Note: This is draft documentation for 8.5.2, and is subject to change in the
final release.
An XPages library is a packaged plugin containing extended controls to be added to the Designer palette, the runtime functionality for those controls and any other extensions to the XPages Runtime. You can install such libraries into Domino Designer, and create applications using the new controls. This is part of the XPages Extensibility documents.
Install the update site to Designer
Enable the Install menu item.
The first time you use any library, you will need to enable the Install menu item in Designer. If your administrator has installed the library for you, you can skip to the next section on Creating Application using the Library.
File, Preferences, Domino Designer, General panel, check the check box "Enable Eclipse plugin install".
(You do not need to restart Designer)
If that check box is not present it is because the Domino administrator has configured a policy setting for the Notes install, to prevent installing plugins to the Notes Client.
Install the update site using the Install menu item.
In Domino Designer, in the File menu, Application, Install. Search for new features to install, Next.
If you have built the update site yourself, choose
Add Folder Location, C:\Program Files\Lotus\Notes\Data\workspace\libraryUpdateSite OK
If the update site has been shared on a web server, choose
Add Remote Location, enter the URL, OK.

Finish. Toggle open the update site, select your feature, Next. Accept the license agreement, Next. Finish.
You'll get a warning about installing an unsigned feature, choose "Install this plug-in", OK.
It asks if you want to restart, choose Restart Now
You can verify that the feature is installed by checking in File, Application, Application Management. It will be in the install location
C:\Program Files\Lotus\Notes\Data\workspace\applications\eclipse
It can also be uninstalled from that dialog.
When installing later versions of the same plugin, this option in the Install dialog doesn't seem to work: "Search for updates of the currently installed features", so just use the "Search for new features to install" option again.
Create an application using the library control
Create an application, and create an XPage. Type in some text.
In the Palette, select Other..., drag onto the page, under Extended Controls, select on the Example Control, click OK.
You will be prompted with a dialog asking if you want to make this application depend on the library. Click Continue.
That will update the option in the Application Properties, Advanced tab, XPage Libraries, to put a check mark beside the library ID.
When that option is enabled, the application will give an error if you attempt to run it in an environment where the library is not present.
(In this screenshot, 2 libraries are installed in Designer, but the current application is only using the first library.)

If the library is not present on the Domino server or Notes Client when you attempt to run the application, you will get an error like:
Unexpected runtime error
The runtime has encountered an unexpected error.
Exception
Cannot find the library com.example.xsp.library, required by the application /myApp.nsf.
Preview in Notes
The Notes Client preview will work because you have manually installed the library into this Designer install. You can use this preview for testing, but your end users will use a different mechanism for installing the application into the Notes Client. The library will be installed for them through the Widget Catalog which is described in this article
Deploying XPage Libraries
Select the XPage in Designer, in the Design menu choose "Preview in Notes".