java 发送 http 请求练习两年半（HttpURLConnection）

1、起一个 springboot 程序做 http 测试：

    @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、写一个 HttpURLConnection 自定义客户端

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);
    }

    /**
     * 创建 http 连接
     *
     * @param url    请求路径
     * @param params 请求参数，可以为空
     * @return http 连接
     */
    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();
    }

    /**
     * 获取 http 请求响应结果
     *
     * @return 响应结果，失败抛异常
     */
    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));
        }
    }

    /**
     * 拼接请求参数
     *
     * @param params 参数 map
     * @return 请求参数字符串
     */
    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、测试 get 和 post 请求

    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、控制台输出结果

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

Process finished with exit code 0

中间遇到一些坑，经常以为 http 会有方法像 openfeign 那样传入请求参数，忽略了路径拼接，

启动的 springboot 接收的 post 的请求体为 List 类型，且 Content-Type 是 json，在测试 post 请求时一直报错，看了 spring 控制台才发现 json 转对象封装 没对上。