Layer files are small XML files provided by modules, which define a virtual filesystem. The layer file defines folders and files that will be merged into the system filesystem that makes up the runtime configuration information NetBeans and its modules use.

Layer files help to make it possible for modules to be dynamically installed. If you've read about FileObjects and FileSystems, you know that you can listen for changes in folders and files in a filesystem. That's exactly what the components of NetBeans whose content is composed from folders in the system filesystem do. So if a module is added at runtime, the system filesystem fires changes; the UI notices that the contents of the folder has changed and updates the UI to reflect the changes.

If you created your module using NetBeans 5.0 or later's New NetBeans Module Project, you already have an XML layer in your module, and you can expand the node for it under Important Files in your module project to see and modify its contents. The way it is declared is simple:

  • In your jar, provide the layer file - e.g. com/foo/mymodule/resources/layer.xml
  • In your module's manifest, include the following line somewhere in the top section:
OpenIDE-Module-Layer: com/foo/mymodule/resources/layer.xml

Source: NetBeans FAQ