@Override
    public void init() throws IOException {
        this.accessToken = Utils.readFileAsString(accessTokenFile.toFile().getPath());
        // always non-null; to remove any newline chars or backend will report err
        this.accessToken = this.accessToken.trim();
    }
<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