The current program is version 2.0.0, a native freeRouting binary. It runs on Linux, Windows and macOS. There is no Java runtime and no Oracle JRE to install.
The usual way to get a working program is the LayoutEditor package. The package includes freeRouting next to the LayoutEditor executable (in the bin folder of the package). Start that binary on its own, or from the LayoutEditor Auto-Router. How the editor calls it is described under Using with LayoutEditor.
Older LayoutEditor packages shipped a Java freeRouting.jar. That file is obsolete. Version 2.0.0 replaces it. You do not need a Java Control Panel, Java Web Start, or an Oracle download to run the current program.
The C++ sources are the archive freeRouting-2.0.0.tar.gz on the ChangeLog page. There is no separate public Git repository; that tarball is the published source. How to compile it with qmake and make on Linux, Windows and macOS is described under Building from Source.
The Java program up to 1.3.2 remains on the ChangeLog page as an archive only. Those tarballs are historical. They are not the current application and they do require a Java runtime if you still build them.
Start freeRouting and open a Specctra dsn file, or pass the file on the command line. freeRouting -h lists the options. Connecting the program to a CAD tool is covered for LayoutEditor, KiCad and Eagle. The manual describes the menus, toolbar and routing modes.