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、使用实例

下面给出HttpClient使用的工具类，该工具类基于HttpClient4.3.X以上版本，该版本与之前的代码写法风格相差较大，用到的包如下：

	<dependency>
		<groupId>org.apache.httpcomponents</groupId>
		<artifactId>httpclient</artifactId>
		<version>4.3.1</version>
	</dependency>

	<dependency>
		<groupId>org.apache.httpcomponents</groupId>
		<artifactId>httpcore</artifactId>
		<version>4.3.1</version>
	</dependency>
	<dependency>
		<groupId>org.apache.httpcomponents</groupId>
		<artifactId>httpmime</artifactId>
		<version>4.3.1</version>
	</dependency>

该工具类使用HttpClient实现SSL(HTTPS)请求/Get请求/Post请求，并模拟了Post Form表单提交/文件上传/文件下载，代码中所有实例都通过测试。具体代码如下：

package zmx.httpclient.test;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
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.security.cert.X509Certificate;
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.SSLContextBuilder;
import org.apache.http.conn.ssl.SSLContexts;
import org.apache.http.conn.ssl.TrustSelfSignedStrategy;
import org.apache.http.conn.ssl.TrustStrategy;
import org.apache.http.entity.mime.HttpMultipartMode;
import org.apache.http.entity.mime.MultipartEntityBuilder;
import org.apache.http.entity.mime.content.FileBody;
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.CharsetUtils;
import org.apache.http.util.EntityUtils;

/**
 * HttpClient工具类
 *
 * @author zhangwenchao
 *
 */
public class HttpClientUtils {

	/**
	 * HttpClient连接SSL，需要导入证书的方法
	 * 若SSL需要导入信任的证书，使用该方法
	 * @param url
	 */
	public static void ssl(String url) {
		CloseableHttpClient httpclient = null;
		try {
			KeyStore trustStore = KeyStore.getInstance(KeyStore
					.getDefaultType());
			FileInputStream instream = new FileInputStream(new File(
					"E:\\tomcat.keystore"));
			try {
				// 加载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和所有自签名的证书
			SSLContext sslcontext = SSLContexts
					.custom()
					.loadTrustMaterial(trustStore,
							new TrustSelfSignedStrategy()).build();
			// 只允许使用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(url);
			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();
				}
			}
		}
	}

	/**
	 * 不需要导入证书，SSL信任所有证书，使用该方法
	 *
	 * @return
	 */
	public static CloseableHttpClient createSSLClientDefault() {
		try {
			SSLContext sslContext = new SSLContextBuilder().loadTrustMaterial(
					null, new TrustStrategy() {
						// 信任所有证书
						public boolean isTrusted(X509Certificate[] chain,
								String authType) throws CertificateException {
							return true;
						}
					}).build();
			SSLConnectionSocketFactory sslsf = new SSLConnectionSocketFactory(
					sslContext);
			return HttpClients.custom().setSSLSocketFactory(sslsf).build();

		} catch (KeyManagementException e) {

			e.printStackTrace();

		} catch (NoSuchAlgorithmException e) {

			e.printStackTrace();

		} catch (KeyStoreException e) {

			e.printStackTrace();

		}
		return HttpClients.createDefault();

	}

	/**
	 * HttpClient连接SSL，不需要导入证书
	 * 信任所有证书，跳过证书验证
	 * @param url
	 */

	public static void ssl2(String url) {
		CloseableHttpClient httpclient = null;
		try {

			httpclient = createSSLClientDefault();
			// 创建http请求(get方式)
			HttpGet httpget = new HttpGet(url);
			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();
		} finally {
			if (httpclient != null) {
				try {
					httpclient.close();
				} catch (IOException e) {
					e.printStackTrace();
				}
			}
		}
	}

	/**
	 * 发送 get请求
	 */
	public static void get(String url) {
		CloseableHttpClient httpclient = HttpClients.createDefault();
		try {
			// 创建httpget.
			HttpGet httpget = new HttpGet(url);
			System.out.println("executing request " + httpget.getURI());
			// 执行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();
			}
		}
	}

	/**
	 * 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
	 */
	public static void post(String url) {
		// 创建默认的httpClient实例.
		CloseableHttpClient httpclient = HttpClients.createDefault();
		// 创建httppost
		HttpPost httppost = new HttpPost(url);
		// 创建参数队列
		List<NameValuePair> formparams = new ArrayList<NameValuePair>();
		formparams.add(new BasicNameValuePair("inputVal", "13301330133"));

		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();
			}
		}
	}

	/**
	 * post方式提交表单（模拟用户登录请求）
	 */
	public static void postForm(String url) {
		// 创建默认的httpClient实例.
		CloseableHttpClient httpclient = HttpClients.createDefault();
		// 创建httppost
		HttpPost httppost = new HttpPost(url);
		// 创建参数队列
		List<NameValuePair> formparams = new ArrayList<NameValuePair>();
		formparams.add(new BasicNameValuePair("j_username", "13301330133"));
		formparams.add(new BasicNameValuePair("j_password", "330133"));
		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();
			}
		}
	}

	/**
	 * 上传文件
	 */
	public static void upload(String url) {
		CloseableHttpClient httpclient = HttpClients.createDefault();
		try {
			HttpPost httppost = new HttpPost(url);
			FileBody bin = new FileBody(new File(
					"C:\\Users\\zhangwenchao\\Desktop\\jinzhongzi.jpg"));
			// StringBody name = new StringBody("这个一测试",
			// ContentType.TEXT_PLAIN);
			HttpEntity reqEntity = MultipartEntityBuilder.create()
					.setMode(HttpMultipartMode.BROWSER_COMPATIBLE)
					.addPart("uploadFile", bin)
					.setCharset(CharsetUtils.get("UTF-8")).build();
			httppost.setEntity(reqEntity);
			System.out.println("executing request: "
					+ httppost.getRequestLine());
			CloseableHttpResponse response = httpclient.execute(httppost);

			// httppost = new
			// HttpPost(response.getLastHeader("location").getValue());
			// response = httpclient.execute(httppost);
			try {
				System.out.println("----------------------------------------");
				System.out.println(response.getStatusLine());
				HttpEntity resEntity = response.getEntity();
				if (resEntity != null) {
					// 响应长度
					System.out.println("Response content length: "
							+ resEntity.getContentLength());
					// 打印响应内容
					System.out.println("Response content: "
							+ EntityUtils.toString(resEntity));
				}
				// 销毁
				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();
			}
		}
	}

	/**
	 * 文件下载
	 */
	public static void download(String url) {
		// 生成一个httpclient对象
		CloseableHttpClient httpclient = HttpClients.createDefault();
		try {
			HttpGet httpget = new HttpGet(url);
			CloseableHttpResponse response = httpclient.execute(httpget);
			HttpEntity resEntity = response.getEntity();
			if (resEntity != null) {
				// 响应长度
				System.out.println("Response content length: "
						+ resEntity.getContentLength());
				InputStream in = resEntity.getContent();
				String fileName = url.substring(url.lastIndexOf("/"));
				File file = new File("E:\\" + fileName);
				try {
					FileOutputStream fout = new FileOutputStream(file);
					int l = -1;
					byte[] tmp = new byte[1024];
					while ((l = in.read(tmp)) != -1) {
						fout.write(tmp, 0, l);
						// 注意这里如果用OutputStream.write(buff)的话，图片会失真，大家可以试试
					}
					fout.flush();
					fout.close();
				} finally {
					// 关闭低层流。
					in.close();
				}
			}
		} catch (ClientProtocolException e) {
			e.printStackTrace();
		} catch (IOException e) {
			e.printStackTrace();
		} finally {
			try {
				httpclient.close();
			} catch (IOException e) {
				e.printStackTrace();
			}
		}

	}

	public static void main(String[] args) throws Exception {

		// HttpClientUtils.ssl2("https://www.baidu.com?word=刘德华"); //模拟访问HTTPS
		// HttpClientUtils.get("http://www.baidu.com?word=刘德华"); //模拟访问get
		// HttpClientUtils.post("http://localhost:8080/BCP/system/user/checkName");//模拟访问post
		// HttpClientUtils.postForm("http://localhost:8080/BCP/j_spring_security_check");//模拟访问post form表单提交
		HttpClientUtils.upload("http://localhost:8080/BCP/all/test/upload"); // 模拟文件上传
		// HttpClientUtils.download("http://localhost:8080/BCP/images/crops/tongyong.jpg");//模拟文件上传

	}

}