HttpClient是一个客户端的HTTP通信实现库。HttpClient的目标是发送和接收HTTP报文。
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
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 { // 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("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"); // 创建参数队列 List<namevaluepair> formparams = new ArrayList<namevaluepair>(); 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请求访问本地应用并根据传递参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"); // 创建参数队列 List<namevaluepair> formparams = new ArrayList<namevaluepair>(); 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()); // 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(); } } } /** * 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(); } } } }</namevaluepair></namevaluepair></namevaluepair></namevaluepair>本实例是采用HttpClient4.3最新版本。该版本与之前的代码写法风格相差较大，大家多留意下。
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