Apache HttpClient 4.x 模擬 HTTP GET/POST 請求完整範例（含 UTF-8 編碼處理）

前言

Apache HttpClient 是 Java 傳統專案常用的 HTTP 客戶端庫（注意：HttpClient 5.x API 已大幅變更，本文針對 4.5.x 版本）。本文示範 GET/POST 基本用法、表單提交、重導向處理、Cookie 策略與自訂 UTF-8 POST 類別。

Maven 依賴

<dependency>
    <groupId>org.apache.httpcomponents</groupId>
    <artifactId>httpclient</artifactId>
    <version>4.5.14</version>
</dependency>
<dependency>
    <groupId>org.apache.httpcomponents</groupId>
    <artifactId>httpmime</artifactId>
    <version>4.5.14</version>
</dependency>

核心程式碼

import org.apache.commons.httpclient.*;
import org.apache.commons.httpclient.methods.*;
import org.apache.commons.httpclient.params.HttpMethodParams;
import org.apache.commons.httpclient.cookie.CookiePolicy;
import java.io.*;

public class HttpClientDemo {
    public static void main(String[] args) {
        HttpClient client = new HttpClient();
        client.getParams().setParameter(HttpMethodParams.RETRY_HANDLER,
            new DefaultHttpMethodRetryHandler(3, false));

        // ---------- GET 請求 ----------
        GetMethod get = new GetMethod("https://httpbin.org/get");
        try {
            int status = client.executeMethod(get);
            if (status == HttpStatus.SC_OK) {
                System.out.println("GET 回應：" + get.getResponseBodyAsString());
            }
        } catch (IOException e) { e.printStackTrace(); }
        finally { get.releaseConnection(); }

        // ---------- POST 表單提交 ----------
        PostMethod post = new UTF8PostMethod("https://httpbin.org/post");
        NameValuePair[] data = {
            new NameValuePair("username", "test_user"),
            new NameValuePair("password", "secret123")
        };
        post.setRequestBody(data);
        post.setRequestHeader("Referer", "https://httpbin.org/");

        try {
            int status = client.executeMethod(post);
            if (status == HttpStatus.SC_OK) {
                System.out.println("POST 回應：" + post.getResponseBodyAsString());
            } else if (status == HttpStatus.SC_MOVED_TEMPORARILY) {
                // 處理 302 重導向
                Header loc = post.getResponseHeader("Location");
                if (loc != null) {
                    PostMethod redirectPost = new UTF8PostMethod(loc.getValue());
                    redirectPost.setRequestBody(data);
                    redirectPost.setRequestHeader("Referer", "https://httpbin.org/post");
                    client.executeMethod(redirectPost);
                    System.out.println("重導向後回應：" + redirectPost.getResponseBodyAsString());
                    redirectPost.releaseConnection();
                }
            }
        } catch (IOException e) { e.printStackTrace(); }
        finally { post.releaseConnection(); }
    }

    // 自訂 UTF-8 POST 類別（解決 4.x 預設 ISO-8859-1 編碼問題）
    static class UTF8PostMethod extends PostMethod {
        public UTF8PostMethod(String url) { super(url); }
        @Override public String getRequestCharSet() { return "UTF-8"; }
    }
}

關鍵點總結

遷移提示：HttpClient 5.x

新專案建議直接使用 org.apache.httpcomponents.client5:httpclient5，API 為 HttpClients.createDefault()、ClassicHttpRequest、ClassicHttpResponse，支援非阻塞 IO 與 HTTP/2。