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
[java]
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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(); }
public void ssl() { CloseableHttpClient httpclient =
null;
try { KeyStore trustStore = KeyStore.getInstance(KeyStore.getDefaultType()); FileInputStream instream =
new FileInputStream(
new File(
"d:\\tomcat.keystore"));
try { trustStore.load(instream,
"123456".toCharArray()); }
catch (CertificateException e) { e.printStackTrace(); }
finally {
try { instream.close(); }
catch (Exception ignore) { } } SSLContext sslcontext = SSLContexts.custom().loadTrustMaterial(trustStore,
new TrustSelfSignedStrategy()).build(); SSLConnectionSocketFactory sslsf =
new SSLConnectionSocketFactory(sslcontext,
new String[] {
"TLSv1" },
null, SSLConnectionSocketFactory.BROWSER_COMPATIBLE_HOSTNAME_VERIFIER); httpclient = HttpClients.custom().setSSLSocketFactory(sslsf).build(); 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(); } } } }
public void postForm() { CloseableHttpClient httpclient = HttpClients.createDefault(); HttpPost httppost =
new HttpPost(
"http://localhost:8080/myDemo/Ajax/serivceJ.action"); 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 {
try { httpclient.close(); }
catch (IOException e) { e.printStackTrace(); } } }
public void post() { CloseableHttpClient httpclient = HttpClients.createDefault(); HttpPost httppost =
new HttpPost(
"http://localhost:8080/myDemo/Ajax/serivceJ.action"); 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 {
try { httpclient.close(); }
catch (IOException e) { e.printStackTrace(); } } }
public void get() { CloseableHttpClient httpclient = HttpClients.createDefault();
try { HttpGet httpget =
new HttpGet(
"http://www.baidu.com/"); System.out.println(
"executing request " + httpget.getURI()); 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(
"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 {
try { httpclient.close(); }
catch (IOException e) { e.printStackTrace(); } } }
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。
