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.
Realm is a mobile database: a replacement for SQLite & ORMs
Lightning Memory Database (LMDB) for Java: a low latency, transactional, sorted, embedded, key-value store
ObjectBox is a superfast lightweight database for objects
Speedment is a Stream ORM Java Toolkit and Runtime
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.
JetBrains Xodus is a transactional schema-less embedded database used by JetBrains YouTrack and JetBrains Hub.