Java发送带JSON的HTTP POST请求返回400 Bad Request问题求助

已定位的问题点 #

• 请求头Fiware-Service的值存在多余前置空格：两处设置的" myHome"前面有一个空格，Fiware平台对服务头校验严格，非法的头格式会直接返回400错误
• POST请求缺少错误响应读取逻辑：400属于异常状态码，直接调用getInputStream()会抛出IO异常，无法获取服务端返回的具体错误提示，无法进一步定位问题
• 变量命名冲突：类级别定义了静态HttpURLConnection connection，两个请求方法中又分别定义了同名局部变量，容易出现资源泄漏、请求状态串用的问题
• 手动拼接JSON字符串存在隐患：虽然当前JSON格式无误，但后续修改容易出现转义错误、语法错误

修复步骤 #

1. 修正Fiware-Service请求头，删除值前面的空格，将两处的" myHome"修改为"myHome"
2. 给POST请求添加状态码判断逻辑，异常状态下读取错误流获取具体报错信息，参考GET请求的处理逻辑
3. 删除类级别定义的静态HttpURLConnection connection变量，所有连接对象使用方法内局部变量即可
4. （可选优化）使用Jackson、Gson等JSON序列化库生成请求JSON，避免手动拼接的语法问题

核心修改代码示例 #

修改后的PostRequest方法 #

public void PostRequest() throws IOException {
    //set the url
    String url = "http://192.168.0.104:4041/iot/devices";
    URL obj = new URL(url);
    //open the connection
    HttpURLConnection connection = (HttpURLConnection) obj.openConnection();
    //set the method type(POST OR GET)
    connection.setRequestMethod("POST");

    //set the request properties(headers etc.)
    connection.setRequestProperty("Content-Type", "application/json; utf-8");
    connection.setRequestProperty("Accept", "application/json");
    // 修复头的多余空格
    connection.setRequestProperty("Fiware-Service","myHome");
    connection.setRequestProperty("Fiware-ServicePath","/environment");

    //make sure that will be able to write content to the connection output stream
    connection.setDoOutput(true);

    //Json formatted input string
    String jsonInputString = "{\n" +
        "    \"devices\": [\n" +
        "        {\n" +
        "            \"device_id\": \"sensor01\",\n" +
        "            \"entity_name\": \"LivingRoomSensor\",\n" +
        "            \"entity_type\": \"multiSensor\",\n" +
        "            \"attributes\": [\n" +
        "                  { \"object_id\": \"t\", \"name\": \"Temperature\", \"type\": \"celsius\" },\n" +
        "                  { \"object_id\": \"l\", \"name\": \"Luminosity\", \"type\": \"lumens\" }\n" +
        "            ]\n" +
        "        }\n" +
        "    ]\n" +
        "}";

    try(OutputStream os = connection.getOutputStream()) {
        byte[] input = jsonInputString.getBytes(StandardCharsets.UTF_8);
        os.write(input, 0, input.length);
    }

    // 新增状态码判断逻辑，获取错误提示
    int status = connection.getResponseCode();
    BufferedReader reader;
    StringBuilder response = new StringBuilder();
    if (status > 299) {
        reader = new BufferedReader(new InputStreamReader(connection.getErrorStream(), StandardCharsets.UTF_8));
    } else {
        reader = new BufferedReader(new InputStreamReader(connection.getInputStream(), StandardCharsets.UTF_8));
    }
    String responseLine = null;
    while ((responseLine = reader.readLine()) != null) {
        response.append(responseLine.trim());
    }
    reader.close();
    //print the result
    System.out.println(response.toString());
}

内容的提问来源于stack exchange，提问作者Nedislav Kolev