Question

如何从包含JSON记录数组的JSON文件中获取第一条记录

示例文件：

[
{"l7ProtocolID":"dhcp","packets_out":1,"bytes_out":400,"start_time":1454281199898,"flow_sample":0,"duration":102,"path":["base","ip","udp","dhcp"],"bytes_in":1200,"l4":[{"client":"68","server":"67","level":0}],"l2":[{"client":"52:54:00:50:04:B2","server":"FF:FF:FF:FF:FF:FF","level":0}],"l3":[{"client":"::ffff:0.0.0.0","server":"::ffff:255.255.255.255","level":0}],"flow_id":"81454281200000731489","applicationID":"dhcp","packets_in":1}
{"l7ProtocolID":"dhcp","packets_out":1,"bytes_out":400,"start_time":1454281199898,"flow_sample":0,"duration":102,"path":["base","ip","udp","dhcp"],"bytes_in":1200,"l4":[{"client":"68","server":"67","level":0}],"l2":[{"client":"52:54:00:50:04:B2","server":"FF:FF:FF:FF:FF:FF","level":0}],"l3":[{"client":"::ffff:0.0.0.0","server":"::ffff:255.255.255.255","level":0}],"flow_id":"81454281200000731489","applicationID":"dhcp","packets_in":1}
Record n.....
]

同样地，文件中可能有1000多条记录。

我想从此文件中获取第一条记录。

Answer 1

以下代码不会将整个文件作为String加载到内存中。虽然，整个阵列都在内存中。例如，Gson一次将大约10KB的文件字节加载到缓冲区中，并解析每一行并添加到数组中。但是，所有1000个对象都将在数组的堆上。

部分流适合大多数情况

public static void readDom() {
    BufferedReader reader = null;
    try {
        reader = new BufferedReader(new FileReader(file));
        Gson gson = new GsonBuilder().create();
        Person[] people = gson.fromJson(reader, Person[].class);

        System.out.println("Object mode: " + people[0]);

    } catch (FileNotFoundException ex) {
        ...
    } finally {
        ...
    }
}

以上代码比下面的代码更有效：

一次性读取（仅适用于小文件）

String fileContents = FileUtils.readAsString(file);
Person[] persons = gson.fromJson(fileContents, Person[].class);

对于一次最多5k-10k行，第一种方法可能没问题。但是，超过10k，即使是第一种方法也可能不是很好。

第三种选择最适合大数据。一次迭代并读取一行，并随时停止。

True Streaming

public static void readStream() {
    try {
        JsonReader reader = new JsonReader(new InputStreamReader(stream, "UTF-8"));
        Gson gson = new GsonBuilder().create();

        // Read file in stream mode
        reader.beginArray();
        while (reader.hasNext()) {
            // Read data into object model
            Person person = gson.fromJson(reader, Person.class);
            if (person.getId() == 0 ) {
                System.out.println("Stream mode: " + person);
            }
            break;
        }
        reader.close();
    } catch (UnsupportedEncodingException ex) {
        ...
    } catch (IOException ex) {
        ...
    }
}

来源：Reading JSON as Stream using GSON

处理JSON解析而不匹配POJO结构

如果您不想创建匹配的POJO对象图结构，可以指示GSON将每一行视为HashMap。

Type type = new TypeToken<Map<String, Object>>(){}.getType();
Map<String, Object> thisRow = gson.fromJson(reader, type);

Answer 2

使用org.JSON.simple库

获得解决方案

public String ReadJsonFromFile(String fileName){
    JSONParser parser = new JSONParser();
    String firstRecord = null;
    try {
        JSONArray jsonArray = (JSONArray) parser.parse(new FileReader(fileName));
        JSONObject jsonObject = (JSONObject) jsonArray.get(0);  
        firstRecord = jsonObject.toString();

    } catch (FileNotFoundException e) {
        LOG.info("JSON -> Can't read from file: File Not Found");
        e.printStackTrace();
    } catch (IOException e) {
        LOG.info("JSON -> Can't read File : IO Exception");
        e.printStackTrace();
    } catch (ParseException e) {
        LOG.info("JSON -> Can't Parse JSON in File");
        e.printStackTrace();
    }

    return firstRecord;

}

从JAVA中的json文件中读取Json记录数组中的第一条记录

2 个答案: