CSV File Parsing Issue - csvParser.iterator().next()

Question

Here is the Java code for reading and parsing a CSV file.

String filePath = dir.resolve(filename).toString();
try (FileReader fileReader = new FileReader(filePath);
     CSVParser csvParser = new CSVParser(fileReader, CSVFormat.DEFAULT
         .withHeader(...))) {

    // Check if there is a next element before calling next()
    if (csvParser.iterator().hasNext()) {
        csvParser.iterator().next();
        
        for (CSVRecord record : csvParser) {
        ...
        }
        ...
    } else {
        // Handle the case where there are no more CSV records available
        log.warn("No more CSV records available");
    }
}

The above code will have different outcomes for the same CSV file. That is the condition (csvParser.iterator().hasNext() will return different values between true and false. I just did a test for the same CSV file and the first two times it goes to the line _"No more CSV records available" (I removed the file when this case occurred) and the data in the CSV file is loaded on the third attempt. To my eyes, I can't find any issues in the CSV file.

I've ruled out issues related to file content, format, and headers, there could be other factors contributing to this inconsistency. The additional points I can't consider to be the cause:

Resource Handling: Ensure that the csvParser and fileReader are properly managed and not subject to unexpected closures or resource issues. Double-check for any resource-related exceptions that might affect the behavior of the CSVParser.

Concurrency: If multiple threads or processes are accessing the same file concurrently, it can lead to unpredictable results. Ensure that you have proper synchronization in place to avoid concurrent access issues.

Library Version: Check if you are using an older or less stable version of the CSV parsing library. Updating to a more recent version or switching to a different library might help resolve the issue if it's a library-specific bug.

JVM or Environment Issues: In rare cases, JVM or environment-specific issues could lead to inconsistent behavior. Make sure your JVM and environment are up to date and correctly configured.

External Factors: Consider other factors that might be affecting the behavior, such as system load, file system issues, or other external factors that can impact file access and I/O operations.

I don't know whether this issue relates to how the file is read or not. I use the WatchService to get a CSV file

try {
    String path = System.getProperty("user.home") + "/my_file";
    Path dir = Paths.get(path);
    WatchService watcher = FileSystems.getDefault().newWatchService();
    dir.register(watcher, StandardWatchEventKinds.ENTRY_CREATE);
    log.warn("Monitoring directory: " + dir);

    while (true) {
        WatchKey key = watcher.take();
        boolean oneLoop = processWatchKeyEvents(key, dir);
        boolean valid = key.reset();
        if (!valid) {
            break;
        }
        if (oneLoop) {
            break;
        }
    }
} catch (IOException | InterruptedException e) {
    log.error("Error in monitoring directory: " + e.getMessage());
}

What is a possible cause of this issue?

Answer 1

It is unwise to handle watch service events immediately especially as the WatchService would normally provide DELETE+CREATE+N x MODIFY events for same file operation. As you are ignoring MODIFY you won't know when it is safe(ish) to access your CSV.

You will be better off collating events and handling in separate process queue once the watch service is quiet, and use combination of poll+take. Have a look at this answer which should provide a better trigger point for you to begin operations on the CSV. In the example setListener just prints the activity, replace with your own handler for CSV.