使用java的HttpClient实现多线程并发

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！