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。