when the response comes back
    vertx.createHttpClient().getNow(8080, "localhost", "/", resp -> {
      System.out.println("Got response " + resp.statusCode());
      resp.bodyHandler(body -> {
        System.out.println("Got data " + body.toString("ISO-8859-1"));
      });
    });
   //this code statement will execute before response comes back
   System.out.println("I am not blocked");
  }
}

you can find the full blown example code from here

回答7:

So, it is probably worth considering that actionscript and Java don't server the same niche. For example, Java does make some things more tedious - but usually that is to give the user more options in how, for example, an HTTP connection is executed, whereas actionscript might abstract details or possible errors away for ease of use. But, your point still stands.

I myself am not aware of an asynchronous HTTP client for Java. Alex Martelli's answer talks about Java's NIO, which is a good answer if you are interested in implementing the HTTP protocol in your own code. NIO will let you use sockets to connect to the web server - but then you have to manually create your own GET requests and parse the incoming HTTP headers/data.

Another option is to use the java.net.URL classes - and you can find many tutorials for those online and on stackoverflow. You can wrap those in threads - so your java program has multiple threads of execution.

But then you run into the problem of synchronization. Which I agree, is a pain, but then it offers a more granular level of flexibility.

(I realize that this doesn't answer your question - and if anybody actually knows of a java facility to do asynchronous http requests, I'd be interested to know!)

回答8:

AFAIK the TCPMon tool takes a similar approach to what you describe. You can take a look at the source code in their SVN browser

Also have a look at WGET-java for the guts of the blocking code.

But do you have to write this in Java? There are a lot of other approaches using JRuby or Rhino to accomplish something like this easily that will run on the JVM but aren't written in Java.

回答9:

httpunit and htmlunit are 2 customizable and configurable Java http clients able to anything a browser such as simulate firefox, headless browsing, scheduled software clients and agents.

回答10:

Take also a look into http://www.javaworld.com/javaworld/jw-03-2008/jw-03-asynchhttp.html This article discusses async HTTP based on a HttpClient named xLightweb

回答11:

I'd recommend firing separate threads for that.

回答12:

I just stumbled upon the asynchronous HTTP client implemented within Geronimo. You might also want to take a look it, at http://svn.apache.org/viewvc/geronimo/sandbox/AsyncHttpClient/ - Caveat: the latest commit seems over a year old.

Another project building an asynchronous HTTP client is xsocket: xsocket.sourceforge.net