无法使用JSONObject更新.json文件中的子节点

import com.google.gson.Gson;
import com.google.gson.JsonObject;
import com.google.gson.JsonParser;
import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;

public class JsonUpdater {
    public static void main(String[] args) {
        String filePath = "path/to/your/file.json";
        String key = "key";
        String value = "value";

        try {
            // 读取.json文件
            BufferedReader reader = new BufferedReader(new FileReader(filePath));
            StringBuilder jsonString = new StringBuilder();
            String line;
            while ((line = reader.readLine()) != null) {
                jsonString.append(line);
            }
            reader.close();

            // 解析JSON
            JsonParser parser = new JsonParser();
            JsonObject jsonObject = parser.parse(jsonString.toString()).getAsJsonObject();

            // 更新子节点
            jsonObject.addProperty(key, value);

            // 将更新后的JSON对象转换为字符串
            Gson gson = new Gson();
            String updatedJsonString = gson.toJson(jsonObject);

            // 写入.json文件
            BufferedWriter writer = new BufferedWriter(new FileWriter(filePath));
            writer.write(updatedJsonString);
            writer.close();

            System.out.println("子节点更新成功！");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}