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，日後我們再討論），它不僅是客戶端傳送Http請求變得容易，而且也方便了開發人員測試介面（基於Http協議的），即提高了開發的效率，也方便提高程式碼的健壯性。因此熟練掌握HttpClient是很重要的必修內容，掌握HttpClient後，相信對於Http協議的瞭解會更加深入。

一、簡介

HttpClient是Apache Jakarta Common下的子專案，用來提供高效的、最新的、功能豐富的支援HTTP協議的客戶端程式設計工具包，並且它支援HTTP協議最新的版本和建議。HttpClient已經應用在很多的專案中，比如Apache Jakarta上很著名的另外兩個開源專案Cactus和HTMLUnit都使用了HttpClient。

二、特性

1. 基於標準、純淨的java語言。實現了Http1.0和Http1.1

2. 以可擴充套件的面向物件的結構實現了Http全部的方法（GET, POST, PUT, DELETE, HEAD, OPTIONS, and TRACE）。

3. 支援HTTPS協議。

4. 通過Http代理建立透明的連線。

5. 利用CONNECT方法通過Http代理建立隧道的https連線。

6. Basic, Digest, NTLMv1, NTLMv2, NTLM2 Session, SNPNEGO/Kerberos認證方案。

7. 外掛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. 連線管理器支援多執行緒應用。支援設定最大連線數，同時支援設定每個主機的最大連線數，發現並關閉過期的連線。

10. 自動處理Set-Cookie中的Cookie。

11. 外掛式的自定義Cookie策略。

12. Request的輸出流可以避免流中內容直接緩衝到socket伺服器。

13. Response的輸入流可以有效的從socket伺服器直接讀取相應內容。

14. 在http1.0和http1.1中利用KeepAlive保持持久連線。

15. 直接獲取伺服器傳送的response code和 headers。

16. 設定連線超時的能力。

17. 實驗性的支援http1.1 response caching。

18. 原始碼基於Apache License 可免費獲取。

三、使用方法

使用HttpClient傳送請求、接收響應很簡單，一般需要如下幾步即可。

1. 建立HttpClient物件。

2. 建立請求方法的例項，並指定請求URL。如果需要傳送GET請求，建立HttpGet物件；如果需要傳送POST請求，建立HttpPost物件。

3. 如果需要傳送請求引數，可呼叫HttpGet、HttpPost共同的setParams(HetpParams params)方法來新增請求引數；對於HttpPost物件而言，也可呼叫setEntity(HttpEntity entity)方法來設定請求引數。

4. 呼叫HttpClient物件的execute(HttpUriRequest request)傳送請求，該方法返回一個HttpResponse。

5. 呼叫HttpResponse的getAllHeaders()、getHeaders(String name)等方法可獲取伺服器的響應頭；呼叫HttpResponse的getEntity()方法可獲取HttpEntity物件，該物件包裝了伺服器的響應內容。程式可通過該物件獲取伺服器的響應內容。

6. 釋放連線。無論執行方法是否成功，都必須釋放連線

四、例項

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連線SSL
	 */
	public void ssl() {
		CloseableHttpClient httpclient = null;
		try {
			KeyStore trustStore = KeyStore.getInstance(KeyStore.getDefaultType());
			FileInputStream instream = new FileInputStream(new File("d:\\tomcat.keystore"));
			try {
				// 載入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();
			// 建立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（模擬使用者登入請求）
	 */
	public void postForm() {
		// 建立預設的httpClient例項.  
		CloseableHttpClient httpclient = HttpClients.createDefault();
		// 建立httppost  
		HttpPost httppost = new HttpPost("http://localhost:8080/myDemo/Ajax/serivceJ.action");
		// 建立引數佇列  
		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 {
			// 關閉連線,釋放資源  
			try {
				httpclient.close();
			} catch (IOException e) {
				e.printStackTrace();
			}
		}
	}

	/**
	 * 傳送 post請求訪問本地應用並根據傳遞引數不同返回不同結果
	 */
	public void post() {
		// 建立預設的httpClient例項.  
		CloseableHttpClient httpclient = HttpClients.createDefault();
		// 建立httppost  
		HttpPost httppost = new HttpPost("http://localhost:8080/myDemo/Ajax/serivceJ.action");
		// 建立引數佇列  
		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 {
			// 關閉連線,釋放資源  
			try {
				httpclient.close();
			} catch (IOException e) {
				e.printStackTrace();
			}
		}
	}

	/**
	 * 傳送 get請求
	 */
	public void get() {
		CloseableHttpClient httpclient = HttpClients.createDefault();
		try {
			// 建立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("--------------------------------------");
				// 列印響應狀態  
				System.out.println(response.getStatusLine());
				if (entity != null) {
					// 列印響應內容長度  
					System.out.println("Response content length: " + entity.getContentLength());
					// 列印響應內容  
					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 {
			// 關閉連線,釋放資源  
			try {
				httpclient.close();
			} catch (IOException e) {
				e.printStackTrace();
			}
		}
	}

	/**
	 * 上傳檔案
	 */
	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();
			}
		}
	}
}