leosimons 2002/11/22 13:47:21
Added: interceptor/src/xdocs interceptor.xml
Log:
Some initial thoughts on interceptor architecture. Submitted by Peter Donald
Revision Changes Path
1.1 jakarta-avalon-excalibur/interceptor/src/xdocs/interceptor.xml
Index: interceptor.xml
===================================================================
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE document PUBLIC "-//APACHE//DTD Documentation V1.1//EN"
"document-v11.dtd">
<document>
<header>
<title>Dynamic Interceptor Chains</title>
</header>
<body>
<section>
<title>Introduction</title>
<p>
Interceptors are objects that sit between the implementation of a
method
and the interface via which that method is called. is An Interceptor
is a
component through which a call to a method will pass. The method
invocation
will first pass from Caller to the Interceptor and then from the
Interceptor
to the Target method and then back through the Interceptor to the
Caller.
</p>
<p>
An Interceptor Chain or Stack is a series of interceptors through
which
an invocation will pass on way down to the method. After the method
completes the invocation will pass back through the chain of
Interceptors
in the reverse order of which they were called.
</p>
<p>
Figure 1 displays such a situation. The Caller invokes a method, it
passes through several Interceptors before invoking the Target method
and
then it passes back through all the Interceptors to the Caller.
</p>
<figure src="images/interceptor.png"
alt="An Interceptor Chain"
width="454" height="340"/>
<p>
An Interceptor can be called simultaneously by multiple threads and by
multiple clients. Thus information pertaining to the particular call
needs to be stored either in ThreadLocal variables (if it does not
need
to be shared) or in the InvocationContext (if it may need to be
accessed
by other Interceptors). The InvocationContext is where all the
information relating to a particular call is stored. The Context may
also
give access to information in different scopes (such as per object or
per session).
</p>
</section>
<section>
<title>Basic Example</title>
<p>
A basic Interceptor is shown in figure 2. It gets the time before and
after the call and displays the duration of the call to standard
output.
It demonstrates the basic format of an Interceptor. Usually an
Interceptor will execute some operations before a call is made on
Target
method and after a call is made.
</p>
<source>
public class MyTimingInterceptor
implements Interceptor
{
public Object invoke( Invocation invocation,
InvocationContext ctx,
InvocationChain chain )
{
final long start = System.currentTimeInMillis();
final Object result = chain.invokeNext( invocation, ctx );
final long end = System.currentTimeInMillis();
System.out.println( "Invocation duration: " + (end - start) );
return result;
}
}
</source>
</section>
<section>
<title>Context Using Example</title>
<p>
Another example is shown in figure 2. It sets the ContextClassLoader
prior to calling the method and then resets it to original value
before
returning to caller. Note that this assumes that the Target method is
in
the same thread as the Interceptor which will be the case unless a
Interceptor later in the chain changes threads. This Interceptor also
demonstrates that values can be retrieved from the InvocationContext.
The
specific values that are available to an Interceptor are determined by
the host application server. In the case of Phoenix see X.
</p>
<source>
public class MyClassLoaderInterceptor
implements Interceptor
{
public Object invoke( Invocation invocation,
InvocationContext ctx,
InvocationChain chain )
{
//Retrieve the ClassLoader object from context.
//Note that the set of keys and values in context is
//container dependent. See Container documentation for relevent
//set of attributes that are valid
final ClassLoader classLoader =
(ClassLoader)ctx.get( "classLoader" );
final ClassLoader oldClassLoader =
Thread.currentThread().getContextClassLoader();
Thread.currentThread().setContextClassLoader( classLoader );
final Object result = chain.invokeNext( invocation, ctx );
Thread.currentThread().setContextClassLoader( oldClassLoader );
return result;
}
}
</source>
</section>
<section>
<title>Constructing Interceptor Chains</title>
<p>
There are numerous policies via which Interceptor chains could be
created.
One such mechanism is to construct an interceptor chain based on
particular
objects <link href="attribute.html">Attributes</link>. Other policies
include
constructing chains in preconfigured arrangments or by using
configuration
files such as;
</p>
<source><![CDATA[
<interceptor-chains>
<intercetor-chain name="MyInterceptorChain">
<!-- Log the call for debugging purposes -->
<interceptor type="org.apache.avalon.LogInterceptor"/>
<!-- Make sure the call is authorized to execute method
and that correct principle has been setup. -->
<interceptor type="org.apache.avalon.AuthorizeInterceptor"/>
<!-- Charge caller for use of the service -->
<interceptor type="org.apache.avalon.AccountingInterceptor">
<!-- configuration passed to the Interceptor. It costs
2c per call -->
<cost>0.02</cost>
</interceptor>
<!-- Make sure the ThreadContext data (like ContextClassLoader)
is setup properly -->
<interceptor type="org.apache.avalon.ThreadContextInterceptor"/>
</intercetor-chain>
</interceptor-chains>
]]></source>
</section>
<section>
<title>Example Interceptors</title>
<p>
<strong>Transaction</strong>: Manage transaction state in a way
similar to EJB
declarative transaction "attributes". May have mandatory, incompatible
etc and
can result in commit or rollback on failure etc.
</p>
<p>
<strong>Security</strong>: Make sure the caller has the right
permissions,
the caller has principle correctly setup and the method is invoked as
correct subject.
</p>
<p>
<strong>Audit</strong>: Record who, when, what and where a method is
called.
</p>
<p>
<strong>Application Isolation</strong>: Make sure caller context does
not interfere with
context of called method. This includes managing things like thread
names, ContextClassLoader
etc.
</p>
<p>
<strong>Stale References</strong>: Make sure stale references are not
used. ie If an object has
been disposed of make sure that no one trys to call the object again.
</p>
<p>
<strong>Pool Objects</strong>: Objects may be pooled with a particular
policy. ie The target object
may be retrieved from a pool prior to method invocation and then
returned to pool after invocation.
</p>
<p>
<strong>Passivate/Activate Objects</strong>: Objects may be passivated
(serialized to disk) if not used
recently and then activated (deserialized from disk) when needed.
</p>
<p>
<strong>Lazy Creation</strong>: Make sure objects are created and
properly initialized before
they can be accessed.
</p>
<p>
<strong>Binding Objects</strong>: Bind objects into a name service or
registry (ie JMX, JNDI, LDAP,
RMI registry etc) the first time they are accessed.
</p>
<p>
<strong>Remoting Objects</strong>: Make sure object is remoted via
RMI, SOAP, AltRMI.
</p>
<p>
<strong>Sub-Component Activator</strong>: Make sure that the first
time a component is accessed,
that all it's sub-components are activated.
</p>
</section>
</body>
</document>
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