Http协议的重要性相信不用我多说了，HttpClient相比传统JDK自带的URLConnection，增加了易用性和灵活性（具体区别，日后我 们再讨论），它不仅是客户端发送Http请求变得容易，而且也方便了开发人员测试接口（基于Http协议的），即提高了开发的效率，也方便提高代码的健壮 性。因此熟练掌握HttpClient是很重要的必修内容，掌握HttpClient后，相信对于Http协议的了解会更加深入。

你可以从官方获取最新版本：http://hc.apache.org/index.html。也可以从这里获取官方使用入门示例。
以下示例，输出所有头内容，和页面返回的内容。

    import java.io.ByteArrayOutputStream;    import java.io.InputStream;    import java.util.ArrayList;    import java.util.List;    import org.apache.http.Header;    import org.apache.http.HttpEntity;    import org.apache.http.NameValuePair;    import org.apache.http.client.entity.UrlEncodedFormEntity;    import org.apache.http.client.methods.CloseableHttpResponse;    import org.apache.http.client.methods.HttpGet;    import org.apache.http.client.methods.HttpPost;    import org.apache.http.impl.client.CloseableHttpClient;    import org.apache.http.impl.client.HttpClients;    import org.apache.http.message.BasicNameValuePair;    public class QuickStart {     public static void main(String[] args) {      try {       CloseableHttpClient httpclient = HttpClients.createDefault();       HttpGet httpGet = new HttpGet("http://javacui.com");       CloseableHttpResponse response1 = httpclient.execute(httpGet);       try {           System.out.println(response1.getStatusLine()); // 读取状态信息                       Header[] hd = response1.getAllHeaders(); // 所有头信息           for(Header h : hd){            System.out.println(h.getName() + "：" + h.getValue());           }                       HttpEntity entity1 = response1.getEntity();           System.out.println(EntityUtils.toString(entity1));       } finally {           response1.close();       }       HttpPost httpPost = new HttpPost("http://javacui.com");       List <NameValuePair> paras = new ArrayList <NameValuePair>(); // 设置表单参数       paras.add(new BasicNameValuePair("username", "name"));       paras.add(new BasicNameValuePair("password", "pass"));       httpPost.setEntity(new UrlEncodedFormEntity(paras));       CloseableHttpResponse response2 = httpclient.execute(httpPost);       try {        System.out.println(response1.getStatusLine()); // 读取状态信息                       Header[] hd = response1.getAllHeaders(); // 所有头信息           for(Header h : hd){            System.out.println(h.getName() + "：" + h.getValue());           }                       HttpEntity entity1 = response1.getEntity();           System.out.println(EntityUtils.toString(entity1));       } finally {           response2.close();       }      } catch (Exception e) {       e.printStackTrace();      }     }           /**      * 读取流      */     public static byte[] readStream(InputStream inStream) throws Exception {      ByteArrayOutputStream outSteam = new ByteArrayOutputStream();      byte[] buffer = new byte[1024];      int len = -1;      while ((len = inStream.read(buffer)) != -1) {       outSteam.write(buffer, 0, len);      }      outSteam.close();      inStream.close();      return outSteam.toByteArray();     }    }     // 结束  

GET和POST请求都输出了获取到的服务器内容，POST请求设置了请求的表单参数。