Java 9 introduced the module system known as Project Jigsaw. Released on September 21, 2017, Java 9 added the Java Platform Module System, or JPMS, to the Java language and runtime.
Project Jigsaw aimed to make the JDK itself more modular and to improve how large applications manage dependencies. Modules declare dependencies with requires and expose selected packages with exports, creating stronger boundaries than the traditional classpath and JAR files.
The project had a long history. Earlier module-system proposals were associated with Java 7 and Java 8, but they were delayed as the design evolved. Java 9 ultimately delivered the standardized system through JSR 376 and several JDK Enhancement Proposals, including JEP 261.
A common mix-up is Java 11, because it was the first long-term-support release after Java 9 and is widely used in production. Java 10 was a short-lived feature release, while Java 8 introduced lambdas and streams, not modules. Java 9 also introduced tools such as jlink for creating customized runtime images.