Java的异步HttpClient

上篇提到了高性能处理的关键是异步，而我们当中许多人依旧在使用同步模式的HttpClient访问第三方Web资源，我认为原因之一是：异步的HttpClient诞生较晚，许多人不知道；另外也可能是大多数Web程序其实不在意这点性能损失了。

而要自己实现一个异步的HttpClient则比较困难，通常都是自己开一个新的工作线程，利用HttpClient的同步去访问，完成后再回调这种形式，这样做其实不是真正的异步，因为依旧会有一个线程处于阻塞中，等待着第三方Web资源的返回。

而如今访问第三方Web资源的情景越来越多，最典型就是使用第三方登录平台，如QQ或微信等，我们需要访问腾讯的服务器去验证登录者的身份，根据我的经验，这个过程可能会阻塞好几秒钟，可看作是一个“长时间调用”，所以最好要使用异步方式。

OK，废话少说，要使用异步的HttpClient，请Maven中带上：

        <dependency>

            <groupId>org.apache.httpcomponents</groupId>

            <artifactId>httpasyncclient</artifactId>

            <version>4.1.1</version>

        </dependency>

接下来是一个完整的Demo代码：

import org.apache.http.HttpResponse;

import org.apache.http.client.methods.HttpGet;

import org.apache.http.concurrent.FutureCallback;

import org.apache.http.impl.nio.client.CloseableHttpAsyncClient;

import org.apache.http.impl.nio.client.HttpAsyncClients;

import org.apache.http.util.EntityUtils;

import java.io.IOException;

import java.util.concurrent.CountDownLatch;

public class Main {

    public static void main(String[] argv) {

        CloseableHttpAsyncClient httpclient = HttpAsyncClients.createDefault();

        httpclient.start();

        final CountDownLatch latch = new CountDownLatch(1);

        final HttpGet request = new HttpGet("https://www.alipay.com/");

        System.out.println(" caller thread id is : " + Thread.currentThread().getId());

        httpclient.execute(request, new FutureCallback<HttpResponse>() {

            public void completed(final HttpResponse response) {

                latch.countDown();

                System.out.println(" callback thread id is : " + Thread.currentThread().getId());

                System.out.println(request.getRequestLine() + "->" + response.getStatusLine());

                try {

                    String content = EntityUtils.toString(response.getEntity(), "UTF-8");

                    System.out.println(" response content is : " + content);

                } catch (IOException e) {

                    e.printStackTrace();

                }

            }

            public void failed(final Exception ex) {

                latch.countDown();

                System.out.println(request.getRequestLine() + "->" + ex);

                System.out.println(" callback thread id is : " + Thread.currentThread().getId());

            }

            public void cancelled() {

                latch.countDown();

                System.out.println(request.getRequestLine() + " cancelled");

                System.out.println(" callback thread id is : " + Thread.currentThread().getId());

            }

        });

        try {

            latch.await();

        } catch (InterruptedException e) {

            e.printStackTrace();

        }

        try {

            httpclient.close();

        } catch (IOException ignore) {

        }

    }

}