1) RxJava: Part of the ReactiveX library for composing asynch and event-based programs using observables.
Web site: http://reactivex.io/
2) Lombok: A library that helps to reduce the boilerplate code.
Web site: https://projectlombok.org/
3) Java Spark: A micro-framework for creating the web applications.
Web site: http://sparkjava.com/
4) Apache Spark: It is a general-purpose cluster computing system which provides lightening fast cluster computing.
Web site: http://spark.apache.org/
5) Retrofit: A library that turns HTTP API into a Java interface.
Web site: http://square.github.io/retrofit/
6) Java GraphQL: Java implementation of the GraphQL.
Web site: https://github.com/graphql-java/graphql-java
7) JavaFX: The rich client platform for Java.
Web site: http://www.oracle.com/technetwork/java/javase/overview/javafx-overview-2158620.html
8) Sql2o: Small java framework that makes it easy to execute SQL statements on a JDBC compliant database.
Web site: http://www.sql2o.org/
9) Speedment: It is a stream ORM Java toolkit and runtime.
Web site: https://github.com/speedment/speedment
10) OrientDB: A distrubuted graph database engine (integrates well with Java)
Web site: https://orientdb.com/orientdb/