OpenAPITools/openapi-generator · critical · RuntimeException

Cannot access tmp folder

Error message

Cannot access tmp folder

What it means

Thrown when Files.createTempDirectory("codegen-tmp") fails (Generator.java:180-188) — the JVM cannot create a directory under java.io.tmpdir. It escapes as a plain RuntimeException (not a ResponseStatusException), so it surfaces as an HTTP 500 on every generate call. Because each generation allocates a fresh temp dir first, a broken tmp location takes the whole generate feature down.

Source

Thrown at modules/openapi-generator-online/src/main/java/org/openapitools/codegen/online/service/Generator.java:187

            try {
                new File(outputFolder).delete();
            } catch (Exception e) {
                LOGGER.error("unable to delete output folder " + outputFolder, e);
            }
        } catch (Exception e) {
            throw new ResponseStatusException(HttpStatus.BAD_REQUEST, "Unable to build target: " + e.getMessage(), e);
        }
        return outputFilename;
    }

    private static File getTmpFolder() {
        try {
            File outputFolder = Files.createTempDirectory("codegen-tmp").toFile();
            outputFolder.deleteOnExit();
            return outputFolder;
        } catch (Exception e) {
            e.printStackTrace();
            throw new RuntimeException("Cannot access tmp folder");
        }
    }
}

View on GitHub (pinned to fcec517be3)

Solutions

1. Verify the service can write to its tmp dir: ls -ld $TMPDIR and touch as the service user
2. In containers, mount a writable volume or tmpfs at /tmp (e.g. docker run --tmpfs /tmp:rw,size=512m)
3. Free disk space or raise the tmpfs size limit on the host
4. As a last resort, point java.io.tmpdir at a known-writable volume via -Djava.io.tmpdir=/var/tmp/openapi-gen

Example fix

# before: container with read-only root fs
 docker run --read-only my-openapi-generator-online   # every generate -> 500 Cannot access tmp folder

# after
 docker run --read-only --tmpfs /tmp:rw,size=512m my-openapi-generator-online

// health check for self-hosted instances: can this JVM create its temp dir?
try {
    Files.createTempDirectory("codegen-tmp-probe").toFile().delete();
} catch (IOException e) {
    throw new IllegalStateException("java.io.tmpdir not writable: " + System.getProperty("java.io.tmpdir"), e);
}

catch (HttpServerErrorException e) {
    // 500 on EVERY generate usually means server tmp is broken — no client-side fix, alert the operator
    alert("openapi-generator-online cannot write tmp; check java.io.tmpdir / disk / container tmpfs");
    throw e;
}

When it happens

Trigger: java.io.tmpdir points to a missing or read-only path (containers with a read-only root filesystem and no tmpfs on /tmp); disk full / tmpfs size limit hit; service user lacks write permission on the tmp dir; security manager or SELinux blocking creation.

Common situations: Docker/Kubernetes images with readOnlyRootFilesystem: true and no --tmpfs /tmp; containers whose /tmp tmpfs is sized for small files while generated SDKs need tens of MB; misconfigured -Djava.io.tmpdir pointing at an unmounted volume.

Related errors

• I/O error while reading file
• File not found or has expired
• File not found
• Framework is required
• Unsupported target %s supplied. %s

AI-assisted analysis of OpenAPITools/openapi-generator@fcec517be3 (2026-08-22). Data as JSON: /api/errors/8ba875cfaeb1cd71. Report an issue: GitHub.