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Re: [aspectj-users] AspectJ Runtime Hooks for Object Persistence
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Ted Neward wrote:
>
> > ... it's not worth using AspectJ; you might as well just use
> javassist.
> >
> You say that with such disdain, Wes. :-)
Please don't pick a fight on my behalf, Ted!
I meant no disrespect for javassist. I think it's a great
package, and I've used it happily. I suppose I should have
stated my recommendation more carefully.
My point was that if he's basically only using this one
(AOP-style) technique (interceptor-proxy), then it might
be better to use a package intended for that. AspectJ as a
language comes with proscriptions designed to handle the
general case (like "be sure to define all aspects before
defining any classes"), but Sean might be perfectly safe
in implementing interceptors on a per-class basis.
The closest actually-working application to what I took Sean to
be asking about (that I know of) is JBoss' use of javassist
(I'm assuming their XML descriptors or class loader can be
controlled via JMX). That's why I recommended it.
Wes
Ted Neward wrote:
>
> > ... it's not worth using AspectJ; you might as well just use
> javassist.
> >
> You say that with such disdain, Wes. :-)
>
> I think Javassist (and other MOP systems) and AspectJ are both equally
> interesting and viable solutions, albeit to differing problems.
>
> Ted Neward
> Author, Instructor, Presenter: Java and .NET
> http://www.neward.net/ted/weblog
> http://www.javageeks.com
> http://www.clrgeeks.com
>
> > -----Original Message-----
> > From: aspectj-users-admin@xxxxxxxxxxx [mailto:aspectj-users-
> > admin@xxxxxxxxxxx] On Behalf Of Wes Isberg
> > Sent: Saturday, March 08, 2003 10:55 AM
> > To: aspectj-users@xxxxxxxxxxx
> > Subject: Re: [aspectj-users] AspectJ Runtime Hooks for Object
> Persistence
> >
> > Hi -
> >
> > > The only issue is how to assign these two aspects at runtime to
> > whatever class I want
> >
> > If/since you're willing to write a class loader and weave at runtime,
> > I'm sure you could
> > figure out how to limit the class targets or regenerate the aspect on
> > the fly. However,
> > I strongly believe that all aspects should be defined before any
> classes
> > are, and that
> > all aspects should apply to all classes. The cases where this is not
> > true are so limited
> > that it's not worth using AspectJ; you might as well just use
> javassist.
> >
> > In any case, with your current interceptor delegate, you must have
> code
> > of the form: {interceptible}.setInterceptor(i);
> > Perhaps that's the point at which to make runtime decisions. That
> does
> > imply
> > every class in your package has the interceptor protocol built in. To
> > reduce the cost
> > for non-intercepted g/setters in the target package, change the
> pointcut
> > so the
> > advice runs only when there is an interceptor (and don't set a default
> > one):
> > before(Interceptable i): getter(i) && if(null !=
> i.getInterceptor())
> > {
> > i.getInterceptor().getField(i,
> > thisJoinPoint.getSignature().getName());
> > }
> >
> >
> > Overall, it looks like you're taking a reflective approach and using
> > AspectJ/AOP to wire it up.
> > Your interceptors are limited to snooping (before advice); around
> advice
> > could change
> > the actual values or flow of control. One alternative design is to
> have
> > the interceptor be the
> > interface, rather than a delegate (assuming you don't have families of
> > interceptors to
> > interpose). Another is to put the snooping logic directly into the
> > advice. Still another
> > is to encapsule the idiom of checking getters/setters in an abstract
> > aspect with template
> > methods for the snooping, and then implement those for each type
> > snooped; that gives
> > you type-safe downwards casts so you can avoid reflection altogether.
> > Also, then
> > you could instantiate the concrete aspects using pertarget to get
> > per-object state (e.g.,
> > for journalling and rollback). And I'm sure there's more ways; I'd be
> > surprised if there
> > were less than 10 ways to do variants of interception in AspectJ.
> >
> > Have fun -
> > Wes
> >
> > P.S. There's a new FAQ entry on the issue of runtime weaving and
> > declarations:
> > http://dev.eclipse.org/viewcvs/indextech.cgi/~checkout~/aspectj-
> > home/doc/faq.html#q:loadtimeWeaving
> >
> > P.P.S. - LCD plain text emails preferred...
> >
> > "Sean R. Drucker" wrote:
> >
> > > AspectJ worked great. Below is what I came up with. The only issue
> > > is how to assign these two aspects at runtime to whatever class I
> want
> > > (as apposed to declaring it statically). Anyone know were I can
> > > look?Thanks... Seanaspect InterceptPointcuts { private
> Interceptor
> > > Interceptable.interceptor = Passthrough.DEFAULT; public void
> > > Interceptable.setInterceptor(Interceptor i) {
> > > interceptor = i == null ? Passthrough.DEFAULT : i;
> > > }
> > > public Interceptor Interceptable.getInterceptor() {
> > > return interceptor;
> > > } declare parents : persistence.objects.* implements
> > > Interceptable;
> > > }aspect Intercept { pointcut getter(Interceptable i):
> > > get(!transient * (persistence.objects.*).*) && target(i);
> > > pointcut setter(Interceptable i, Object value):
> > > set(!transient * (persistence.objects.*).*) && target(i) &&
> > > args(value); before(Interceptable i): getter(i) {
> > > i.getInterceptor().getField(i,
> > > thisJoinPoint.getSignature().getName());
> > > } before(Interceptable i, Object value): setter(i, value) {
> > > i.getInterceptor().setField(i,
> > > thisJoinPoint.getSignature().getName(), value);
> > > }}
> > >
> > > -----Original Message-----
> > > From: aspectj-users-admin@xxxxxxxxxxx
> > > [mailto:aspectj-users-admin@xxxxxxxxxxx]On Behalf Of Sean R.
> > > Drucker
> > > Sent: Saturday, March 01, 2003 9:48 AM
> > > To: aspectj-users@xxxxxxxxxxx
> > > Subject: [aspectj-users] AspectJ Runtime Hooks for Object
> > > Persistence
> > > Can anyone point me in the right direction for doing the
> > > following (can AspectJ do this?): -At runtime, using
> > > weaving, load a class and declareall field get and set
> > > pointcuts for all non-transient fields in the class.
> > > Obviously, field point-cuts from outside the class can not
> > > be created as they require access to the calling code. This
> > > is not an issue. -At runtime, be able to hook/unhook the
> > > object with a single object to intercept the field
> > > pointcuts. Only one hook object can be attached to the
> > > weaved object at a time. Thanks... Sean Reasons for doing
> > > this (if you care): -Developing a large object persistence
> > > project (>100 classes).-Depending on the usage of the
> > > persistence object, the fields will be loaded using
> > > different fetching schemes. Therefore, the fields that need
> > > to be intercepted (to do the background fetching) can vary
> > > at runtime. Sample: public class Employee {
> > > private String name;
> > > private transient int setCount; public String
> > > getName() {
> > > return name;
> > > }
> > > public void setName(String name) {
> > > setCount++;
> > > this.name = name; // edge case
> > > assert ((this.name = this.name) == this.name);
> > > }} Could be weaved as follows:
> > > public class Employee implements Interceptable {
> > > private String name;
> > > private transient int setCount; public String
> > > getName() {
> > > return (String)interceptor.getField(this, "name",
> > > name);
> > > }
> > > public void setName(String name) {
> > > setCount++;
> > > this.name = (String)interceptor.setField(this,
> > > "name", this.name, name); // edge case
> > > assert ((this.name =
> > > (String)interceptor.setField(this, "name", this.name,
> > > (String)interceptor.getField(this, "name",
> > > this.name))) ==
> > > (String)interceptor.getField(this, "name",
> > > this.name));
> > > } // implement hook
> > > public void setInterceptor(Interceptor interceptor) {
> > > this.interceptor = interceptor == null ?
> > > Passthrough.DEFAULT : interceptor;
> > > }
> > > public Interceptor getInterceptor() {
> > > return interceptor;
> > > } private Interceptor interceptor =
> > > Passthrough.DEFAULT;}Passthrough/Interceptor: public class
> > > Passthrough implements Interceptor { public static final
> > > Interceptor DEFAULT = new Passthrough(); public Object
> > > getField(Object obj, String field, Object currentValue) {
> > > return currentValue;
> > > } public Object setField(Object obj, String field,
> > > Object currentValue, Object newValue) {
> > > return newValue;
> > > }
> > > }Thanks...
> > >
> >
> > _______________________________________________
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> > aspectj-users@xxxxxxxxxxx
> > http://dev.eclipse.org/mailman/listinfo/aspectj-users
>
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