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 two sub-parts:
that keeps track of the JAR files on the file system containing extensions code.
The manager can detect new, updated and deleted files and is used to
load metadata information about the extensions and register them in the
Registry so that they can be used.
The manager is also used to install plug-ins.
Let's start by looking at the extensions manager and related classes.
The BASE application is using a single manager and a single registry (handled by the
The manager has been configured to look for extensions and plug-ins in the directory
specified by the plugins.dir setting in
base.config. Theoretically, a single manager can handle
multiple directories, but we do not use that feature. The BASE core also
include some special files that are added with the
method. They contain definitions for the core extensions and core plug-ins
and are shipped as XML files that reside inside the BASE core JAR files.
method is called to initiate a check for new, updated and deleted files. The
manager uses the
to find information about each JAR or XML file it find in the directory.
After the scan, the
method can be used to find more information about each individual file, for example,
if it is a new or modified file, if it contains valid extension definitions and
information about the author, etc. This information is used by the installation
wizard in the web client to display a dialog were the user can select which extensions
to intall. See Figure 22.2, “Extensions and plug-ins installation wizard”.
Note that no installation or other actions take place at this stage.
ExtensionsManager.processFiles() method is called to actually do
something. It needs an
ExtensionsFileProcessor implementation as an argument. As you can see
in the diagram above there are multiple implementations (all are not
shown in the diagram), each with a very specific task. A processor is usually
also paired with a filter to target it at files that fulfil some criteria, for example,
only at valid extension files that has been updated. Typically, the
ExtensionsManager.processFiles() method need to be
called multiple times with different processor implementations to perform a full
installation of an extension or plug-in. Here is a list of the various processors
currently in use in BASE.
Is used to register
extensions with the registry. Can be paired with different filters
depending on when it is used. At BASE startup an
is used so that only installed extensions are registered.
Is used to extract files from the JAR file to a local directory. This is currently used by the web client to extract JSP files, images, etc. that are needed by web client extensions.
The opposite of
Is used to install and register plug-ins.
Is used to disable plug-ins from extensions that have been removed.
This is usually the final
processor that is called and reset the timestamp on all processed files
so that the next time
is called it will know what has been modified.
This list contains the core processors only. The web client part is using some additional processors to perform, for example, servlet registration.
The result of the processing can be collected with a
and is then displayed to the user as in Figure 22.3, “Extensions and plug-ins installation results”.
All of the above is only used when BASE is starting up and initializing the extensions
system or when the server administrator is performing a manual installation or update.
The next diagram shows the part of the extensions system that is used when
actually using the extensions for what they are intended to do (the web client adds some
extra features to this as well, but that is discussed later).
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
is an ID and a definition of an
class. The other options (name, description, renderer factory, etc.) are optional.
that extends a specific extension point must provide an
instance that can create actions of the type the extension point requires.
Example 29.1. The menu extensions point
net.sf.basedb.clients.web.menu.extensions extension point
objects. An extension for this extension point must provide a factory that
MenuItemAction:s. BASE ships with default
factory implementations, for example the
class, but an extension may provide it's own factory implementation if it wants to.
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
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
is also called. They have the same possibility of setting attributes
on the context, but can't disable an extension.
After this, an
object is created and returned to the extension point. Note that
ActionFactory.getActions() has not been
called yet, so we don't know if the extensions are actually
going to generate any actions. The
is not called until we have got ourselves an
ExtensionsInvoker.iterate() method and
starts to iterate. The call to
will propagate down to
and the generated actions are then available with the
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.
|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
The core provides two implementations. We usually don't want the
errors to show up in the gui so the
is the default implementation that only writes to the log file. The
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
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 the
class which is used to administrate the extension system. It is more or
less a wrapper around the
provided by the core, but adds permission control and a few other things.
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
class. All web client extension points use the
JspContext class instead of
The JSP context provides some extra information about the current request.
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 menu. See Section 27.8, “Extension points defined by BASE” for more information about the extension points defined by BASE.
Initialise the extensions system by calling
ExtensionsControl.init(). This will result in
an initial scan for installed extensions. This means that
the extension system is up an running as soon as the first user log's
in to BASE. The processing scheme is slightly different from what is done
when the core is setting up the initial manager. The major additions are:
Act as a proxy for custom servlets defined by the extensions. URL:s
.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
and then invoke the custom servlet. More information can be found in
Section 27.7, “Custom servlets”.
Using extensions only involves calling the
ExtensionsControl.useExtensions() methods. This
object as described in the previous section.
To render the actions it is possible to either use the
and generate HTML from the information in each action. Or
(the better way) is to use a renderer together with the
To get information about the installed extensions,
change settings, enabled/disable extensions, performing a manual
installation, etc. use the
method. This will create a permission-controlled object. All
users has read permission, administrators has write permission.
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.
is mapped to handle the compilation
which are regular JSP files with a different extension. The difference is that
the XJSP compiler include the extension's JAR file on the class path, which
means that the JSP file can use classes that would otherwise be impossible.
This feature is experimental and requires installing an extra JAR into Tomcat's lib
directory. See Section 22.1.4, “Installing the X-JSP compiler” for