Before learning what's new in Java 9, we need to know one of the important components of Java, which is Java Development Kit (JDK).

JDK is a collection of Java Standard Edition (Java SE), Java Enterprise Edition (Java EE), Java Micro Edition platforms (Java ME), and different tools such as javac, Java console, JAR, JShell, Jlink, and it provides all the libraries for developing, debugging, and monitoring for building Java-based applications.

Java 9 has come up with almost over 100 new features and enhancements in different categories of JDK, such as tools, security, deployment, performance tuning, API changes of core libraries, and javadoc.