说明：以下的代码基于httpclient4.5.2实现。
我们要使用java的HttpClient实现get请求抓取网页是一件比较容易实现的工作：
public static String get(String url) { CloseableHttpResponseresponse = null; BufferedReader in = null; String result = ""; try { CloseableHttpClienthttpclient = HttpClients.createDefault(); HttpGethttpGet = new HttpGet(url); response = httpclient.execute(httpGet); in = new BufferedReader(new InputStreamReader(response.getEntity().getContent())); StringBuffersb = new StringBuffer(""); String line = ""; String NL = System.getProperty("line.separator"); while ((line = in.readLine()) != null) { sb.append(line + NL); } in.close(); result = sb.toString(); } catch (IOException e) { e.printStackTrace(); } finally { try { if (null != response) response.close(); } catch (IOException e) { e.printStackTrace(); } } return result; }要多线程执行get请求时上面的方法也堪用。不过这种多线程请求是基于在每次调用get方法时创建一个HttpClient实例实现的。每个HttpClient实例使用一次即被回收。这显然不是一种最优的实现。
HttpClient提供了多线程请求方案，可以查看官方文档的《 Pooling connection manager 》这一节。HttpCLient实现多线程请求是基于内置的连接池实现的，其中有一个关键的类即PoolingHttpClientConnectionManager，这个类负责管理HttpClient连接池。在PoolingHttpClientConnectionManager中提供了两个关键的方法：setMaxTotal和setDefaultMaxPerRoute。setMaxTotal设置连接池的最大连接数，setDefaultMaxPerRoute设置每个路由上的默认连接个数。此外还有一个方法setMaxPerRoute――单独为某个站点设置最大连接个数，像这样：
HttpHosthost = new HttpHost("locahost", 80); cm.setMaxPerRoute(new HttpRoute(host), 50);根据文档稍稍调整下我们的get请求实现：
package com.zhyea.robin; import org.apache.http.client.methods.CloseableHttpResponse;import org.apache.http.client.methods.HttpGet;import org.apache.http.impl.client.CloseableHttpClient;import org.apache.http.impl.client.HttpClients;import org.apache.http.impl.conn.PoolingHttpClientConnectionManager; import java.io.BufferedReader;import java.io.IOException;import java.io.InputStreamReader; public class HttpUtil { private static CloseableHttpClienthttpClient; static { PoolingHttpClientConnectionManagercm = new PoolingHttpClientConnectionManager(); cm.setMaxTotal(200); cm.setDefaultMaxPerRoute(20); cm.setDefaultMaxPerRoute(50); httpClient = HttpClients.custom().setConnectionManager(cm).build(); } public static String get(String url) { CloseableHttpResponseresponse = null; BufferedReaderin = null; String result = ""; try { HttpGethttpGet = new HttpGet(url); response = httpClient.execute(httpGet); in = new BufferedReader(new InputStreamReader(response.getEntity().getContent())); StringBuffersb = new StringBuffer(""); String line = ""; String NL = System.getProperty("line.separator"); while ((line = in.readLine()) != null) { sb.append(line + NL); } in.close(); result = sb.toString(); } catch (IOException e) { e.printStackTrace(); } finally { try { if (null != response) response.close(); } catch (IOException e) { e.printStackTrace(); } } return result; } public static void main(String[] args) { System.out.println(get("https://www.baidu.com/")); }}这样就差不多了。不过对于我自己而言，我更喜欢httpclient的fluent实现，比如我们刚才实现的http get请求完全可以这样简单的实现：
package com.zhyea.robin; import org.apache.http.client.fluent.Request;import java.io.IOException; public class HttpUtil { public static String get(String url) { String result = ""; try { result = Request.Get(url) .connectTimeout(1000) .socketTimeout(1000) .execute().returnContent().asString(); } catch (IOException e) { e.printStackTrace(); } return result; } public static void main(String[] args) { System.out.println(get("https://www.baidu.com/")); }}我们要做的只是将以前的httpclient依赖替换为fluent-hc依赖：
<dependency> <groupId>org.apache.httpcomponents</groupId> <artifactId>fluent-hc</artifactId> <version>4.5.2</version></dependency>
并且这个fluent实现天然就是采用PoolingHttpClientConnectionManager完成的。它设置的maxTotal和defaultMaxPerRoute的值分别是200和100：
CONNMGR = new PoolingHttpClientConnectionManager(sfr); CONNMGR.setDefaultMaxPerRoute(100); CONNMGR.setMaxTotal(200);
唯一一点让人不爽的就是Executor没有提供调整这两个值的方法。不过这也完全够用了，实在不行的话，还可以考虑重写Executor方法，然后直接使用Executor执行get请求：
Executor.newInstance().execute(Request.Get(url)) .returnContent().asString();
就这样！