java 發送 http 請求練習兩年半（HttpURLConnection）

1、起一個 springboot 程式做 http 測驗：

    @GetMapping("/http/get")
    public ResponseEntity<String> testHttpGet(@RequestParam("param") String param) {
        System.out.println(param);
        return ResponseEntity.ok("---------> revive http get request --------->");
    }

    @PostMapping("/http/post")
    public ResponseEntity<String> testHttpPost(@RequestBody List<Object> body) {
        System.out.println(body);
        return ResponseEntity.ok("---------> receive http post request --------->");
    }

2、寫一個 HttpURLConnection 自定義客戶端

import java.io.BufferedReader;
import java.io.DataOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.ConnectException;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLEncoder;
import java.nio.charset.StandardCharsets;
import java.util.Map;

public class MyHttpClient {

    private final HttpURLConnection connection;

    private MyHttpClient(String url, Map<String, String> params) throws IOException {
        connection = buildConnection(url, params);
    }

    private MyHttpClient(String url, Map<String, String> params, String jsonBody) throws IOException {
        connection = buildConnection(url, params);
        connection.setRequestMethod("POST");
        connection.setRequestProperty("Content-Type", "application/json;charset=utf-8");
        connection.setDoOutput(true);

        try (DataOutputStream outputStream = new DataOutputStream(connection.getOutputStream())) {
            outputStream.writeBytes(jsonBody);
            outputStream.flush();
        }
    }

    public static MyHttpClient get(String url, Map<String, String> params) throws IOException {
        return new MyHttpClient(url, params);
    }

    public static MyHttpClient post(String url, Map<String, String> params, String jsonBody) throws IOException {
        return new MyHttpClient(url, params, jsonBody);
    }

    /**
     * 創建 http 連接
     *
     * @param url    請求路徑
     * @param params 請求引數，可以為空
     * @return http 連接
     */
    private HttpURLConnection buildConnection(String url, Map<String, String> params) throws IOException {
        String requestParams = getParamsString(params);
        return (HttpURLConnection) new URL(requestParams != null ? url + "?" + requestParams : url).openConnection();
    }

    /**
     * 獲取 http 請求回應結果
     *
     * @return 回應結果，失敗拋例外
     */
    public String getResponse() throws IOException {
        int responseCode = connection.getResponseCode();
        if (responseCode >= HttpURLConnection.HTTP_OK && responseCode < HttpURLConnection.HTTP_MULT_CHOICE) {
            BufferedReader in = new BufferedReader(new InputStreamReader(connection.getInputStream()));
            String inputLine;
            StringBuilder response = new StringBuilder();

            while ((inputLine = in.readLine()) != null) {
                response.append(inputLine);
            }

            in.close();

            connection.disconnect();
            return response.toString();
        } else {
            connection.disconnect();
            InputStream errorStream = connection.getErrorStream();
            if (errorStream == null) {
                throw new ConnectException("request fail");
            }
            throw new ConnectException(new String(errorStream.readAllBytes(), StandardCharsets.UTF_8));
        }
    }

    /**
     * 拼接請求引數
     *
     * @param params 引數 map
     * @return 請求引數字串
     */
    public static String getParamsString(Map<String, String> params) {
        if (params == null || params.isEmpty()) {
            return null;
        }

        StringBuilder result = new StringBuilder();

        for (Map.Entry<String, String> entry : params.entrySet()) {
            result.append(URLEncoder.encode(entry.getKey(), StandardCharsets.UTF_8));
            result.append("=");
            result.append(URLEncoder.encode(entry.getValue(), StandardCharsets.UTF_8));
            result.append("&");
        }

        String resultString = result.toString();
        return resultString.length() > 0
                ? resultString.substring(0, resultString.length() - 1)
                : resultString;
    }
}

3、測驗 get 和 post 請求

    public static void main(String[] args) throws IOException {

        MyHttpClient myHttpClient = MyHttpClient.get("http://127.0.0.1:8083/springboot/http/get",
                Map.of("param", "1"));
        String resultGet = myHttpClient.getResponse();
        System.out.println(resultGet);


        MyHttpClient httpClient = MyHttpClient.post("http://127.0.0.1:8083/springboot/http/post",
                null,
                "[1,2,3,4,5]");
        String resultPost = httpClient.getResponse();
        System.out.println(resultPost);

    }

4、控制臺輸出結果

---------> revive http get request --------->
---------> receive http post request --------->

Process finished with exit code 0

中間遇到一些坑，經常以為 http 會有方法像 openfeign 那樣傳入請求引數，忽略了路徑拼接，

啟動的 springboot 接收的 post 的請求體為 List 型別，且 Content-Type 是 json，在測驗 post 請求時一直報錯，看了 spring 控制臺才發現 json 轉物件封裝 沒對上，

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