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。

一、简介

HttpClient是Apache Jakarta Common下的子项目，用来提供高效的、最新的、功能丰富的支持HTTP协议的客户端编程工具包，并且它支持HTTP协议最新的版本和建议。HttpClient已经应用在很多的项目中，比如Apache Jakarta上很著名的另外两个开源项目Cactus和HTMLUnit都使用了HttpClient。

下载地址: http://hc.apache.org/downloads.cgi

二、特性

1. 基于标准、纯净的java语言。实现了Http1.0和Http1.1

2. 以可扩展的面向对象的结构实现了Http全部的方法（GET, POST, PUT, DELETE, HEAD, OPTIONS, and TRACE）。

3. 支持HTTPS协议。

4. 通过Http代理建立透明的连接。

5. 利用CONNECT方法通过Http代理建立隧道的https连接。

6. Basic, Digest, NTLMv1, NTLMv2, NTLM2 Session, SNPNEGO/Kerberos认证方案。

7. 插件式的自定义认证方案。

8. 便携可靠的套接字工厂使它更容易的使用第三方解决方案。

9. 连接管理器支持多线程应用。支持设置最大连接数，同时支持设置每个主机的最大连接数，发现并关闭过期的连接。

10. 自动处理Set-Cookie中的Cookie。

11. 插件式的自定义Cookie策略。

12. Request的输出流可以避免流中内容直接缓冲到socket服务器。

13. Response的输入流可以有效的从socket服务器直接读取相应内容。

14. 在http1.0和http1.1中利用KeepAlive保持持久连接。

15. 直接获取服务器发送的response code和 headers。

16. 设置连接超时的能力。

17. 实验性的支持http1.1 response caching。

18. 源代码基于Apache License 可免费获取。

三、使用方法

使用HttpClient发送请求、接收响应很简单，一般需要如下几步即可。

1. 创建HttpClient对象。

2. 创建请求方法的实例，并指定请求URL。如果需要发送GET请求，创建HttpGet对象；如果需要发送POST请求，创建HttpPost对象。

3. 如果需要发送请求参数，可调用HttpGet、HttpPost共同的setParams(HetpParams params)方法来添加请求参数；对于HttpPost对象而言，也可调用setEntity(HttpEntity entity)方法来设置请求参数。

4. 调用HttpClient对象的execute(HttpUriRequest request)发送请求，该方法返回一个HttpResponse。

5. 调用HttpResponse的getAllHeaders()、getHeaders(String name)等方法可获取服务器的响应头；调用HttpResponse的getEntity()方法可获取HttpEntity对象，该对象包装了服务器的响应内容。程序可通过该对象获取服务器的响应内容。

6. 释放连接。无论执行方法是否成功，都必须释放连接

四、实例

  1 package com.test;
  2 
  3 import java.io.File;
  4 import java.io.FileInputStream;
  5 import java.io.IOException;
  6 import java.io.UnsupportedEncodingException;
  7 import java.security.KeyManagementException;
  8 import java.security.KeyStore;
  9 import java.security.KeyStoreException;
 10 import java.security.NoSuchAlgorithmException;
 11 import java.security.cert.CertificateException;
 12 import java.util.ArrayList;
 13 import java.util.List;
 14 
 15 import javax.net.ssl.SSLContext;
 16 
 17 import org.apache.http.HttpEntity;
 18 import org.apache.http.NameValuePair;
 19 import org.apache.http.ParseException;
 20 import org.apache.http.client.ClientProtocolException;
 21 import org.apache.http.client.entity.UrlEncodedFormEntity;
 22 import org.apache.http.client.methods.CloseableHttpResponse;
 23 import org.apache.http.client.methods.HttpGet;
 24 import org.apache.http.client.methods.HttpPost;
 25 import org.apache.http.conn.ssl.SSLConnectionSocketFactory;
 26 import org.apache.http.conn.ssl.SSLContexts;
 27 import org.apache.http.conn.ssl.TrustSelfSignedStrategy;
 28 import org.apache.http.entity.ContentType;
 29 import org.apache.http.entity.mime.MultipartEntityBuilder;
 30 import org.apache.http.entity.mime.content.FileBody;
 31 import org.apache.http.entity.mime.content.StringBody;
 32 import org.apache.http.impl.client.CloseableHttpClient;
 33 import org.apache.http.impl.client.HttpClients;
 34 import org.apache.http.message.BasicNameValuePair;
 35 import org.apache.http.util.EntityUtils;
 36 import org.junit.Test;
 37 
 38 public class HttpClientTest {
 39 
 40     @Test
 41     public void jUnitTest() {
 42         get();
 43     }
 44 
 45     /**
 46      * HttpClient连接SSL
 47      */
 48     public void ssl() {
 49         CloseableHttpClient httpclient = null;
 50         try {
 51             KeyStore trustStore = KeyStore.getInstance(KeyStore.getDefaultType());
 52             FileInputStream instream = new FileInputStream(new File("d:\\tomcat.keystore"));
 53             try {
 54                 // 加载keyStore d:\\tomcat.keystore  
 55                 trustStore.load(instream, "123456".toCharArray());
 56             } catch (CertificateException e) {
 57                 e.printStackTrace();
 58             } finally {
 59                 try {
 60                     instream.close();
 61                 } catch (Exception ignore) {
 62                 }
 63             }
 64             // 相信自己的CA和所有自签名的证书
 65             SSLContext sslcontext = SSLContexts.custom().loadTrustMaterial(trustStore, new TrustSelfSignedStrategy()).build();
 66             // 只允许使用TLSv1协议
 67             SSLConnectionSocketFactory sslsf = new SSLConnectionSocketFactory(sslcontext, new String[] { "TLSv1" }, null,
 68                     SSLConnectionSocketFactory.BROWSER_COMPATIBLE_HOSTNAME_VERIFIER);
 69             httpclient = HttpClients.custom().setSSLSocketFactory(sslsf).build();
 70             // 创建http请求(get方式)
 71             HttpGet httpget = new HttpGet("https://localhost:8443/myDemo/Ajax/serivceJ.action");
 72             System.out.println("executing request" + httpget.getRequestLine());
 73             CloseableHttpResponse response = httpclient.execute(httpget);
 74             try {
 75                 HttpEntity entity = response.getEntity();
 76                 System.out.println("----------------------------------------");
 77                 System.out.println(response.getStatusLine());
 78                 if (entity != null) {
 79                     System.out.println("Response content length: " + entity.getContentLength());
 80                     System.out.println(EntityUtils.toString(entity));
 81                     EntityUtils.consume(entity);
 82                 }
 83             } finally {
 84                 response.close();
 85             }
 86         } catch (ParseException e) {
 87             e.printStackTrace();
 88         } catch (IOException e) {
 89             e.printStackTrace();
 90         } catch (KeyManagementException e) {
 91             e.printStackTrace();
 92         } catch (NoSuchAlgorithmException e) {
 93             e.printStackTrace();
 94         } catch (KeyStoreException e) {
 95             e.printStackTrace();
 96         } finally {
 97             if (httpclient != null) {
 98                 try {
 99                     httpclient.close();
100                 } catch (IOException e) {
101                     e.printStackTrace();
102                 }
103             }
104         }
105     }
106 
107     /**
108      * post方式提交表单（模拟用户登录请求）
109      */
110     public void postForm() {
111         // 创建默认的httpClient实例.  
112         CloseableHttpClient httpclient = HttpClients.createDefault();
113         // 创建httppost  
114         HttpPost httppost = new HttpPost("http://localhost:8080/myDemo/Ajax/serivceJ.action");
115         // 创建参数队列  
116         List<namevaluepair> formparams = new ArrayList<namevaluepair>();
117         formparams.add(new BasicNameValuePair("username", "admin"));
118         formparams.add(new BasicNameValuePair("password", "123456"));
119         UrlEncodedFormEntity uefEntity;
120         try {
121             uefEntity = new UrlEncodedFormEntity(formparams, "UTF-8");
122             httppost.setEntity(uefEntity);
123             System.out.println("executing request " + httppost.getURI());
124             CloseableHttpResponse response = httpclient.execute(httppost);
125             try {
126                 HttpEntity entity = response.getEntity();
127                 if (entity != null) {
128                     System.out.println("--------------------------------------");
129                     System.out.println("Response content: " + EntityUtils.toString(entity, "UTF-8"));
130                     System.out.println("--------------------------------------");
131                 }
132             } finally {
133                 response.close();
134             }
135         } catch (ClientProtocolException e) {
136             e.printStackTrace();
137         } catch (UnsupportedEncodingException e1) {
138             e1.printStackTrace();
139         } catch (IOException e) {
140             e.printStackTrace();
141         } finally {
142             // 关闭连接,释放资源  
143             try {
144                 httpclient.close();
145             } catch (IOException e) {
146                 e.printStackTrace();
147             }
148         }
149     }
150 
151     /**
152      * 发送 post请求访问本地应用并根据传递参数不同返回不同结果
153      */
154     public void post() {
155         // 创建默认的httpClient实例.  
156         CloseableHttpClient httpclient = HttpClients.createDefault();
157         // 创建httppost  
158         HttpPost httppost = new HttpPost("http://localhost:8080/myDemo/Ajax/serivceJ.action");
159         // 创建参数队列  
160         List<namevaluepair> formparams = new ArrayList<namevaluepair>();
161         formparams.add(new BasicNameValuePair("type", "house"));
162         UrlEncodedFormEntity uefEntity;
163         try {
164             uefEntity = new UrlEncodedFormEntity(formparams, "UTF-8");
165             httppost.setEntity(uefEntity);
166             System.out.println("executing request " + httppost.getURI());
167             CloseableHttpResponse response = httpclient.execute(httppost);
168             try {
169                 HttpEntity entity = response.getEntity();
170                 if (entity != null) {
171                     System.out.println("--------------------------------------");
172                     System.out.println("Response content: " + EntityUtils.toString(entity, "UTF-8"));
173                     System.out.println("--------------------------------------");
174                 }
175             } finally {
176                 response.close();
177             }
178         } catch (ClientProtocolException e) {
179             e.printStackTrace();
180         } catch (UnsupportedEncodingException e1) {
181             e1.printStackTrace();
182         } catch (IOException e) {
183             e.printStackTrace();
184         } finally {
185             // 关闭连接,释放资源  
186             try {
187                 httpclient.close();
188             } catch (IOException e) {
189                 e.printStackTrace();
190             }
191         }
192     }
193 
194     /**
195      * 发送 get请求
196      */
197     public void get() {
198         CloseableHttpClient httpclient = HttpClients.createDefault();
199         try {
200             // 创建httpget.  
201             HttpGet httpget = new HttpGet("http://www.baidu.com/");
202             System.out.println("executing request " + httpget.getURI());
203             // 执行get请求.  
204             CloseableHttpResponse response = httpclient.execute(httpget);
205             try {
206                 // 获取响应实体  
207                 HttpEntity entity = response.getEntity();
208                 System.out.println("--------------------------------------");
209                 // 打印响应状态  
210                 System.out.println(response.getStatusLine());
211                 if (entity != null) {
212                     // 打印响应内容长度  
213                     System.out.println("Response content length: " + entity.getContentLength());
214                     // 打印响应内容  
215                     System.out.println("Response content: " + EntityUtils.toString(entity));
216                 }
217                 System.out.println("------------------------------------");
218             } finally {
219                 response.close();
220             }
221         } catch (ClientProtocolException e) {
222             e.printStackTrace();
223         } catch (ParseException e) {
224             e.printStackTrace();
225         } catch (IOException e) {
226             e.printStackTrace();
227         } finally {
228             // 关闭连接,释放资源  
229             try {
230                 httpclient.close();
231             } catch (IOException e) {
232                 e.printStackTrace();
233             }
234         }
235     }
236 
237     /**
238      * 上传文件
239      */
240     public void upload() {
241         CloseableHttpClient httpclient = HttpClients.createDefault();
242         try {
243             HttpPost httppost = new HttpPost("http://localhost:8080/myDemo/Ajax/serivceFile.action");
244 
245             FileBody bin = new FileBody(new File("F:\\image\\sendpix0.jpg"));
246             StringBody comment = new StringBody("A binary file of some kind", ContentType.TEXT_PLAIN);
247 
248             HttpEntity reqEntity = MultipartEntityBuilder.create().addPart("bin", bin).addPart("comment", comment).build();
249 
250             httppost.setEntity(reqEntity);
251 
252             System.out.println("executing request " + httppost.getRequestLine());
253             CloseableHttpResponse response = httpclient.execute(httppost);
254             try {
255                 System.out.println("----------------------------------------");
256                 System.out.println(response.getStatusLine());
257                 HttpEntity resEntity = response.getEntity();
258                 if (resEntity != null) {
259                     System.out.println("Response content length: " + resEntity.getContentLength());
260                 }
261                 EntityUtils.consume(resEntity);
262             } finally {
263                 response.close();
264             }
265         } catch (ClientProtocolException e) {
266             e.printStackTrace();
267         } catch (IOException e) {
268             e.printStackTrace();
269         } finally {
270             try {
271                 httpclient.close();
272             } catch (IOException e) {
273                 e.printStackTrace();
274             }
275         }
276     }
277 }</namevaluepair></namevaluepair></namevaluepair></namevaluepair>

本实例是采用HttpClient4.3最新版本。(转载)