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

1、起一个 springboot 程序做 http 测试：

1.     @GetMapping("/http/get")
2.     public ResponseEntity<String> testHttpGet(@RequestParam("param") String param) {
3.         System.out.println(param);
4.         return ResponseEntity.ok("---------> revive http get request --------->");
5.     }
7.     @PostMapping("/http/post")
8.     public ResponseEntity<String> testHttpPost(@RequestBody List<Object> body) {
9.         System.out.println(body);
10.         return ResponseEntity.ok("---------> receive http post request --------->");
11.     }

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2、写一个 HttpURLConnection 自定义客户端

1. import java.io.BufferedReader;
2. import java.io.DataOutputStream;
3. import java.io.IOException;
4. import java.io.InputStream;
5. import java.io.InputStreamReader;
6. import java.net.ConnectException;
7. import java.net.HttpURLConnection;
8. import java.net.URL;
9. import java.net.URLEncoder;
10. import java.nio.charset.StandardCharsets;
11. import java.util.Map;
13. public class MyHttpClient {
15.     private final HttpURLConnection connection;
17.     private MyHttpClient(String url, Map<String, String> params) throws IOException {
18.         connection = buildConnection(url, params);
19.     }
21.     private MyHttpClient(String url, Map<String, String> params, String jsonBody) throws IOException {
22.         connection = buildConnection(url, params);
23.         connection.setRequestMethod("POST");
24.         connection.setRequestProperty("Content-Type", "application/json;charset=utf-8");
25.         connection.setDoOutput(true);
27.         try (DataOutputStream outputStream = new DataOutputStream(connection.getOutputStream())) {
28.             outputStream.writeBytes(jsonBody);
29.             outputStream.flush();
30.         }
31.     }
33.     public static MyHttpClient get(String url, Map<String, String> params) throws IOException {
34.         return new MyHttpClient(url, params);
35.     }
37.     public static MyHttpClient post(String url, Map<String, String> params, String jsonBody) throws IOException {
38.         return new MyHttpClient(url, params, jsonBody);
39.     }
41.     /**
42.      * 创建 http 连接
43.      *
44.      * @param url    请求路径
45.      * @param params 请求参数，可以为空
46.      * @return http 连接
47.      */
48.     private HttpURLConnection buildConnection(String url, Map<String, String> params) throws IOException {
49.         String requestParams = getParamsString(params);
50.         return (HttpURLConnection) new URL(requestParams != null ? url + "?" + requestParams : url).openConnection();
51.     }
53.     /**
54.      * 获取 http 请求响应结果
55.      *
56.      * @return 响应结果，失败抛异常
57.      */
58.     public String getResponse() throws IOException {
59.         int responseCode = connection.getResponseCode();
60.         if (responseCode >= HttpURLConnection.HTTP_OK && responseCode < HttpURLConnection.HTTP_MULT_CHOICE) {
61.             BufferedReader in = new BufferedReader(new InputStreamReader(connection.getInputStream()));
62.             String inputLine;
63.             StringBuilder response = new StringBuilder();
65.             while ((inputLine = in.readLine()) != null) {
66.                 response.append(inputLine);
67.             }
69.             in.close();
71.             connection.disconnect();
72.             return response.toString();
73.         } else {
74.             connection.disconnect();
75.             InputStream errorStream = connection.getErrorStream();
76.             if (errorStream == null) {
77.                 throw new ConnectException("request fail");
78.             }
79.             throw new ConnectException(new String(errorStream.readAllBytes(), StandardCharsets.UTF_8));
80.         }
81.     }
83.     /**
84.      * 拼接请求参数
85.      *
86.      * @param params 参数 map
87.      * @return 请求参数字符串
88.      */
89.     public static String getParamsString(Map<String, String> params) {
90.         if (params == null || params.isEmpty()) {
91.             return null;
92.         }
94.         StringBuilder result = new StringBuilder();
96.         for (Map.Entry<String, String> entry : params.entrySet()) {
97.             result.append(URLEncoder.encode(entry.getKey(), StandardCharsets.UTF_8));
98.             result.append("=");
99.             result.append(URLEncoder.encode(entry.getValue(), StandardCharsets.UTF_8));
100.             result.append("&");
101.         }
103.         String resultString = result.toString();
104.         return resultString.length() > 0
105.                 ? resultString.substring(0, resultString.length() - 1)
106.                 : resultString;
107.     }
108. }

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3、测试 get 和 post 请求

1.     public static void main(String[] args) throws IOException {
3.         MyHttpClient myHttpClient = MyHttpClient.get("http://127.0.0.1:8083/springboot/http/get",
4.                 Map.of("param", "1"));
5.         String resultGet = myHttpClient.getResponse();
6.         System.out.println(resultGet);
9.         MyHttpClient httpClient = MyHttpClient.post("http://127.0.0.1:8083/springboot/http/post",
10.                 null,
11.                 "[1,2,3,4,5]");
12.         String resultPost = httpClient.getResponse();
13.         System.out.println(resultPost);
15.     }

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4、控制台输出结果

1. ---------> revive http get request --------->
2. ---------> receive http post request --------->
4. Process finished with exit code 0

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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。

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