public String validateString(String name, boolean isRequired) throws ValidateException {
        String value = get(name);

        if (value == null) {
            if (isRequired)
                throw new ConfigException(String.format("The OAuth configuration option %s value must be non-null", name));
            else
                return null;
        }

        value = value.trim();

        if (value.isEmpty()) {
            if (isRequired)
                throw new ConfigException(String.format("The OAuth configuration option %s value must not contain only whitespace", name));
            else
                return null;
        }

        return value;
    }
Validates that the configured URL that:

<li>
    <ul>is well-formed</ul>
    <ul>contains a scheme</ul>
    <ul>uses either HTTP, HTTPS, or file protocols</ul>
</li>

No effort is made to connect to the URL in the validation step.
clients/src/main/java/org/apache/kafka/common/security/oauthbearer/internals/secured/ConfigurationUtils.java