29.6. Extensions API

29.6.1. The core part
29.6.2. The web client part

29.6.1. The core part

The Extensions API is divided into two parts. A core part and a web client specific part. The core part can be found in the net.sf.basedb.util.extensions package and it's sub-packages, and consists of three sub-parts:

  • A set of interface definitions which forms the core of the Extensions API. The interfaces defines, for example, what an Extension is and what an ActionFactory should do.

  • A Registry that is used to keep track of installed extensions. The registry also provides functionality for invoking and using the extensions.

  • Utility classes that are useful when implementation a client application that can be extendable. The most useful example is the XmlLoader which can read extension definitions from XML files and create the proper factories, etc.

Figure 29.23. The core part of the Extensions API

The core part of the Extensions API

The Registry is one of the main classes in the extension system. All extension points and extensions must be registered before they can be used. Typically, you will first register extension points and then extensions, beacuse an extension can't be registered until the extension point it is extending has been registered.

An ExtensionPoint is an ID and a definition of an Action class. The other options (name, description, renderer factory, etc.) are optional. An Extension that extends a specific extension point must provide an ActionFactory instance that can create actions of the type the extension point requires.

Example 29.1. The menu extensions point

The net.sf.basedb.clients.web.menu.extensions extension point requires MenuItemAction objects. An extension for this extension point must provide a factory that can create MenuItemAction:s. BASE ships with default factory implementations, for example the FixedMenuItemFactory class, but an extension may provide it's own factory implementation if it wants to.

Call the Registry.useExtensions() method to use extensions from one or several extension points. This method will find all extensions for the given extension points. If a filter is given, it checks if any of the extensions or extension points has been disabled. It will then call ActionFactory.prepareContext() for all remaining extensions. This gives the action factory a chance to also disable the extension, for example, if the logged in user doesn't have a required permission. The action factory may also set attributes on the context. The attributes can be anything that the extension point may make use of. Check the documentation for the specific extension point for information about which attributes it supports. If there are any renderer factories, their RendererFactory.prepareContext() is also called. They have the same possibility of setting attributes on the context, but can't disable an extension.

After this, an ExtensionsInvoker object is created and returned to the extension point. Note that the ActionFactory.getActions() has not been called yet, so we don't know if the extensions are actually going to generate any actions. The ActionFactory.getActions() is not called until we have got ourselves an ActionIterator from the ExtensionsInvoker.iterate() method and starts to iterate. The call to ActionIterator.hasNext() will propagate down to ActionFactory.getActions() and the generated actions are then available with the ActionIterator.next() method.

The ExtensionsInvoker.renderDefault() and ExtensionsInvoker.render() are just convenience methods that will make it easer to render the actions. The first method will of course only work if the extension point is providing a renderer factory, that can create the default renderer.

[Note] Be aware of multi-threading issues

When you are creating extensions you must be aware that multiple threads may access the same objects at the same time. In particular, any action factory or renderer factory has to be thread-safe, since only one exists for each extension. Action and renderer objects should be thread-safe if the factories re-use the same objects.

Any errors that happen during usage of an extension is handled by an ErrorHandler. The core provides two implementations. We usually don't want the errors to show up in the gui so the LoggingErrorHandlerFactory is the default implementation that only writes to the log file. The RethrowErrorHandlerFactory error handler can be used to re-throw exceptions which usually means that they trickle up to the gui and are shown to the user. It is also possible for an extension point to provide its own implementation of an ErrorHandlerFactory.

29.6.2. The web client part

The web client specific parts of the Extensions API can be found in the net.sf.basedb.client.web.extensions package and it's subpackages. The top-level package contains classes used to administrate the extension system. Here is for example the ExtensionsControl class which is the master controller for the web client extensions. It:

  • Keeps track of installed extensions and which JAR or XML file they are installed from.

  • Can, manually or automatically, find and install new or updated extensions and uninstall deleted extensions.

  • Adds permission control to the extension system, so that only an administrator is allowed to change settings, enable/disable extensions, etc.

In the top-level package there are also some abstract classes that may be useful to extend for developers creating their own extensions. For example, we recommend that all action factories extend the AbstractJspActionFactory class.

The sub-packages to net.sf.basedb.client.web.extensions are mostly specific to a single extension point or to a specific type of extension point. The net.sf.basedb.client.web.extensions.menu package, for example, contains classes that are/can be used for extensions adding menu items to the Extensions menu.

Figure 29.24. The web client part of the Extensions API

The web client part of the Extensions API

When the Tomcat web server is starting up, the ExtensionsServlet is automatically loaded. This servlet has as two purposes:

  • Initialise the extensions system by calling ExtensionsControl.init(). This will result in an initial scan for installed extensions, which is equivalent to doing a manual scan with the force update setting to false. This means that the extension system is up an running as soon as the first user log's in to BASE.

  • Act as a proxy for custom servlets defined by the extensions. URL:s ending with .servlet has been mapped to the ExtensionsServlet. When a request is made it will extract the name of the extension's JAR file from the URL, get the corresponding ExtensionsFile and ServletWrapper and then invoke the custom servlet. More information can be found in Section 27.7, “Custom servlets”.

Using extensions only involves calling the ExtensionsControl.createContext() and ExtensionsControl.useExtensions() methods. This returns an ExtensionsInvoker object as described in the previous section.

To render the actions it is possible to either use the ExtensionsInvoker.iterate() method and generate HTML from the information in each action. Or (the better way) is to use a renderer together with the Render taglib.

To get information about the installed extensions, change settings, enabled/disable extensions, performing a manual scan, etc. use the ExtensionsControl.get() method. This will create a permission-controlled object. All users has read permission, administrators has write permission.

[Note] Note

The permission we check for is WRITE permission on the web client item. This means it is possible to give a user permissions to manage the extension system by assigning WRITE permission to the web client entry in the database. Do this from AdministrateClients.

The XJspCompiler is mapped to handle the compilation .xjsp files which are regular JSP files with a different extension. This feature is experimental and requires installing an extra JAR into Tomcat's lib directory. See Section 23.2, “Installing the X-JSP compiler” for more information.