RE: [aspectj-users] Any useful examples

["Vankeirsbilck, Rudi. (BE - Merelbeke)" <ruvankeirsbilck@xxxxxx>]

I don't think this is the example Edward was looking for. There is a
clean way to solve your problem with plain OO and that is NOT to use
System.getProperty directly but introduce a PropertyManager (maybe
singleton) that has one public method getProperty and implements it
pretty much in the same way as your aspect is doing. But that is besides
the point because there is a non-aop solution to every problem.

The point that needs to be made is that AOP is just another way of
modularizing your code and it does this in a partucular that makes it
easy to deal with cross cutting code. Like with any modularizing
technique, it makes your code more readable, understandable and
maintainable and that is why you would use AOP.

Rudi.

-----Original Message-----
From: R. Dale Asberry [mailto:lists@xxxxxxxxxxxxxxx] 
Sent: vrijdag 8 augustus 2003 13:07
To: aspectj-users@xxxxxxxxxxx
Subject: RE: [aspectj-users] Any useful examples


An app that I'm working on uses properties defined on the command line.
For usability, the application defaults some of them to a best-guess
value so that a user doesn't have to know the ins and outs of the app
just to get it running.  Unfortunately, due to timing issues (e.g., the
app works with RMI which doesn't set some properties until after the
RMISecurityManager is set), these defaults can't be easily set in a
single place.  The calls to the defaulting mechanism got scattered
throughout the code.  At this point, if I add another property that
needs a default, where do I put the call? What if it depends on the
value of another property?  Simply, I have to play with it until I find
a place that works!  So, this suggests that setting properties to
default values is a cross-cutting concern.  I pull the defaulting method
into an aspect method, put the calls to the method into advice, and
define the pointcut to match on certain calls to System.getProperty().
All of my properties that get defaulted are now in ONE place!!  Want to
add another default?  Just put it in the advice with the others.  There
is no simple way to do this using OO.

> -----Original Message-----
> From: aspectj-users-admin@xxxxxxxxxxx 
> [mailto:aspectj-users-admin@xxxxxxxxxxx]On Behalf Of Kenworthy, Edward
> (EDS)
> Sent: Friday, August 08, 2003 2:55 AM
> To: 'aspectj-users@xxxxxxxxxxx'
> Subject: [aspectj-users] Any useful examples
>
>
> Been following Aspects for a year/18 months but never really dived in,

> principally because I still don't see the point ;-)
...


Here's the code:

------------------------------------
pointcut getProperty(String pProperty):
	call(static String System.getProperty(String)) &&
	args(pProperty) &&
	within(org.jini.judy.*) &&
	!withincode(boolean
org.jini.judy.JudyFunctionalAspects.propertyExists(String)) &&
	!withincode(void
org.jini.judy.JudyFunctionalAspects.defaultTo*(String,
String));

before(String pProperty): getProperty(pProperty)
{
	if("webservice.storage.home".equals(pProperty))
	{
		defaultToValue("webservice.storage.home",
				System.getProperty("judy.home") +
				System.getProperty("file.separator") +
				"webservices");
	}
	else if("judy.scripts.home".equals(pProperty))
	{
		defaultToValue("judy.scripts.home",
				System.getProperty("judy.home") +
				System.getProperty("file.separator") +
				"scripts");
	}
	else if("ant.home".equals(pProperty))
	{
		defaultToValue("ant.home",
				System.getProperty("judy.home") +
				System.getProperty("file.separator") +
				"lib");
	}
	else if("java.security.policy".equals(pProperty))
	{
		defaultToValue("java.security.policy",
				System.getProperty("judy.home") +
				System.getProperty("file.separator") +
				"conf" +
				System.getProperty("file.separator") +
				"all.policy");
	}
	else if("judy.axis.codebase.server".equals(pProperty))
	{
		defaultToProperty("judy.axis.codebase.server",
"java.rmi.server.codebase");
	}
	else if("judy.home".equals(pProperty))
	{
		defaultToProperty("judy.home", "user.dir");
	}
}

private boolean propertyExists(String pProperty)
{
	String lProperty = System.getProperty(pProperty);
	if("".equals(lProperty) || lProperty == null)
	{
		return false;
	}
	return true;
}

private void defaultToProperty(String pProperty, String
pDefaultProperty) {
	if(!propertyExists(pProperty))
	{
		System.out.println(pProperty + " is not defined.\n\t" +
			"Defaulting to " + pDefaultProperty + " [" +
			System.getProperty(pDefaultProperty) + "]");
		System.setProperty(pProperty,
System.getProperty(pDefaultProperty));
	}
}

private void defaultToValue(String pProperty, String pDefaultValue) {
	if(!propertyExists(pProperty))
	{
		System.out.println(pProperty + " is not defined.\n\t" +
			"Defaulting to [" +
			pDefaultValue + "]");
		System.setProperty(pProperty, pDefaultValue);
	}
}
------------------------------------

An interesting (but non-obvious) point to note is that the pointcut
matches on System.getProperty() calls in the before() advice.  This
makes the order of defaulting unimportant!  Another nice side-effect is
that the console is spitting out that a property is being defaulted.  If
the default isn't working, the app user knows what to change the value
to, as well as, which property needs to be set.

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