java - 当 JSON 为 null 时尝试从中提取

Question

将尝试在这里解释我的问题。我有一个程序，应该解析我从网络爬虫收到的传入 JSON 文件。

    public static void Scan(Article article) throws Exception
{
    //When running program, creates a error text-file inside java Project folder
    File file = new File("errorlogg.txt");
    FileWriter fileWriter = new FileWriter(file, true);

    // if file doesn't exists, then create it
    if (!file.exists()) 
    {
        file.createNewFile();
    }

    //Setting up an URL HttpURLConnection given DOI
    URL urlDoi = new URL (article.GetElectronicEdition());

    //Used for debugging 
    System.out.println("Initial DOI: " + urlDoi);

    //Transform from URL to String
    String doiCheck = urlDoi.toString();

    //Redirect from IEEE Xplore toe IEEE Computer Society
    if(doiCheck.startsWith("http://dx."))
    {
        doiCheck = doiCheck.replace("http://dx.doi.org/", "http://doi.ieeecomputersociety.org/");
        urlDoi = new URL(doiCheck);
    }

    HttpURLConnection connDoi = (HttpURLConnection) urlDoi.openConnection();

    // Make the logic below easier to detect redirections
    connDoi.setInstanceFollowRedirects(false);  

    String doi = "{\"url\":\"" + connDoi.getHeaderField("Location") + "\",\"sessionid\":\"abc123\"}";

    //Setting up an URL to translation-server
    URL url = new URL("http://127.0.0.1:1969/web");
    URLConnection conn = url.openConnection();

    conn.setDoOutput(true);
    conn.setRequestProperty("Content-Type", "application/json");

    OutputStreamWriter writer = new OutputStreamWriter(conn.getOutputStream());

    writer.write(doi);
    writer.flush();

    String line;
    BufferedReader reader = new BufferedReader(new InputStreamReader(conn.getInputStream()));



    while ((line = reader.readLine()) != null ) 
    {
        //Used to see of we get something from stream
        System.out.println(line);

        //Incoming is JSONArray, so create new array and parse fill it with incoming information
        JSONArray jsonArr = new JSONArray(line);
        JSONObject obj = jsonArr.getJSONObject(0);

        //Check if names from DBLP is the same as translators get
        //AuthorName, from field creators
        JSONArray authorNames = obj.getJSONArray("creators");
        ArrayList<Author> TranslationAuthors = new ArrayList<Author>();

这是我正在谈论的代码片段。正如你所看到的，当我从缓冲区读取器获得一些信息时，我想运行这段代码。我的问题是，当我没有获得有效的 JSON 时，我的程序似乎不会跳过。相反，它运行到这行代码:

        JSONArray authorNames = obj.getJSONArray("creators")

然后被迫退出，因为它无法获取“creators”字段，因为没有字段。我怎样才能确保我的程序不会遇到这个问题？我如何轻松地将其放入我创建的错误日志文件中，以至于我无法收集任何信息。

Answer 1

我认为您正在使用 org.json.JSONObject？如果是这样，则有一个 has 方法，可用于在 key 不存在时避免 JSONException 。

 JSONArray authorNames = null;
 if (obj.has("creators")) {
   authorNames = obj.getJSONArray("creators");
 }