Serialization via Kryo for various "tommyettinger libraries," and also libGDX. The main repo page is here.
In alphabetical order:
The docs for kryo-simple-graphs are still available for an older version here, but kryo-simple-graphs is limited to an older version of Simple-Graphs by earlygrey. A fork of simple-graphs, gand, has an up-to-date kryo-more serializer for its graphs and data structures.