JSON转换为CSV错误-java.lang.NullPointerException

Question

I am trying to convert a JSON file to a CSV file using Java. I am newbie.

I am following https://github.com/Arkni/json-to-csv tutorial.

I'm getting a NullPointerException.

Any help is appreciated.

My code:

test.java

List<Map<String, String>> flatJson;
flatJson = JSONFlattener.parseJson(new File("/C:/Users/file/path/input.json"), "UTF-8");
!!---CSVWriter.writeToFile(CSVWriter.getCSV(flatJson, ";"), "/C:/Users/file/path/output.csv");--!!

Error_line: CSVWriter.writeToFile(CSVWriter.getCSV(flatJson, ";"), "/C:/Users/file/path/output.csv");

CSVWriter.java:

 private static final Logger LOGGER = Logger.getLogger(CSVWriter.class);

    public static String getCSV(List<Map<String, String>> flatJson) {
        return getCSV(flatJson, ",");
    }

    public static String getCSV(List<Map<String, String>> flatJson, String separator) {
       !!!-- ***Set<String> headers = collectHeaders(flatJson);***---!!!!
        String csvString = StringUtils.join(headers.toArray(), separator) + "\n";

     !!!!---   ***for (Map<String, String> map : flatJson) {***---!!!
            csvString = csvString + getSeperatedColumns(headers, map, separator) + "\n";
        }

        return csvString;
    }

    public static void writeToFile(String csvString, String fileName) {
        try {
            FileUtils.write(new File(fileName), csvString);
        } catch (IOException e) {
            LOGGER.error("CSVWriter#writeToFile(csvString, fileName) IOException: ", e);
        }
    }

    private static String getSeperatedColumns(Set<String> headers, Map<String, String> map, String separator) {
        List<String> items = new ArrayList<String>();
        for (String header : headers) {
            String value = map.get(header) == null ? "" : map.get(header).replaceAll("[\\,\\;\\r\\n\\t\\s]+", " "); 
            items.add(value);
        }

        return StringUtils.join(items.toArray(), separator);
    }

    private static Set<String> collectHeaders(List<Map<String, String>> flatJson) {
        Set<String> headers = new LinkedHashSet<String>();

        for (Map<String, String> map : flatJson) {
            headers.addAll(map.keySet());
        }

        return headers;
    }

Error_line:

Set<String> headers = collectHeaders(flatJson);

for (Map<String, String> map : flatJson) {

ERROR:

Exception in thread "main" java.lang.NullPointerException
    at test2.CSVWriter.collectHeaders(CSVWriter.java:134)
    at test2.CSVWriter.getCSV(CSVWriter.java:49)
    at test2.test2.main(test.java:20)

Answer 1

I suspect the JSON file you are trying to read either does not exist, there is some other I/O error reading it in or it contains malformed JSON.

Unfortunately, the error-handling in the parseJson method you are using leaves a lot to be desired. Here's the entire text of it ( source ):

public static List<Map<String, String>> parseJson(URI uri, String encoding) {
    List<Map<String, String>> flatJson = null;
    String json = "";

    try {
        json = IOUtils.toString(uri, encoding);
        flatJson = parseJson(json);
    } catch (IOException e) {
        LOGGER.error("JsonFlattener#ParseJson(uri, encoding) IOException: ", e);
    } catch (Exception ex) {
        LOGGER.error("JsonFlattener#ParseJson(uri, encoding) Exception: ", ex);
    }

    return flatJson;
}

If an exception is thrown reading in the file or parsing the JSON, it writes a message to LOGGER (which is a Log4J logger) and then returns null . As you describe yourself as a newbie, I'm going to guess Log4J isn't quite set up correctly (you may have seen a message about log4j.properties or log4j.xml file not being found), so the logging isn't working, the error message gets lost and you don't find out what the problem is.

As a first step I would simply avoid calling this method. Instead of writing

List<Map<String, String>> flatJson;
flatJson = JSONFlattener.parseJson(new File("/C:/Users/file/path/input.json"), "UTF-8");
CSVWriter.writeToFile(CSVWriter.getCSV(flatJson, ";"), "/C:/Users/file/path/output.csv");

write the following instead:

List<Map<String, String>> flatJson;
String json = IOUtils.toString(new File("/C:/Users/file/path/input.json"), "UTF-8");
flatJson = JSONFlattener.parseJson(json);
CSVWriter.writeToFile(CSVWriter.getCSV(flatJson, ";"), "/C:/Users/file/path/output.csv");

You may need to add a line import org.apache.commons.io.IOUtils; and handle some more exceptions, but those exceptions should hopefully tell you what is going wrong.

However, having looked further at the source code of this library, this may not help too much. There are other places where it simply catches Exception , writes a generic message to the log and returns null . You may have to look elsewhere for a better alternative library.