@Override
    public Map<String, String> getHeaders() {
        if (combinedHeaders == null) {
            synchronized (this) {
                if (combinedHeaders == null) {
                    this.combinedHeaders = Collections.unmodifiableMap(generateHeaders());
                }
            }
        }

        return combinedHeaders;
    }
A wrapper class for a set of headers to be included in a Glide request, allowing headers to be
constructed lazily.

<p> Ideally headers are constructed once and then re-used for multiple loads, rather then being
constructed individually for each load. </p>

<p> This class is thread safe. </p>
core/src/main/java/com/bumptech/glide/load/model/LazyHeaders.java