前段时间有个需求在springmvc mapping的url跳转前完成一个统计的业务。显然需要进行异步的处理，不然出错或者异常会影响到后面的网页跳转。异步的方式也就是非阻塞式的，当异步调用成功与否程序会接着往下执行，不必等到输入输出处理完毕才返回。

主要用到httpasyncclient-4.0.1.jar，httpclient-4.3.2.jar,httpcore-4.3.2.jar,httpcore-nio-4.3.2.jar,commons-logging-1.1.3.jar。

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<HttpResponse> 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");
	}
}

 
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<HttpResponse>() {
					//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");
	}
}