Java异步HTTP请求

需要用到的包（包版本应该可能不同）：

httpcore-4.1.4.jar

httpsayncclient-4.0-alpha3.jar

httpcore-nio-4.2-alpha3.jar

代码（来自http://hc.apache.org/httpcomponents-asyncclient-dev/httpasyncclient/examples/org/apache/http/examples/nio/client/AsyncClientHttpExchangeStreaming.java）：

 

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 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file!
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
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 * This software consists of voluntary contributions made by many
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 */
package sync_http;

import java.io.IOException;
import java.nio.CharBuffer;
import java.util.concurrent.Future;

import org.apache.http.HttpResponse;
import org.apache.http.impl.nio.client.DefaultHttpAsyncClient;
import org.apache.http.nio.IOControl;
import org.apache.http.nio.client.HttpAsyncClient;
import org.apache.http.nio.client.methods.AsyncCharConsumer;
import org.apache.http.nio.client.methods.HttpAsyncMethods;
import org.apache.http.protocol.HttpContext;

public class AsyncClientHttpExchangeStreaming {

    public static void main(String[] args) throws Exception {
        HttpAsyncClient httpclient = new DefaultHttpAsyncClient();
        httpclient.start();
        try {
            Future future = httpclient.execute(
                    HttpAsyncMethods.createGet("http://www.baidu.com/"),
                    new MyResponseConsumer(), null);
            Boolean result = future.get();
            if (result != null && result.booleanValue()) {
                System.out.println("Request successfully executed");
            } else {
                System.out.println("Request failed");
            }
            System.out.println("Shutting down");
        } finally {
            httpclient.shutdown();
        }
        System.out.println("Done");
    }

    static class MyResponseConsumer extends AsyncCharConsumer {

        private int times = 0;

        private String getTimes() {
            return "\n\n### 第" + ++times + "步\n###";
        }

        @Override
        protected void onResponseReceived(final HttpResponse response) {
            System.out.println(getTimes() + "onResponseReceived");
        }

        @Override
        protected void onCharReceived(final CharBuffer buf, final IOControl ioctrl) throws IOException {
            System.out.println(getTimes() + "onCharReceived");
            while (buf.hasRemaining()) {
                System.out.print(buf.get());
            }
        }

        @Override
        protected void releaseResources() {
            System.out.println(getTimes() + "releaseResources");
        }

        @Override
        protected Boolean buildResult(final HttpContext context) {
            System.out.println(getTimes() + "buildResult");
            return Boolean.TRUE;
        }

    }

}

更多官方例子，参看：http://hc.apache.org/httpcomponents-asyncclient-dev/examples.html

 

java.util.concurrent中主要包括三类工具，Executor Freamework,并发集合（Concurrent Collection），以及同步器（Synchronizer）。下面的例子是利用java.util.concurrent.Future只请求一个url异步请求。Future接口表示异步计算的结果。它提供了检查计算是否完成的方法，以等待计算的完成，并获取计算的结果。计算完成后只能使用 get 方法来获取结果，如有必要，计算完成前可以阻塞此方法。取消则由 cancel 方法来执行。还提供了其他方法，以确定任务是正常完成还是被取消了。一旦计算完成，就不能再取消计算。如果为了可取消性而使用 Future 但又不提供可用的结果，则可以声明 Future 形式类型、并返回 null 作为底层任务的结果。

import java.util.concurrent.Future;
import org.apache.http.HttpResponse;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.nio.client.CloseableHttpAsyncClient;
import org.apache.http.impl.nio.client.HttpAsyncClients;
/**
 * This example demonstrates a basic asynchronous HTTP request / response
 * exchange. Response content is buffered in memory for simplicity.
 */
public class AsyncClientHttpExchange {
 public static void main(final String[] args) throws Exception {
  CloseableHttpAsyncClient httpclient = HttpAsyncClients.createDefault();// 默认的配置
  try {
   httpclient.start();
   HttpGet request = new HttpGet("http://www.apache.org/");
   Future future = httpclient.execute(request, null);
   HttpResponse response = future.get();// 获取结果
   System.out.println("Response: " + response.getStatusLine());
   System.out.println("Shutting down");
  } finally {
   httpclient.close();
  }
  System.out.println("Done");
 }
}

同步器（Synchronizer）是一些使线程能够等待另一个线程的对象，允许它们协作，最常用的同步器是CountDownLatch和Semaphore。较不常用的是CyclicBarrier和Exchanger。Semaphore类是一个计数信号量。从概念上讲，信号量维护了一个许可集。如有必要，在许可可用前会阻塞每一个 acquire()，然后再获取该许可。每个 release() 添加一个许可，从而可能释放一个正在阻塞的获取者。但是，不使用实际的许可对象，Semaphore 只对可用许可的号码进行计数，并采取相应的行动。 CountDownLatch是一个同步辅助类，在完成一组正在其他线程中执行的操作之前，它允许一个或多个线程一直等待。用给定的计数 初始化 CountDownLatch。由于调用了 countDown() 方法，所以在当前计数到达零之前，await 方法会一直受阻塞。之后，会释放所有等待的线程，await 的所有后续调用都将立即返回。这种现象只出现一次——计数无法被重置。如果需要重置计数，可使用 CyclicBarrier。倒计数索存器（CountDownLatch）是一次性的障碍。它的唯一构造器带有一个int类型的参数，这个参数是指允许所有的等待线程处理之前，必须在锁存器上调用countDown()方法的次数。这一点非常有用。下面是异步请求一组url的例子，利用callback借口完成独立的操作。

 
import java.util.concurrent.CountDownLatch;
import org.apache.http.HttpResponse;
import org.apache.http.client.config.RequestConfig;
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;
/**
 * This example demonstrates a fully asynchronous execution of multiple HTTP
 * exchanges where the result of an individual operation is reported using a
 * callback interface.
 */
public class AsyncClientHttpExchangeFutureCallback {
 public static void main(final String[] args) throws Exception {
  RequestConfig requestConfig = RequestConfig.custom()
    .setSocketTimeout(3000).setConnectTimeout(3000).build();
  CloseableHttpAsyncClient httpclient = HttpAsyncClients.custom()
    .setDefaultRequestConfig(requestConfig).build();
  try {
   httpclient.start();
   final HttpGet[] requests = new HttpGet[] {
     new HttpGet("http://www.apache.org/"),
     new HttpGet("https://www.verisign.com/"),
     new HttpGet("http://www.google.com/"),
     new HttpGet("http://www.baidu.com/") };
   final CountDownLatch latch = new CountDownLatch(requests.length);
   for (final HttpGet request : requests) {
    httpclient.execute(request, new FutureCallback() {
     //无论完成还是失败都调用countDown()
     @Override
     public void completed(final HttpResponse response) {
      latch.countDown();
      System.out.println(request.getRequestLine() + "->"
        + response.getStatusLine());
     }
     @Override
     public void failed(final Exception ex) {
      latch.countDown();
      System.out.println(request.getRequestLine() + "->" + ex);
     }
     @Override
     public void cancelled() {
      latch.countDown();
      System.out.println(request.getRequestLine()
        + " cancelled");
     }
    });
   }
   latch.await();
   System.out.println("Shutting down");
  } finally {
   httpclient.close();
  }
  System.out.println("Done");
 }
}