Before Java 9, the runtime library was contained in a single massive file called rt.jar . Java 9 replaced this with a modular system where code is stored in .jmod files or a specific internal format.
is a specialized Java archive file introduced in Java 9 as part of the new modular runtime. It provides the Java Runtime File System (JRT-FS) , which allows tools—including those running on older Java versions—to access and explore the internal modular structure of a modern JDK or JRE as if it were a standard file system. Where to "Download" jrt-fs.jar jrt-fs.jar download
You can find it in the lib directory of your Java installation: Windows: C:\Program Files\Java\jdk-[version]\lib\jrt-fs.jar Linux/macOS: /usr/lib/jvm/java-[version]/lib/jrt-fs.jar Before Java 9, the runtime library was contained
You generally jrt-fs.jar from a separate website. It is bundled automatically with all JDK 9 and later installations. It provides the Java Runtime File System (JRT-FS)