Http協議的重要性相信不用我多說了，HttpClient相比傳統JDK自帶的URLConnection，增加了易用性和靈活性（具體區別，日后我們再討論），它不僅是客戶端發送Http請求變得容易，而且也方便了開發人員測試接口（基于Http協議的），即提高了開發的效率，也方便提高代碼的健壯性。因此熟練掌握HttpClient是很重要的必修內容，掌握HttpClient后，相信對于Http協議的了解會更加深入。
一、簡介
HttpClient是Apache Jakarta Common下的子項目，用來提供高效的、最新的、功能豐富的支持HTTP協議的客戶端編程工具包，并且它支持HTTP協議最新的版本和建議。HttpClient已經應用在很多的項目中，比如Apache Jakarta上很著名的另外兩個開源項目Cactus和HTMLUnit都使用了HttpClient。
下載地址: http://hc.apache.org/downloads.cgi
二、特性
1. 基于標準、純凈的java語言。實現了Http1.0和Http1.1
2. 以可擴展的面向對象的結構實現了Http全部的方法（GET, POST, PUT, DELETE, HEAD, OPTIONS, and TRACE）。
3. 支持HTTPS協議。
4. 通過Http代理建立透明的連接。
5. 利用CONNECT方法通過Http代理建立隧道的https連接。
6. Basic, Digest, NTLMv1, NTLMv2, NTLM2 Session, SNPNEGO/Kerberos認證方案。
7. 插件式的自定義認證方案。
8. 便攜可靠的套接字工廠使它更容易的使用第三方解決方案。
9. 連接管理器支持多線程應用。支持設置最大連接數，同時支持設置每個主機的最大連接數，發現并關閉過期的連接。
10. 自動處理Set-Cookie中的Cookie。
11. 插件式的自定義Cookie策略。
12. Request的輸出流可以避免流中內容直接緩沖到socket服務器。
13. Response的輸入流可以有效的從socket服務器直接讀取相應內容。
14. 在http1.0和http1.1中利用KeepAlive保持持久連接。
15. 直接獲取服務器發送的response code和 headers。
16. 設置連接超時的能力。
17. 實驗性的支持http1.1 response caching。
18. 源代碼基于Apache License 可免費獲取。
三、使用方法
使用HttpClient發送請求、接收響應很簡單，一般需要如下幾步即可。
1. 創建HttpClient對象。
2. 創建請求方法的實例，并指定請求URL。如果需要發送GET請求，創建HttpGet對象；如果需要發送POST請求，創建HttpPost對象。
3. 如果需要發送請求參數，可調用HttpGet、HttpPost共同的setParams(HetpParams params)方法來添加請求參數；對于HttpPost對象而言，也可調用setEntity(HttpEntity entity)方法來設置請求參數。
4. 調用HttpClient對象的execute(HttpUriRequest request)發送請求，該方法返回一個HttpResponse。
5. 調用HttpResponse的getAllHeaders()、getHeaders(String name)等方法可獲取服務器的響應頭；調用HttpResponse的getEntity()方法可獲取HttpEntity對象，該對象包裝了服務器的響應內容。程序可通過該對象獲取服務器的響應內容。
6. 釋放連接。無論執行方法是否成功，都必須釋放連接
四、實例
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連接SSL */ public void ssl() { CloseableHttpClient httpclient = null; try { KeyStore trustStore = KeyStore.getInstance(KeyStore.getDefaultType()); FileInputStream instream = new FileInputStream(new File("d://tomcat.keystore")); try { // 加載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和所有自簽名的證書 SSLContext sslcontext = SSLContexts.custom().loadTrustMaterial(trustStore, new TrustSelfSignedStrategy()).build(); // 只允許使用TLSv1協議 SSLConnectionSocketFactory sslsf = new SSLConnectionSocketFactory(sslcontext, new String[] { "TLSv1" }, null, SSLConnectionSocketFactory.BROWSER_COMPATIBLE_HOSTNAME_VERIFIER); httpclient = HttpClients.custom().setSSLSocketFactory(sslsf).build(); // 創建http請求(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方式提交表單（模擬用戶登錄請求） */ public void postForm() { // 創建默認的httpClient實例. CloseableHttpClient httpclient = HttpClients.createDefault(); // 創建httppost HttpPost httppost = new HttpPost("http://localhost:8080/myDemo/Ajax/serivceJ.action"); // 創建參數隊列 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 { // 關閉連接,釋放資源 try { httpclient.close(); } catch (IOException e) { e.printStackTrace(); } } } /** * 發送 post請求訪問本地應用并根據傳遞參數不同返回不同結果 */ public void post() { // 創建默認的httpClient實例. CloseableHttpClient httpclient = HttpClients.createDefault(); // 創建httppost HttpPost httppost = new HttpPost("http://localhost:8080/myDemo/Ajax/serivceJ.action"); // 創建參數隊列 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 { // 關閉連接,釋放資源 try { httpclient.close(); } catch (IOException e) { e.printStackTrace(); } } } /** * 發送 get請求 */ public void get() { CloseableHttpClient httpclient = HttpClients.createDefault(); try { // 創建httpget. HttpGet httpget = new HttpGet("http://www.baidu.com/"); System.out.println("executing request " + httpget.getURI()); // 執行get請求. CloseableHttpResponse response = httpclient.execute(httpget); try { // 獲取響應實體 HttpEntity entity = response.getEntity(); System.out.println("--------------------------------------"); // 打印響應狀態 System.out.println(response.getStatusLine()); if (entity != null) { // 打印響應內容長度 System.out.println("Response content length: " + entity.getContentLength()); // 打印響應內容 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 { // 關閉連接,釋放資源 try { httpclient.close(); } catch (IOException e) { e.printStackTrace(); } } } /** * 上傳文件 */ 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> 本實例是采用HttpClient4.3最新版本。該版本與之前的代碼寫法風格相差較大，大家多留意下。
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