protected void setCredentialsExtractorIfUndefined(final CredentialsExtractor credentialsExtractor) {
        if (this.credentialsExtractor == null) {
            this.credentialsExtractor = credentialsExtractor;
        }
    }
<p>setCredentialsExtractorIfUndefined.</p>

@param credentialsExtractor a {@link CredentialsExtractor} object
pac4j-core/src/main/java/org/pac4j/core/client/BaseClient.java