If you use a Unix-based operating system, this concept will be familiar; if not, it may require some explanation.
NetBeans uses virtual files to refer to the users files on disk, and to refer to its own configuration files. If you used the NetBeans IDE 3.6 or earlier, you may remember that the way you constructed your classpath used to be by "mounting" filesystems - folders on your disk.
Filesystems are gone from the UI, but are alive and well under the hood in NetBeans.
A Filesystem is a hierarchical tree of folders and files. A filesystem has a root folder, which may contain files and other folders. "Files" (FileObjects) in a Filesystem may be actual files on disk, or entries in JAR file, or entries in an XML file, or anything else that walks and talks like a file that someone has implemented the Filesystem interface for.
In the NetBeans source base there are implementations of Filesystems for
- XML files
- Remote FTP filesystems
- CVS filesystems
- A filesystem that takes an array of other filesystems and proxies them
Filesystems are used both to represent user files on disk, and also to represent configuration data internal to NetBeans - the System Filesystem. This is one of the reason that it takes only minimal code to create a GUI view of the system filesystem - the same file recognition code that recognizes user files, gives them actions, icons and display names is what recognizes internal configuration data.
Especially in the case of the System Filesystem, it can be useful to think of a Filesystem as a namespace in which objects live - for the System Filesystem, the fact that the entries in it are referred to as files is incidental.
Source: NetBeans FAQ