我使用POST从Java向API发送数据。
我尝试做的是在POST请求中发送一个特定的变量到API，然后使用它的值。但是当前值为空。API肯定被调用了。
我的Java看起来像这样：

String line;
StringBuffer jsonString = new StringBuffer();

try {
    URL url = new URL("https://www.x.com/api.php");
    String payload = "{\"variable1\":\"value1\",\"variable2\":\"value2\"}";

    HttpURLConnection connection = (HttpURLConnection) url.openConnection();

    connection.setDoInput(true);
    connection.setDoOutput(true);
    connection.setRequestMethod("POST");
    connection.setRequestProperty("Accept", "application/json");
    connection.setRequestProperty("Content-Type", "application/json; charset=UTF-8");
    OutputStreamWriter writer = new OutputStreamWriter(connection.getOutputStream(), "UTF-8");
    writer.write(payload);
    writer.close();

    BufferedReader br = new BufferedReader(new InputStreamReader(connection.getInputStream()));
    while ((line = br.readLine()) != null) {
        jsonString.append(line);
    }
    br.close();
    connection.disconnect();
}

这是基于：How to send Request payload to REST API in java?
当前值没有被正确读取。我在Java中正确发送了它吗？我必须做些什么来解码它吗？