@Override
    public String retrieve() throws IOException {
        if (accessToken == null)
            throw new IllegalStateException("Access token is null; please call init() first");

        return accessToken;
    }
<code>FileTokenRetriever</code> is an {@link AccessTokenRetriever} that will load the contents,
interpreting them as a JWT access key in the serialized form.

@see AccessTokenRetriever
clients/src/main/java/org/apache/kafka/common/security/oauthbearer/internals/secured/FileTokenRetriever.java