@Override
    public void initialize(Subject subject, CallbackHandler callbackHandler, Map<String, ?> sharedState, Map<String, ?> options) {
        this.callbackHandler = callbackHandler;
        fileName = (String) options.get(FILE_OPTIONS);
        if (Utils.isBlank(fileName)) {
            throw new ConfigException("Property Credentials file must be specified");
        }

        if (!CREDENTIAL_PROPERTIES.containsKey(fileName)) {
            log.trace("Opening credential properties file '{}'", fileName);
            Properties credentialProperties = new Properties();
            try {
                try (InputStream inputStream = Files.newInputStream(Paths.get(fileName))) {
                    log.trace("Parsing credential properties file '{}'", fileName);
                    credentialProperties.load(inputStream);
                }
                CREDENTIAL_PROPERTIES.putIfAbsent(fileName, credentialProperties);
                if (credentialProperties.isEmpty())
                    log.warn("Credential properties file '{}' is empty; all requests will be permitted",
                        fileName);
            } catch (IOException e) {
                log.error("Error loading credentials file ", e);
                throw new ConfigException("Error loading Property Credentials file");
            }
        } else {
            log.trace(
                "Credential properties file '{}' has already been opened and parsed; will read from cached, in-memory store",
                fileName);
        }
    }
{@link PropertyFileLoginModule} authenticates against a properties file.
The credentials should be stored in the format {username}={password} in the properties file.
The absolute path of the file needs to specified using the option <b>file</b>

<p><b>NOTE: This implementation is NOT intended to be used in production since the credentials are stored in PLAINTEXT in the
properties file.</b>
connect/basic-auth-extension/src/main/java/org/apache/kafka/connect/rest/basic/auth/extension/PropertyFileLoginModule.java