If you can somehow find a class implementing ExplorerManager.Provider then you can get the Explorer manager. This provider might in fact be a TopComponent in the TopComponent.Registry, if for example it was actually a ExplorerPanel.
But this is bad style - for example, if someone wrote a TopComponent that included an ExplorerPanel only as a subcomponent, and manually managed the node selection, this trick would fail.
Rather, if you know which top component you care about, you can just call TopComponent.getActivatedNodes() and this will work correctly even for non-Explorer components with a node selection, such as Editor panes open on Java sources.
Better still is to be agnostic about which top component should be providing the activated nodes, and just listen to changes in the TopComponent.Registry.PROP_ACTIVATED_NODES (or TopComponent.Registry.PROP_CURRENT_NODES as appropriate).
But best of all is not to have to ever directly pay attention to the node selection. If you only need to know the node selection in order to make some user action enabled or not, you should simply extend NodeAction; this class does all the dirty work for you of listening to changes in the node selection and updating its state automatically.
If you just want to write some code that is sensitive to the global selection (not an action), you probably want to use Utilities.actionsGlobalContext().
Source: NetBeans FAQ