Question

我有一个文本文件，每15-16分钟就会有一些json数据更新。这些json数据之间用####行分隔。该文件的片段是：

[{"accountId":"abc","items":[{"serviceName":"XYZ","dataCenter":"TG","startTimeUtc":"2017-04-05T19:57:33.509+0000","endTimeUtc":"2017-04-05T19:57:33.509+0000","usage":[{"resourceName":"XYZ_EE_PAAS_GATEWAYS","quantity":7,"units":"number"}]}]},{"accountId":"XYZp1cm9mbe","items":[{"serviceName":"XYZ","dataCenter":"TG","startTimeUtc":"2017-04-05T19:57:33.509+0000","endTimeUtc":"2017-04-05T19:57:33.509+0000","usage":[{"resourceName":"XYZ_EE_PAAS_GATEWAYS","quantity":6,"units":"number"}]}]}]
######################
[{"accountId":"abc","items":[{"serviceName":"XYZ","dataCenter":"TG","startTimeUtc":"2017-04-05T19:59:33.523+0000","endTimeUtc":"2017-04-05T19:59:33.523+0000","usage":[{"resourceName":"XYZ_EE_PAAS_GATEWAYS","quantity":7,"units":"number"}]}]},{"accountId":"XYZp1cm9mbe","items":[{"serviceName":"XYZ","dataCenter":"TG","startTimeUtc":"2017-04-05T19:59:33.523+0000","endTimeUtc":"2017-04-05T19:59:33.523+0000","usage":[{"resourceName":"XYZ_EE_PAAS_GATEWAYS","quantity":6,"units":"number"}]}]}]
######################
[{"accountId":"abc","items":[{"serviceName":"XYZ","dataCenter":"TG","startTimeUtc":"2017-04-05T20:01:33.531+0000","endTimeUtc":"2017-04-05T20:01:33.531+0000","usage":[{"resourceName":"XYZ_EE_PAAS_GATEWAYS","quantity":7,"units":"number"}]}]},{"accountId":"XYZp1cm9mbe","items":[{"serviceName":"XYZ","dataCenter":"TG","startTimeUtc":"2017-04-05T20:01:33.531+0000","endTimeUtc":"2017-04-05T20:01:33.531+0000","usage":[{"resourceName":"XYZ_EE_PAAS_GATEWAYS","quantity":6,"units":"number"}]}]}]
######################

此文件每15-16分钟更新一次，新条目。我想读取文件并在json对象中存储除####行之外的最新条目。如何在java中做到这一点？我不想使用15分钟的间隔，因为它不是恒定的。

我的简单要求是在任何时候我都会读取文件并想要检索###行上方的最后一个json。

Answer 1

使用Java 8，你可以这样做：

public JsonObject retrieveLastEntry(Path path) throws IOException {
  String[] jsonLines = Files.lines(path)
    .filter(line -> !line.equals("######################")
    .toArray();
  String lastJsonLine = jsonLines[jsonLines.length - 1];
  return MyFavoriteJsonParser.parse(lastJsonLine);
}

MyFavoriteJsonParser指的是您想要使用的任何JSON库（可能需要查看this question）。这里可能没有什么性能考虑因素。如果您的文件非常大（远远超过几MB），那么.toArray()调用可能不适合您。实际上，如果性能非常关键，您甚至可能需要考虑向后解析文件。但性能优化的黄金法则是首先使用一个简单的解决方案，看看它是否（以及在哪里）可能不够高效。

但是，如果您的JSON跨行，则Stream API不是最佳选择。在这种情况下，定期迭代来拯救：

public JsonObject retrieveLastEntry(File file) throws IOException {
  String lastJson = "";
  StringBuffer sb = new StringBuffer();
  try (BufferedReader reader = new BufferedReader(new InputStreamReader(new FileReader(file), "UTF-8")))) {
    String line;
    while ((line = reader.readLine()) != null) {
      if (line.equals("######################") {
        lastJson = sb.toString(); sb.setLength(0);
      } else {
        sb.append(line).append('\n');
      }
   }
   return MyFavoriteJsonParser.parse(lastJsonLine);
}

基本思想是聚合###...之间的行，并在达到新的分隔符时将它们放入变量中。您仍然可能想要考虑根本没有条目并正确处理IOException的情况。

我认为这几乎是惯用的做法。

如何在Java中用分离的json对象解析文本文件？

1 个答案: