OpenAPITools/openapi-generator · error · RuntimeException

Failed to generate .openapi-generator-ignore when the…

Error message

Failed to generate .openapi-generator-ignore when the option `openapiGeneratorIgnoreList` is enabled:

What it means

DefaultGenerator.generateOpenapiGeneratorIgnoreFile (line 1023) runs when the openapiGeneratorIgnoreList option is non-empty: it writes a .openapi-generator-ignore file containing the user's entries, then rebuilds the ignore processor from it. The catch of IOException from Files.newBufferedWriter/write/close is rethrown with this message. This is virtually always a filesystem condition, not a spec problem: the output directory does not exist, is not writable, the disk is full, or the file is locked by another process.

Solutions

Ensure the output directory exists and is writable by the generating process (mkdir -p <out>; ls -ld <out>) before running.
Close any editor/IDE/antivirus holding .openapi-generator-ignore open, or generate into a fresh output directory (-o /tmp/fresh-out) to confirm.
Check disk space (df -h) and mount options (read-only volumes) if running in a container.
If the ignore list is not actually needed, drop the openapiGeneratorIgnoreList option so the default ignore-file path (error [7]) or none applies instead.

Example fix

# before
openapi-generator generate -g java -i api.yaml -o out \
  --additional-properties openapiGeneratorIgnoreList=src/main/old/
# after: pre-create and verify the output directory first
mkdir -p out && openapi-generator generate -g java -i api.yaml -o out \
  --additional-properties openapiGeneratorIgnoreList=src/main/old/

Defensive patterns

Strategy: validation

Validate before calling

// Pre-flight the output directory before enabling openapiGeneratorIgnoreList
Path out = Path.of(config.outputFolder());
Files.createDirectories(out);
Path ignore = out.resolve(".openapi-generator-ignore");
if (Files.exists(ignore) && !Files.isWritable(ignore)) {
    throw new IllegalStateException("Ignore file not writable: " + ignore);
}
if (!Files.isWritable(out)) {
    throw new IllegalStateException("Output dir not writable: " + out);
}

Try / catch

try {
    generator.opts(input).generate();
} catch (RuntimeException e) {
    if (e.getMessage() != null && e.getMessage().contains("openapi-generator-ignore")
            && e.getCause() instanceof java.io.IOException) {
        // filesystem condition: surface actionable message, do not retry blindly
        throw new GenerationFailure("Cannot write ignore file - check output dir permissions", e.getCause());
    }
    throw e;
}

Prevention

Create and permission the output directory in the build script before generation.
On CI, write output to a directory the build user owns.
Close/stop file watchers that lock .openapi-generator-ignore on Windows.

When it happens

Trigger: Setting openapiGeneratorIgnoreList (e.g. --additional-properties openapiGeneratorIgnoreList=<entries>, or config.setOpenapiGeneratorIgnoreList(...)) while the configured output folder does not exist or is read-only; the target .openapi-generator-ignore being held open by an editor/IDE/antivirus on Windows; generating into a read-only Docker volume; ENOSPC.

Common situations: CI pipelines generating into a workspace owned by another user; containerized runs with a read-only bind mount for the output; local runs where the previous output folder is open in a file watcher; disk-full conditions on shared agents.

Related errors

AI-assisted analysis of OpenAPITools/openapi-generator@fcec517be3 (2026-08-22). Data as JSON: /api/errors/19bd2f4d052af8ab. Report an issue: GitHub.

Appendix: source

Thrown at modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultGenerator.java:1084

                    "#docs/*.md",
                    "# Then explicitly reverse the ignore rule for a single file:",
                    "#!docs/README.md",
                    "",
                    "# The following entries are pre-populated based on the input obtained via",
                    "# the option `openapiGeneratorIgnoreList` (--openapi-generator-ignore-list in CLI for example).",
                    "");
            Writer fileWriter = Files.newBufferedWriter(ignoreFile.toPath(), StandardCharsets.UTF_8);
            fileWriter.write(header);
            // add entries provided by the users
            for (String entry : config.getOpenapiGeneratorIgnoreList()) {
                fileWriter.write(entry);
                fileWriter.write("\n");
            }
            fileWriter.close();
            // re-create ignore processor based on the newly-created .openapi-generator-ignore
            this.ignoreProcessor = new CodegenIgnoreProcessor(ignoreFile);
        } catch (IOException e) {
            throw new RuntimeException("Failed to generate .openapi-generator-ignore when the option `openapiGeneratorIgnoreList` is enabled: ", e);
        }
    }

    private void generateSupportingFiles(List<File> files, Map<String, Object> bundle) {
        if (!generateSupportingFiles) {
            // TODO: process these anyway and report via dryRun?
            LOGGER.info("Skipping generation of supporting files.");
            return;
        }

        Set<String> supportingFilesToGenerate = getPropertyAsSet(CodegenConstants.SUPPORTING_FILES);
        for (SupportingFile support : config.supportingFiles()) {
            try {
                String outputFolder = config.outputFolder();
                if (StringUtils.isNotEmpty(support.getFolder())) {
                    outputFolder += File.separator + support.getFolder();
                }
                File of = new File(outputFolder);

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