Java 發送 HTTP 請求兩大方式：HttpClient 與 HttpURLConnection

前言

Java 原生提供 HttpURLConnection，輕量零依賴；Apache HttpClient 功能豐富、API 友善。兩者皆可發送 GET/POST 請求並處理回應。本文提供兩套完整、可直接複製使用的工具類。

方式一：Apache HttpClient 4.5+ (推薦)

Maven 依賴：org.apache.httpcomponents:httpclient:4.5.14

import org.apache.http.HttpEntity;
import org.apache.http.NameValuePair;
import org.apache.http.client.entity.UrlEncodedFormEntity;
import org.apache.http.client.methods.CloseableHttpResponse;
import org.apache.http.client.methods.HttpPost;
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 java.util.ArrayList;
import java.util.List;

public class HttpClientUtil {
    /** 發送 POST 表單請求
     * @param url  目標 URL
     * @param params 參數鍵值對
     * @return 回應字串
     */
    public static String post(String url, List params) {
        try (CloseableHttpClient client = HttpClients.createDefault()) {
            HttpPost httpPost = new HttpPost(url);
            httpPost.setEntity(new UrlEncodedFormEntity(params, "UTF-8")); // 編碼自行調整
            
            // 設定超時 (可選)
            RequestConfig config = RequestConfig.custom()
                    .setConnectTimeout(5000)
                    .setSocketTimeout(10000)
                    .build();
            httpPost.setConfig(config);

            try (CloseableHttpResponse response = client.execute(httpPost)) {
                HttpEntity entity = response.getEntity();
                if (entity != null) {
                    return EntityUtils.toString(entity, "UTF-8");
                }
            }
        } catch (Exception e) {
            throw new RuntimeException("HTTP POST 失敗: " + url, e);
        }
        return null;
    }

    // 使用範例
    public static void main(String[] args) {
        List params = new ArrayList<>();
        params.add(new BasicNameValuePair("user", "admin"));
        params.add(new BasicNameValuePair("action", "login"));
        String result = post("https://api.example.com/endpoint", params);
        System.out.println("回應：" + result);
    }
}

方式二：原生 HttpURLConnection (零依賴)

import java.io.*;
import java.net.HttpURLConnection;
import java.net.URL;
import java.nio.charset.StandardCharsets;
import java.util.Map;

public class HttpUrlConnectionUtil {
    /** 通用請求方法
     * @param urlString 目標 URL
     * @param method    "GET" 或 "POST"
     * @param params    參數 Map (GET 會拼接到 URL，POST 寫入 Body)
     * @return 回應字串
     */
    public static String request(String urlString, String method, Map params) {
        HttpURLConnection conn = null;
        try {
            // 處理 GET 參數拼接
            String url = urlString;
            if ("GET".equalsIgnoreCase(method) && params != null && !params.isEmpty()) {
                url += (url.contains("?") ? "&" : "?") + buildQueryString(params);
            }
            
            URL urlObj = new URL(url);
            conn = (HttpURLConnection) urlObj.openConnection();
            conn.setRequestMethod(method);
            conn.setConnectTimeout(5000);
            conn.setReadTimeout(10000);
            conn.setRequestProperty("Content-Type", "application/x-www-form-urlencoded; charset=UTF-8");
            conn.setRequestProperty("User-Agent", "Java-HttpClient/1.0");

            // POST 寫入 Body
            if ("POST".equalsIgnoreCase(method) && params != null && !params.isEmpty()) {
                conn.setDoOutput(true);
                try (OutputStream os = conn.getOutputStream()) {
                    os.write(buildQueryString(params).getBytes(StandardCharsets.UTF_8));
                }
            }

            // 讀取回應
            int code = conn.getResponseCode();
            InputStream is = (code >= 200 && code < 300) ? conn.getInputStream() : conn.getErrorStream();
            try (BufferedReader br = new BufferedReader(new InputStreamReader(is, StandardCharsets.UTF_8))) {
                StringBuilder sb = new StringBuilder();
                String line;
                while ((line = br.readLine()) != null) sb.append(line);
                return sb.toString();
            }
        } catch (Exception e) {
            throw new RuntimeException("HTTP 請求失敗: " + urlString, e);
        } finally {
            if (conn != null) conn.disconnect();
        }
    }

    private static String buildQueryString(Map params) {
        StringBuilder sb = new StringBuilder();
        for (Map.Entry e : params.entrySet()) {
            if (sb.length() > 0) sb.append('&');
            sb.append(e.getKey()).append('=').append(e.getValue());
        }
        return sb.toString();
    }
}

選型建議

常見坑點提醒

• ⚠️ 編碼一致性：請求 Body、URL 參數、回應讀取皆指定 UTF-8
• ⚠️ 資源釋放：HttpClient 用 try-with-resources；HttpURLConnection 必須 disconnect()
• ⚠️ 超時設定：務必設定連線與讀取超時，避免執行緒無限期阻塞
• ⚠️ HTTPS 憑證：測試環境若遇自簽憑證，需配置 TrustManager 或信任全部 (僅測試用)

根據專案需求選擇合適工具，並建立統一的 HTTP 工具類，即可輕鬆應對各類介面串接。