Java实现JSON转CSV时触发NullPointerException求助

Hey there! Let's work through that NullPointerException you're hitting when converting JSON to CSV with the json-to-csv library. Here are the most likely issues and fixes to try out:

Common Causes & Fixes #

1. Your flatJson variable might be null #

The JSONFlattener.parseJson() method will return null if it can't read or parse your JSON file properly. Passing a null flatJson to CSVWriter.getCSV() will immediately trigger a NullPointerException.

Fix: Add a validity check before calling CSV methods, and wrap the parsing step in error handling to catch exactly what's going wrong:

List<Map<String, String>> flatJson = null;
try {
    // Fix the file path here too (see next point!)
    flatJson = JSONFlattener.parseJson(new File("C:/Users/file/path/input.json"), "UTF-8");
} catch (IOException e) {
    e.printStackTrace();
    System.err.println("Couldn't read the JSON file: " + e.getMessage());
} catch (Exception e) {
    e.printStackTrace();
    System.err.println("Failed to parse JSON content: " + e.getMessage());
}

// Verify flatJson is valid before proceeding
if (flatJson == null) {
    System.err.println("JSON parsing failed - flatJson is null!");
    return;
}

// Now execute the CSV writer code safely
CSVWriter.writeToFile(CSVWriter.getCSV(flatJson, ";"), "C:/Users/file/path/output.csv");

2. Your file path has an error #

Windows file paths shouldn't start with a leading slash. Your current path /C:/Users/... is invalid—remove the first slash to make it C:/Users/file/path/input.json. Alternatively, use escaped double backslashes like C:\\Users\\file\\path\\input.json.

An invalid path means the library can't locate your JSON file, so parseJson() returns null, leading directly to the NPE.

3. Your JSON file has syntax issues #

If your input.json has invalid JSON (missing commas, unclosed quotes, mismatched brackets), the parser will fail silently and return null.

Fix: Use a JSON validator tool to check your file for syntax errors. Fix any issues (like missing brackets or quote mismatches) before trying to parse it again.

Quick Debugging Tip #

To narrow down the root cause, print the flatJson variable right after parsing:

System.out.println("Parsed flatJson content: " + flatJson);

If it prints null, you know the problem lies in the parsing step (either path or JSON format). If it shows a populated list of maps, the issue might be within the CSVWriter methods themselves—but that's far less likely.

内容的提问来源于stack exchange，提问作者chris_evans