Redisson - Redis Java client with features of In-Memory Data Grid. Over 50 Redis based Java objects and services: Set, Multimap, SortedSet, Map, List, Queue, Deque, Semaphore, Lock, AtomicLong, Map Reduce, Publish / Subscribe, Bloom filter, Spring Cache, Tomcat, Scheduler, JCache API, Hibernate, MyBatis, RPC, local cache ...
H2 is an embeddable RDBMS written in Java.
OrientDB is the most versatile DBMS supporting Graph, Document, Reactive, Full-Text, Geospatial and Key-Value models in one Multi-Model product. OrientDB can run distributed (Multi-Master), supports SQL, ACID Transactions, Full-Text indexing and Reactive Queries. OrientDB Community Edition is Open Source using a liberal Apache 2 license.
Realm is a mobile database: a replacement for SQLite & ORMs
ObjectBox is a superfast lightweight database for objects
Transactional schema-less embedded database used by JetBrains YouTrack and JetBrains Hub.