Java请求遇Connection timed out错误，疑Payload有误，附代码求助

我用Java发起请求时碰到了java.net.ConnectException: Connection timed out错误，怀疑是请求Payload创建有问题。我参考下面的Curl请求写了代码，附上我的Java代码，帮忙排查下。

参考的Curl请求 #

curl --location --request POST 'https://Vishal.net/v1/oauth20/token' \
--header 'Content-Type: application/x-www-form-urlencoded' \
--data-urlencode 'grant_type=client_credentials' \
--data-urlencode 'client_secret=d4j8Q~po9Tm3PEShtcHcaeBZW1cuYtHKcbA0FcwG' \
--data-urlencode 'client_id=9572e860-7dd7-45ff-a9c7-e9c09a915a4c' \
--data-urlencode 'scope=api://e3981169-cbc9-4e08-aa35-620714eab5bb/.default'

我的Java代码 #

public String getToken() throws IOException, ServletException{
            String output = "";
            URL url = null;
            try {
                url = new URL(baseUrl+tokenSubpath);
                createLogFile(baseUrl+tokenSubpath);
            } catch (MalformedURLException e) {
                // TODO Auto-generated catch block
                return"Error: "+ e.toString();
            }
            System.setProperty("sun.net.http.allowRestrictedHeaders", "true");
            HttpURLConnection connection = null;
            try {
                //If we need to connect through proxy uncomment the below line
                connection = (HttpURLConnection) url.openConnection();
            } catch (IOException e) {
                // TODO Auto-generated catch block
                return "Error: "+e.toString();
            }
            connection.setInstanceFollowRedirects(false);
            connection.setDoOutput(true); // Triggers POST.
            try {
                connection.setRequestMethod("POST");
            } catch (IOException e) {
                // TODO Auto-generated catch block
                return "Error: "+e.toString();
            }
            String Bearer = new String(Base64.getEncoder().encode(authUserPass.getBytes()));
              //connection.setRequestProperty("Authorization", "Basic "+Bearer); //vishal bedre 8/8/2022
            connection.setRequestProperty("Authorization", "");
            
            connection.setRequestProperty("Content-Type", "application/x-www-form-urlencoded;charset=UTF-8");
            connection.setRequestProperty("Accept", "application/json");
            connection.setRequestProperty("Content-Length", "");  //vishal bedre 8/8/2022
            connection.setRequestProperty("Host", "");  //vishal bedre 8/8/2022
            connection.setRequestProperty("Accept", "*/*");  //vishal bedre 8/8/2022
            connection.setUseCaches(false);
            
            String urlParameters="grant_type=client_credentials&client_secret=d4j8Q~po9Tm3PEShtcHcaeBZW1cuYtHKcbA0FcwG&client_id=9572e860-7dd7-45ff-a9c7-e9c09a915a4c&scope=api://e3981169-cbc9-4e08-aa35-620714eab5bb/.default";
            
            try(OutputStream os = connection.getOutputStream()) {
                //os.write(postData);
                OutputStreamWriter osw = new OutputStreamWriter(os, "UTF-8");    
                //osw.write("grant_type=client_credentials");
                osw.write(urlParameters);
                osw.flush();
                osw.close();
                os.close();  //don't forget to close the OutputStream
                connection.connect();           
            } catch (IOException e) {
                // TODO Auto-generated catch block
                return "Error: "+e.toString();
            }
}

排查方向 #

• 优先排查网络层面：Connection timed out多数是网络不通导致，先确认Java程序所在机器能访问目标域名https://Vishal.net，比如在同一机器执行原Curl命令看是否能成功。如果机器需要代理，代码里未配置代理也会触发超时。
• 修正错误的请求头配置：
  • 删掉connection.setRequestProperty("Authorization", "");，空的Authorization头可能被服务器拦截，原Curl请求也未携带该头，无需设置。
  • 重复设置了Accept头，保留一个即可（比如保留application/json）。
  • 删掉手动设置的空Content-Length和Host头，HttpURLConnection会自动处理这两个字段，手动设空值可能引发异常。
• 规范Payload编码：原Curl用了--data-urlencode对参数编码，Java代码里直接拼接参数可能存在编码隐患，建议对每个参数值单独用URLEncoder.encode(参数值, "UTF-8")编码后再拼接。
• 清理冗余代码：try-with-resources会自动关闭OutputStream，无需手动调用os.close()和osw.close()；connection.connect()无需手动调用，获取输出流或响应时会自动发起连接。

内容的提问来源于stack exchange，提问作者Vishal Bedre