What is the user directory, what is in it, and why do I need one?

The userdir is a directory passed to NetBeans on startup by the launch script. You can specify it on the command line, e.g.:

--userdir /tmp/some/folder/that/need/not/exist/yet

It contains configuration data that allows NetBeans to restore its state after shutdown; it also contains log files and other miscellaneous artifacts of running NetBeans. The config/ subfolder of the userdir is where changes made at runtime to the system filesystem (DevFaqSystemFilesystem) are written.

It is expected that applications built on the NetBeans platform will provide a custom launch script which will specify the userdir such that it does not overlap with any other platform-based application. This is done for you if you use the stock launchers created by "suite" projects in the IDE.

Only one running copy of NetBeans may use a single userdir at a time; during startup, the platform checks a lock file and if it is present, tries to contact the running instance via a socket, to determine if the lock file is stale or not. If it does not make contact, it will display a warning and ask the user if startup should continue. This same protocol also supports command-line options.

You can find the path to the userdir at runtime if you need to, using the system property netbeans.user. Most modules should not do this; use FileUtil.getConfigFile to work with the config/ subdirectory.