Java发送带JSON body的POST请求返回400错误如何解决

问题根因分析 #

• 输出流未主动刷新关闭：请求体写入OutputStream后没有调用flush()或close()，部分场景下会导致请求体未完整发送到服务端
• 错误响应读取逻辑缺失：HTTP 400属于客户端错误响应，此时直接调用getInputStream()会直接抛出IO异常，无法读取服务端返回的具体错误提示，无法定位根因
• 响应读取逻辑错误：使用in.ready()判断是否结束读取是错误用法，该方法仅判断当前流是否有可读取的字节，不等同于流已传输完成，会出现响应读取不全的问题
• Content-Type属性缺失字符集声明：部分服务端会严格校验请求头的字符集，未声明utf-8可能导致请求体解析失败

修复后的代码 #

public class PostRequest {

    public void sendPost(String Url) throws IOException {
        FileWriter myWriter = null;
        String name = "{\"messageName\": \"URLFound\", \"businessKey\": \"3\"}";
        
        try {
            myWriter = new FileWriter("C:\\Users\\test\\Desktop\\camunda test save\\Post.txt");
            // 这里建议先校验Url参数是否以/开头，避免拼接错误
            URL url = new URL("http://localhost:8080/engine-rest" + Url);
            HttpURLConnection http = (HttpURLConnection) url.openConnection();
            
            http.setRequestMethod("POST");
            http.setDoOutput(true);
            // 补充charset声明
            http.setRequestProperty("Content-Type", "application/json; utf-8");
            // 可选添加Accept头，声明接收JSON响应
            http.setRequestProperty("Accept", "application/json");
            
            // try-with-resources自动关闭输出流，自动flush
            try (OutputStream os = http.getOutputStream()) {
                byte[] input = name.getBytes("utf-8");
                os.write(input, 0, input.length);
            }
            
            // 先获取响应码，区分正常响应和错误响应
            int responseCode = http.getResponseCode();
            StringBuilder response = new StringBuilder();
            // 错误响应读ErrorStream
            try (BufferedReader in = new BufferedReader(
                    new InputStreamReader(responseCode < 400 ? http.getInputStream() : http.getErrorStream()))) {
                String line;
                // 正确的流结束判断逻辑
                while ((line = in.readLine()) != null) {
                    response.append(line);
                }
            }
            
            // 写入结果，把响应码也写上方便排查
            myWriter.append("响应码：").append(String.valueOf(responseCode))
                    .append("\n响应内容：").append(response.toString())
                    .append("\n请求URL：").append(url.toString());

        } catch (Exception e) {
            if (myWriter != null) {
                myWriter.append("请求异常：").append(e.toString());
            }
            e.printStackTrace();
        } finally {
            if (myWriter != null) {
                myWriter.close();
            }
        }
    }
}

额外排查建议 #

修复后运行代码，查看Post.txt中记录的服务端错误响应内容，即可明确是请求体字段错误、参数缺失还是鉴权等其他问题导致的400报错。

内容的提问来源于stack exchange，提问作者Hadi Hoteit