功能介紹：

1）實現了所有 HTTP 的方法（GET,POST,PUT,HEAD 等）

2）支援自動轉向

3）支援 HTTPS 協議

4）支援代理伺服器等

HttpClient最重要的功能是執行HTTP方法：執行一個HTTP方法涉及一個或多個HTTP請求/ HTTP響應資訊交流，通常是由HttpClient內部處理。使用者提供一個請求物件，HttpClient傳送請求到目標伺服器，希望伺服器返回一個相應的響應物件，或者丟擲一個異常（如果執行失敗）。

示例工具類：

package com.forezp;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.UnsupportedEncodingException;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLEncoder;
import java.util.Map;

public class HttpClientUtils {
         /**
         * 向指定URL傳送GET方法的請求
         *
         * @param url   傳送請求的URL 例如：http://localhost:8080/demo/login
         * @param param 請求引數 例：{ "userName":"admin", "password":"123456" }
         * @return URL 所代表遠端資源的響應結果
         */
        public static String sendGet(String url, Map<String, Object> param) {
            StringBuilder result = new StringBuilder();
            BufferedReader in = null;
            try {
                URL realUrl = createURL(url, param);
                HttpURLConnection conn = (HttpURLConnection) realUrl.openConnection();
                conn.setRequestProperty("accept", "*/*");
                conn.setRequestProperty("connection", "Keep-Alive");
                conn.setRequestProperty("Charset", "UTF-8");
                conn.connect();
                in = new BufferedReader(new InputStreamReader(
                        conn.getInputStream()));
                String line;
                while ((line = in.readLine()) != null) {
                    result.append(line);
                }
            } catch (Exception e) {
                System.out.println("傳送GET請求出現異常！" + e);
                e.printStackTrace();
            } finally {
                try {
                    if (in != null) {
                        in.close();
                    }
                } catch (Exception e2) {
                    e2.printStackTrace();
                }
            }
            return result.toString();
        }

        /**
         * 向指定 URL 傳送POST方法的請求
         *
         * @param url   傳送請求的URL 例如：http://localhost:8080/demo/login
         * @param param 請求引數 例：{ "userName":"admin", "password":"123456" }
         * @return  所代表遠端資源的響應結果
         */
        public static String sendPost(String url, Map<String, Object> param) {
            BufferedReader in = null;
            StringBuilder result = new StringBuilder();
            try {
                URL realUrl = createURL(url, param);
                // 開啟和URL之間的連線
                HttpURLConnection conn = (HttpURLConnection) realUrl.openConnection();
                // 傳送POST請求必須設定
                conn.setDoOutput(true);
                conn.setDoInput(true);
                conn.setRequestMethod("POST");
                conn.setUseCaches(false);
                conn.setInstanceFollowRedirects(true);
                // 設定通用的請求屬性
                conn.setRequestProperty("Charset", "UTF-8");
                conn.setRequestProperty("Content-Type", "application/json; charset=UTF-8");
                conn.connect();
                // 定義BufferedReader輸入流來讀取URL的響應
                in = new BufferedReader(
                        new InputStreamReader(conn.getInputStream()));
                String line;
                while ((line = in.readLine()) != null) {
                    result.append(line);
                }
            } catch (Exception e) {
                System.out.println("傳送 POST 請求出現異常！" + e);
                e.printStackTrace();
            } finally {
                try {
                    if (in != null) {
                        in.close();
                    }
                } catch (IOException ex) {
                    ex.printStackTrace();
                }
            }
            return result.toString();
        }


        private static URL createURL(String url, Map<String, Object> param) throws MalformedURLException {
            if (param == null || param.size() == 0){
                return new URL(url);
            }
            StringBuilder sb = new StringBuilder(url);
            sb.append("?");
            for (String key : param.keySet()) {
                sb.append(key);
                sb.append("=");
                sb.append(encode(String.valueOf( param.get(key))));
                sb.append("&");
            }
            return new URL(sb.substring(0, sb.length() - 1));
        }

        private static String encode(String text) {
            try {
                return URLEncoder.encode(text, "UTF-8");
            } catch (UnsupportedEncodingException e) {
                e.printStackTrace();
            }
            return null;
        }
    }