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，添加�了易用性和灵活性（详细差别，日后我们再讨论），它不仅是client发送Http请求变得easy，并且也方便了开发者測试接口（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是非常重要的必修内容，掌握HttpClient后，相信对于Http协议的了解会更加深入。

一、简单介绍

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client编程工具包，而且它支持HTTP协议最新的版本号和建议。HttpClient已经应用在非常多的项目中，比方Apache Jakarta上非常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所有的方法（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. 插件式的自己定义认证方案。

8. 便携可靠的套接字工厂使它更easy的使用第三方解决方式。

9. 连接管理器支持多线程应用。支持设置最大连接数，同一时候支持设置每一个主机的最大连接数，发现并关闭过期的连接。

10. 自己主动处理Set-Cookie中的Cookie。

11. 插件式的自己定义Cookie策略。

12. Request的输出流能够避免流中内容直接缓冲到socketserver。

13. Response的输入流能够有效的从socketserver直接读取对应内容。

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

15. 直接获取server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. 源码基于Apache License 可免费获取。

三、用法

使用HttpClient发送请求、接收响应非常easy，一般须要例如以下几步就可以。

1. 创建HttpClient对象。

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

3. 假设须要发送请求參数，可调用HttpGet、HttpPost共同的setParams(HetpParams params)方法来加入�请求參数；对于HttpPost对象而言，也可调用setEntity(HttpEntity entity)方法来设置请求參数。

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server的响应头；调用HttpResponse的getEntity()方法可获取HttpEntity对象，该对象包装了server的响应内容。程序可通过该对象获取server的响应内容。

6. 释放连接。不管运行方法是否成功，都必须释放连接

四、实例

package com.test;

import java.io.File;

import java.io.FileInputStream;

import java.io.IOException;

import java.io.UnsupportedEncodingException;

import java.security.KeyManagementException;

import java.security.KeyStore;

import java.security.KeyStoreException;

import java.security.NoSuchAlgorithmException;

import java.security.cert.CertificateException;

import java.util.ArrayList;

import java.util.List;

import javax.net.ssl.SSLContext;

import org.apache.http.HttpEntity;

import org.apache.http.NameValuePair;

import org.apache.http.ParseException;

import org.apache.http.client.ClientProtocolException;

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.conn.ssl.SSLConnectionSocketFactory;

import org.apache.http.conn.ssl.SSLContexts;

import org.apache.http.conn.ssl.TrustSelfSignedStrategy;

import org.apache.http.entity.ContentType;

import org.apache.http.entity.mime.MultipartEntityBuilder;

import org.apache.http.entity.mime.content.FileBody;

import org.apache.http.entity.mime.content.StringBody;

import org.apache.http.impl.client.CloseableHttpClient;

import org.apache.http.impl.client.HttpClients;

import org.apache.http.message.BasicNameValuePair;

import org.apache.http.util.EntityUtils;

import org.junit.Test;

public class HttpClientTest {

	@Test

	public void jUnitTest() {

		get();

	}

	/**

	 * HttpClient连接SSL

	 */

	public void ssl() {

		CloseableHttpClient httpclient = null;

		try {

			KeyStore trustStore = KeyStore.getInstance(KeyStore.getDefaultType());

			FileInputStream instream = new FileInputStream(new File("d:\\tomcat.keystore"));

			try {

				// 载入keyStore d:\\tomcat.keystore

				trustStore.load(instream, "123456".toCharArray());

			} catch (CertificateException e) {

				e.printStackTrace();

			} finally {

				try {

					instream.close();

				} catch (Exception ignore) {

				}

			}

			// 相信自己的CA和全部自签名的证书

			SSLContext sslcontext = SSLContexts.custom().loadTrustMaterial(trustStore, new TrustSelfSignedStrategy()).build();

			// 仅仅同意使用TLSv1协议

			SSLConnectionSocketFactory sslsf = new SSLConnectionSocketFactory(sslcontext, new String[] { "TLSv1" }, null,

					SSLConnectionSocketFactory.BROWSER_COMPATIBLE_HOSTNAME_VERIFIER);

			httpclient = HttpClients.custom().setSSLSocketFactory(sslsf).build();

			// 创建http请求(get方式)

			HttpGet httpget = new HttpGet("https://localhost:8443/myDemo/Ajax/serivceJ.action");

			System.out.println("executing request" + httpget.getRequestLine());

			CloseableHttpResponse response = httpclient.execute(httpget);

			try {

				HttpEntity entity = response.getEntity();

				System.out.println("----------------------------------------");

				System.out.println(response.getStatusLine());

				if (entity != null) {

					System.out.println("Response content length: " + entity.getContentLength());

					System.out.println(EntityUtils.toString(entity));

					EntityUtils.consume(entity);

				}

			} finally {

				response.close();

			}

		} catch (ParseException e) {

			e.printStackTrace();

		} catch (IOException e) {

			e.printStackTrace();

		} catch (KeyManagementException e) {

			e.printStackTrace();

		} catch (NoSuchAlgorithmException e) {

			e.printStackTrace();

		} catch (KeyStoreException e) {

			e.printStackTrace();

		} finally {

			if (httpclient != null) {

				try {

					httpclient.close();

				} catch (IOException e) {

					e.printStackTrace();

				}

			}

		}

	}

	/**

	 * post方式提交表单（模拟用户登录请求）

	 */

	public void postForm() {

		// 创建默认的httpClient实例.

		CloseableHttpClient httpclient = HttpClients.createDefault();

		// 创建httppost

		HttpPost httppost = new HttpPost("http://localhost:8080/myDemo/Ajax/serivceJ.action");

		// 创建參数队列

		List formparams = new ArrayList();

		formparams.add(new BasicNameValuePair("username", "admin"));

		formparams.add(new BasicNameValuePair("password", "123456"));

		UrlEncodedFormEntity uefEntity;

		try {

			uefEntity = new UrlEncodedFormEntity(formparams, "UTF-8");

			httppost.setEntity(uefEntity);

			System.out.println("executing request " + httppost.getURI());

			CloseableHttpResponse response = httpclient.execute(httppost);

			try {

				HttpEntity entity = response.getEntity();

				if (entity != null) {

					System.out.println("--------------------------------------");

					System.out.println("Response content: " + EntityUtils.toString(entity, "UTF-8"));

					System.out.println("--------------------------------------");

				}

			} finally {

				response.close();

			}

		} catch (ClientProtocolException e) {

			e.printStackTrace();

		} catch (UnsupportedEncodingException e1) {

			e1.printStackTrace();

		} catch (IOException e) {

			e.printStackTrace();

		} finally {

			// 关闭连接,释放资源

			try {

				httpclient.close();

			} catch (IOException e) {

				e.printStackTrace();

			}

		}

	}

	/**

	 * 发送 post请求訪问本地应用并依据传递參数不同返回不同结果

	 */

	public void post() {

		// 创建默认的httpClient实例.

		CloseableHttpClient httpclient = HttpClients.createDefault();

		// 创建httppost

		HttpPost httppost = new HttpPost("http://localhost:8080/myDemo/Ajax/serivceJ.action");

		// 创建參数队列

		List formparams = new ArrayList();

		formparams.add(new BasicNameValuePair("type", "house"));

		UrlEncodedFormEntity uefEntity;

		try {

			uefEntity = new UrlEncodedFormEntity(formparams, "UTF-8");

			httppost.setEntity(uefEntity);

			System.out.println("executing request " + httppost.getURI());

			CloseableHttpResponse response = httpclient.execute(httppost);

			try {

				HttpEntity entity = response.getEntity();

				if (entity != null) {

					System.out.println("--------------------------------------");

					System.out.println("Response content: " + EntityUtils.toString(entity, "UTF-8"));

					System.out.println("--------------------------------------");

				}

			} finally {

				response.close();

			}

		} catch (ClientProtocolException e) {

			e.printStackTrace();

		} catch (UnsupportedEncodingException e1) {

			e1.printStackTrace();

		} catch (IOException e) {

			e.printStackTrace();

		} finally {

			// 关闭连接,释放资源

			try {

				httpclient.close();

			} catch (IOException e) {

				e.printStackTrace();

			}

		}

	}

	/**

	 * 发送 get请求

	 */

	public void get() {

		CloseableHttpClient httpclient = HttpClients.createDefault();

		try {

			// 创建httpget.

			HttpGet httpget = new HttpGet("http://www.baidu.com/");

			System.out.println("executing request " + httpget.getURI());

			// 运行get请求.

			CloseableHttpResponse response = httpclient.execute(httpget);

			try {

				// 获取响应实体

				HttpEntity entity = response.getEntity();

				System.out.println("--------------------------------------");

				// 打印响应状态

				System.out.println(response.getStatusLine());

				if (entity != null) {

					// 打印响应内容长度

					System.out.println("Response content length: " + entity.getContentLength());

					// 打印响应内容

					System.out.println("Response content: " + EntityUtils.toString(entity));

				}

				System.out.println("------------------------------------");

			} finally {

				response.close();

			}

		} catch (ClientProtocolException e) {

			e.printStackTrace();

		} catch (ParseException e) {

			e.printStackTrace();

		} catch (IOException e) {

			e.printStackTrace();

		} finally {

			// 关闭连接,释放资源

			try {

				httpclient.close();

			} catch (IOException e) {

				e.printStackTrace();

			}

		}

	}

	/**

	 * 上传文件

	 */

	public void upload() {

		CloseableHttpClient httpclient = HttpClients.createDefault();

		try {

			HttpPost httppost = new HttpPost("http://localhost:8080/myDemo/Ajax/serivceFile.action");

			FileBody bin = new FileBody(new File("F:\\image\\sendpix0.jpg"));

			StringBody comment = new StringBody("A binary file of some kind", ContentType.TEXT_PLAIN);

			HttpEntity reqEntity = MultipartEntityBuilder.create().addPart("bin", bin).addPart("comment", comment).build();

			httppost.setEntity(reqEntity);

			System.out.println("executing request " + httppost.getRequestLine());

			CloseableHttpResponse response = httpclient.execute(httppost);

			try {

				System.out.println("----------------------------------------");

				System.out.println(response.getStatusLine());

				HttpEntity resEntity = response.getEntity();

				if (resEntity != null) {

					System.out.println("Response content length: " + resEntity.getContentLength());

				}

				EntityUtils.consume(resEntity);

			} finally {

				response.close();

			}

		} catch (ClientProtocolException e) {

			e.printStackTrace();

		} catch (IOException e) {

			e.printStackTrace();

		} finally {

			try {

				httpclient.close();

			} catch (IOException e) {

				e.printStackTrace();

			}

		}

	}

}

本实例是採用HttpClient4.3最新版本号。该版本号与之前的代码写法风格相差较大，大家多留意下。