crafterm 2002/07/17 09:30:11
Modified: fortress/src/xdocs book.xml
Added: fortress/src/xdocs lifecycle-extensions.xml
Log:
Added document explaining how to use the lifecycle extension support in
Fortress, and link to it from the table of contents.
Any feedback/comments about the document, etc, is greatly appreciated :)
Revision Changes Path
1.6 +1 -0 jakarta-avalon-excalibur/fortress/src/xdocs/book.xml
Index: book.xml
===================================================================
RCS file: /home/cvs/jakarta-avalon-excalibur/fortress/src/xdocs/book.xml,v
retrieving revision 1.5
retrieving revision 1.6
diff -u -r1.5 -r1.6
--- book.xml 17 Jul 2002 14:10:03 -0000 1.5
+++ book.xml 17 Jul 2002 16:30:11 -0000 1.6
@@ -18,6 +18,7 @@
<menu-item label="Features" href="features.html"/>
<menu-item type="external" id="download" label="Download"
href="http://jakarta.apache.org/builds/jakarta-avalon-excalibur/release"/>
<menu-item type="external" id="api-docs" label="API Docs" href="api/"/>
+ <menu-item type="external" id="extensions" label="Lifecycle Extensions"
href="lifecycle-extensions.html"/>
</menu>
1.1
jakarta-avalon-excalibur/fortress/src/xdocs/lifecycle-extensions.xml
Index: lifecycle-extensions.xml
===================================================================
<?xml version="1.0"?>
<document>
<header>
<title>Writing Lifecycle Extensions</title>
<authors>
<person name="Marcus Crafter" email="[EMAIL PROTECTED]"/>
</authors>
</header>
<body>
<s1 title="Introduction">
<p>
Avalon Framework defines a set of standard interfaces often termed as
<b>Lifecycle</b>
metainfo which tells the ComponentManager how a particular Component
should be treated
during it's life.
</p>
<p>
This metainfo allows the developer to separate the various concerns
involved when
writing a Component, often termed SoC and IoC (Separation of Concerns and
Inversion of
Control) and is one of primary advantages of using Avalon.
</p>
<p>
Sometimes it's useful to extend this development paradigm from the
framework level
into the application domain, to create customized lifecycle extensions
that are called
upon in addition to the standard set defined by the Avalon Framework.
Such custom lifecycle
stages can further enable domain specific logic, and allows the developer
to reuse the same
development and thinking paradigm as the standard lifecycle stages.
</p>
<p>
This document describes how to add new lifecycle extensions to
<strong>Fortress</strong>.
This document assumes a knowledge of what an Avalon lifecycle is, and a
basic understanding
of the standard lifecycle interfaces Avalon Framework defines. References
in this document to
Component and ComponentManager can also be freely interpreted as Service
and ServiceManager
by the reader.
</p>
<p>
<strong>Note</strong>, as at the time of writing, Fortress is the only
Avalon container that
supports lifecycle extensions, which means Components that use this
feature will most likely
only work as expected with Fortress, and not with the other Avalon
containers
(ExcaliburComponentManager, Phoenix, Merlin, Tweety, etc).
</p>
<p>
Support for lifecycle extensions in the other Avalon containers is
technically possible but
has not yet been discussed. Please check with the Avalon developer
mailing list if you use
one of these containers and would like to use lifecycle extensions.
</p>
</s1>
<s1 title="Overview">
<p>
Adding new lifecycle extensions to Fortress is straightforward. An
overview of the process
follows:
</p>
<ol>
<li>Define a new component interface</li>
<p>
Create a new interface defining the operations that should be called
upon components
that implement this interface.
</p>
<li>Define an extension object that calls upon the methods defined in
the new interface,
during one or more of the pre-defined phases of component's
lifecycle</li>
<p>
Create a class that implements <code>LifecycleExtension</code>, that
tests any given
component for the above defined interface (and others if applicable),
invoking methods
defined in that interface.
</p>
<li>Register the extension object with Fortress'
<code>LifecycleExtensionManager</code></li>
<p>
Create an instance of the class defined in the previous step, and
register it with a
<code>LifecycleExtensionManager</code>, using either the default
manager available inside
of your container, or an externally created manager that is later given
to the container
to use.
</p>
<li>Implement the new component interface on your component</li>
<p>
Add the new <code>implements</code> clause to your Component, or
Component implementation,
and write any methods defined in the implemented interface.
</p>
<li><code>lookup()/select()/release()</code> components as normal</li>
<p>
Proceed as normal. Checking for extensions is done implicitly within
Fortress. Once
lifecycle extensions are registered they will be invoked on any
implementing components
during the 4 phases defined later in this document.
</p>
</ol>
<p>
The rest of this document describes this process in greater detail.
</p>
</s1>
<s1 title="Lifecycle phases">
<p>
A Component's lifecycle can be broken down to the following phases:
</p>
<ol>
<li>Creation</li>
<p>
When the Component is actually instantiated.
</p>
<li>Access</li>
<p>
When the Component is accessed via a ComponentManager/Selector
(<code>lookup()/select()</code>)
</p>
<li>Release</li>
<p>
When the Component is released via a ComponentManager/Selector
(<code>release()</code>)
</p>
<li>Destruction</li>
<p>
When the Component is decomissioned, ready for garbage collection.
</p>
</ol>
<p>
A Component will go through it's Creation and Destruction phase only
once. Since
<code>ComponentHandler</code> classes can implement different handling
strategies
(Poolable, ThreadSafe, etc), the access and release phases of a component
can be
done multiple times.
</p>
<p>
Lifecycle extensions can be added to any of the above defined phases.
This allows
you to choose when your particular extension will be executed.
</p>
</s1>
<s1 title="Interfaces and Classes">
<p>
Support for lifecycle extensions in Fortress is done using the following
classes/interfaces.
</p>
<s2 title="The Component Extension Interface">
<p>
The component extension interface is what the developer writes. It
defines the new
interface that components will implement to receive additional
functionality. There is no
particular base interface the developer needs to extend, and the
interface can be kept
separate from the Container itself.
</p>
</s2>
<s2 title="The LifecycleExtension Interface">
<p>
Component extensions are invoked via a Lifecycle extension object.
Lifecycle extension
objects are managed via a <code>LifecycleExtensionManager</code> class
and essentially
test for a particular Component extension interface, and appropriately
call methods defined
in that interface.
</p>
<p>
All Lifecycle extension objects must implement the
<code>LifecycleExtension</code> interface.
The <code>LifecycleExtension</code> interface defines 4 methods that are
called upon to allow
an implementor to extend a component's lifecycle.
</p>
<p>
The 4 methods (<code>create</code>, <code>destroy</code>,
<code>access</code> and
<code>release</code>) are invoked by a manager class from inside
Fortress. Each method
accepts 2 parameters, the particular Component instance being extended,
and the container
<code>Context</code>.
</p>
<p>
The container <code>Context</code> is passed as a parameter to provide
access to any
miscellaneous objects that might be needed during extension code (to make
use of this feature
the container's Context will need to be prefilled with references and
passed to the
<code>ContextBuilder</code> during Fortress' startup sequence).
</p>
<p>
Each method may throw an exception to indicate an error, which will be
logged, but will
not terminate other extensions from being executed on that Component.
</p>
<source>
/**
* LifecycleExtension interface. This interface defines the methods that
* a LifecycleExtensionManager can call on a particular concrete
* LifecycleExtensionMarker class.
*/
public interface LifecycleExtension
{
/**
* Create, called when the given component is being
* instantiated.
*
* @param component a Component instance
* @param context a Context instance
* @exception Exception if an error occurs
*/
void create( Object component, Context context )
throws Exception;
/**
* Destroy, called when the given component is being
* decomissioned.
*
* @param component a Component instance
* @param context a Context instance
* @exception Exception if an error occurs
*/
void destroy( Object component, Context context )
throws Exception;
/**
* Access, called when the given component is being
* accessed (ie. via lookup() or select()).
*
* @param component a Component instance
* @param context a Context instance
* @exception Exception if an error occurs
*/
void access( Object component, Context context )
throws Exception;
/**
* Release, called when the given component is being
* released (ie. by a CM or CS).
*
* @param component a Component instance
* @param context a Context instance
* @exception Exception if an error occurs
*/
void release( Object component, Context context )
throws Exception;
}
</source>
<p>
Many extensions will not require implementation of every method defined
in the
above interface, for that reason, there's a
<code>AbstractLifecycleExtension</code>
convenience class available which provides default (empty)
implementations of each
method which you can extend from. This allows you to implement only the
methods
necessary for your particular extension.
</p>
</s2>
<s2 title="The LifecycleExtensionManager class">
<p>
The <code>LifecycleExtensionManager</code> class provides default
management of
extension implementations and an API which ComponentManager/Selector's
can call
upon to execute them.
</p>
<p>
The LifecycleExtensionManager class API is too big to list here, instead
have a look at
the following <link
href="http://jakarta.apache.org/avalon/excalibur/fortress/api/org/apache/excalibur/fortress/lifecycle/LifecycleExtensionManager.html";>link</link>.
It essentially defines
4 methods for executing extension objects at the various phases of a
component's lifecycle,
and several methods for registering extension objects with the manager.
</p>
<p>
The <code>LifecycleExtensionManager</code> class will operate safely in
multithreaded
environments, and allows you to add/remove extensions to a running system.
</p>
<p>
By default, all Fortress based containers will be initialized with a
default
<code>LifecycleExtensionManager</code> that contains no extensions. You
can alternatively
provide a pre-configured LifecycleExtensionManager to your Container via
the
ContextBuilder class (<code>ContainerConstants.EXTENSION_MANAGER</code>
key) if you like.
</p>
<p>
To add a new lifecycle extension object to the manager simply call the
method
<code>LifecycleExtensionManager.addExtension()</code>. Methods also exist
for removing
and iterating through the currently available extensions.
</p>
</s2>
<s2 title="FortressComponentManager/FortressComponentSelector">
<p>
Fortress' inbuilt Component Manager/Selector/Factory code will
automatically call
upon the LifecycleExtensionManager class at each phase in a Component's
life at the
following predefined times:
</p>
<ol>
<li>Access</li>
<p>
Called inside the ComponentManager, after the component has been
retrieved
from it's handler, but before it's returned to the invoker of
<code>lookup()/select()</code>.
</p>
<li>Release</li>
<p>
Called inside the ComponentManager, before the component is passed back
to
it's handler to be disposed/pooled/etc.
</p>
<li>Creation</li>
<p>
Called inside the ComponentFactory, before <code>initialize()</code>.
</p>
<li>Destruction</li>
<p>
Called inside the ComponentFactory, after <code>dispose()</code>.
</p>
</ol>
<p>
<strong>Note</strong>, components created via Fortress' ComponentHandler
classes directly
will bypass the logic for <code>access</code> and <code>release</code>
extensions. This is
because the code performing this logic is located in the
ComponentManager/Selector classes
(independent from all handlers).
</p>
</s2>
</s1>
<s1 title="An Example">
<p>
Let's look at a simple example. The following is also available as a
working sample
in Fortress' examples directory.
</p>
<p>
Our example implements a Lifecycle extension for passing a
<code>SecurityManager</code> to
Components. We'll call it the <code>SecurityManageable</code> interface.
</p>
<s2 title="Define the component extension interface">
<p>
First we define the new Component extension interface.
</p>
<source>
/**
* Simple custom lifecycle extension interface for supplying a component
* with a security manager.
*/
public interface SecurityManageable
{
/**
* Pass a SecurityManager object to the component
*
* @param manager a SecurityManager value
*/
void secure( SecurityManager manager )
throws SecurityException;
}
</source>
</s2>
<s2 title="Create the lifecycle extensions class">
<p>
Next we define the actual extension implementation which invokes the
<code>secure()</code>
method. We extend from <code>AbstractLifecycleExtension</code> since we
only want
<code>secure()</code> to be invoked upon each access (ie. lookup()) to
the component, and
don't need to implement the other 3 LifecycleExtension methods (create,
release, and
destroy).
</p>
<source>
/**
* Some custom extensions for this container's components.
*/
public class Extensions
extends AbstractLifecycleExtension
{
/**
* Access, called when the given component is being
* accessed (ie. via lookup() or select()).
*
* @param component a Component instance
* @param context a Context instance
* @exception Exception if an error occurs
*/
public void access( Object component, Context context )
throws Exception
{
if ( component instanceof SecurityManageable )
{
// pass in a simple security manager, a real system might want
to pass
// in specialized/custom security managers
( ( SecurityManageable ) component ).secure( new
SecurityManager() );
}
}
}
</source>
<p>
<strong>Note</strong>, an extension class may run components through any
given number of
extensions, and are not limited to just one.
</p>
</s2>
<s2 title="Register the lifecycle extensions class">
<p>
We then inform our container about the extension. This could be done in
several different
ways, for simplicity we'll extend <code>initialize()</code> and add it to
the
<code>LifecycleExtensionManager</code> there.
</p>
<p>
(an alternative might be to initialize a LifecycleExtensionManager before
creating the
container and pass it in via the
<code>ContextBuilder.setExtensionManager()</code> method,
or to create a LifecycleExtensionManager subclass that includes the
extension preset)
</p>
<source>
/**
* Simple container that includes custom lifecycle extensions.
*/
public final class ExtendedContainer
extends AbstractContainer
{
public void initialize()
throws Exception
{
super.initialize();
m_extManager.addExtension( new Extensions() );
}
}
</source>
</s2>
<s2 title="Use the new component interface">
<p>
To use the new SecurityManageable lifecycle extension, we simply implement
SecurityManageable just as we do with any other Avalon lifecycle
interfaces
(assuming a predefined Component interface
<code>ExtendedComponent</code>).
</p>
<source>
/**
* ExtendedComponentImpl, demonstrating the use of a custom
* lifecycle stage SecurityManageable. This code does
* a simple access check for several files on the file system and logs
* the results accordingly.
*/
public class ExtendedComponentImpl
extends AbstractLogEnabled
implements ExtendedComponent, SecurityManageable
{
/**
* Pass a SecurityManager object to the component
*
* @param manager a SecurityManager value
*/
public void secure( final SecurityManager manager )
throws SecurityException
{
getLogger().debug( "Received SecurityManager instance: " + manager
);
final String[] files = { "/tmp", "/vmlinuz", "/usr/lib/libc.a" };
for ( int i = 0; i < files.length; ++i )
{
try
{
manager.checkRead( files[ i ] );
getLogger().info( "Thread can read " + files[ i ] );
}
catch ( SecurityException e )
{
getLogger().info( "Thread can not read " + files[ i ] );
}
}
}
}
</source>
</s2>
<p>
As you can see, it's a straightforward process to implement a new
extension.
</p>
<p>
That's it for the documentation so far, if you have any particular
questions, comments,
please send an email to the avalon developer's mailing
<link href="mailto:[email protected]";>list</link>.
</p>
</s1>
</body>
<footer>
<legal>
Copyright (c) @year@ The Jakarta Apache Project All rights reserved.
$Revision: 1.1 $ $Date: 2002/07/17 16:30:11 $
</legal>
</footer>
</document>
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