Dynamic AOP with JBoss AOP
JBoss AOP is an AOP implementation developed by JBoss. While it came out of the desire to use AOP in the JBoss application server, it is an independent framework like AspectJ that can be used in any Java program. To see how it compares to AspectJ, let's jump right in to the same example recoded for AspectJ. We'll use all of the same code we used with the AspectJ example with a few exceptions. Here's what advice looks like in JBoss AOP:
public class ContextPasser implements Interceptor {
public String getName() {
return getClass().getName();
}
public Object invoke(Invocation invocation) throws Throwable {
ServerSideContext.instance().setContext(
ClientSideContext.instance().getContext());
return invocation.invokeNext();
}
}
JBoss AOP advice is simply a Java class that implements the interface org.jboss.aop.Interceptor. This interface has one trivial method: getName(), which is used for display, and one interesting method, invoke(Invocation), which is where we put the same context-passing code we put in the AspectJ advice. The last line of invoke(Invocation) returns control to the framework. Here it's just a bit of boilerplate, but in a different situation, we could replace the value returned from the actual method call with something else.
That's it! Advice is just a Java class in JBoss AOP, so there is no new syntax to learn and all of your development tools work with it like any other Java code. That takes care of the first objection we raised above.
But where's the equivalent to AspectJ's pointcut expression, which binds the advice to a method call? JBoss AOP provides two different ways to do this. The first uses a configuration file, usually called jboss-aop.xml:
<aop>
<bind pointcut="execution(* *->@contextual(..))">
<interceptor class="ContextPasser"/>
</bind>
</aop>
This file is usually read at class load time, so we can add and remove advice and change the methods to which advice applies without recompiling. If we wish, we can compile our aspects instead, just as we did with AspectJ; this file will then be interpreted at compile time rather than at load time.
The other way to attach our advice is even more flexible. We still need a pointcut in jboss-aop.xml to tell JBoss AOP what methods we might want to advise:
<aop>
<prepare expr="execution(* *->@contextual(..))"/>
</aop>
But this doesn't actually advise any classes, only prepares classes to be advised. (The need to prepare classes proves somewhat annoying, and it may disappear in a future version of the Java VM that has better support for runtime class redefinition.) Having prepared our class, we can advise it in our own code at any time:
public class ProxyFactory {
private static final ProxyFactory INSTANCE = new ProxyFactory();
public static ProxyFactory instance() {
return INSTANCE;
}
public Object getProxy(String name) {
Advised advised = (Advised) new ContextRetrieverImpl();
advised._getInstanceAdvisor().insertInterceptor(new ContextPasser());
return advised;
}
}
In the AspectJ example, we didn't show ProxyFactory because it only constructed and returned an instance of ContextRetrieverImpl. Here we actually instantiate the advice and attach it to the instance we want to advise. With this method, we're completely free to use our advised objects in any context we like. We can have one instance that is advised and another that isn't, or we can add or subtract advice from a single instance as necessary.
I have yet to say anything about how we can free AOP from the issues associated with attaching advice to objects using matching expressions. Careful readers will have noticed that the matching expression used in jboss-aop.xml doesn't actually mention a class or method name. JBoss AOP does allow definition of pointcuts that refer to specific classes, methods, etc., just like AspectJ, but in this example, we chose not to do that. Instead, we use the notation @contextual, which refers to an annotation we placed on the method we want to advise. Here it is in ContextRetrieverImpl:
public class ContextRetrieverImpl implements ContextRetriever {
/** @@contextual */
public Object get(Object key) {
return ServerSideContext.instance().get(key);
}
}
A JBoss AOP annotation looks like a javadoc tag but with a double @ sign. JBoss AOP's AnnotationC tool parses Java source and compiles the annotations it finds into a file, usually called metadata-aop.xml. This gives us the most flexibility possible in attaching advice to our code: when AspectJ-style pointcuts do the job, we can use them, attaching our advice without editing the code being advised. If we need to advise a set of methods for which it's unreasonable to write a pointcut, we can either put annotations on the target methods as shown here, or (if modifying the code to be advised is not an option) we can write metadata-aop.xml ourselves.
If you've read about the features coming in J2SE 1.5 (aka Tiger), you might have noticed that JBoss AOP annotations behave much like J2SE 1.5 annotations (which are specified in JSR 175). In fact, JBoss AOP will support JSR 175 annotations when the final version of J2SE 1.5 becomes available, making its own annotation system unnecessary and removing the need for precompilation.
In software, as elsewhere, power and flexibility come at a price. The design tradeoffs that JBoss AOP makes are typical of those made in the new generation of AOP frameworks. Attaching advice at runtime breaks the separation between your code and the AOP framework. The clean separation of advice targeting from code semantics that annotation allows requires code changes, or the complexity of an additional layer of indirection, and so on. But the only good deal is the one that gets you what you need, and the intense effort that has been expended on the new frameworks makes it clear that these features are needed in the application server world.
The competition
JBoss AOP is given here as an example, but it isn't the only serious second-generation AOP framework out there. The following chart lists more AOP implementations for the Java platform and highlights their features.
AOP frameworks Feature/issue AspectJ AspectWerkz JBoss AOP Spring dynaaop Weaving time Compile/load-time Compile/load-time Compile/load/run Runtime Runtime Transparency Transparent Transparent Choice Factory Factory Per-instance aspects No No Yes Yes Yes Constructor, field, throw, and cflow interception All All Some Some None Annotations No Yes Yes Yes No Standalone Yes Yes Yes No Yes AOP Alliance No No No Yes Yes Affiliation IBM BEA JBoss Spring ?
The frameworks are listed roughly in order of "heaviness," from AspectJ on the left—with its own language and supporting tools, rigid separation of aspects from Java code, and static compilation—to lighter-weight frameworks on the right, which do everything at runtime, albeit with more impact on client code. One can actually think of Java's dynamic proxies as a simple AOP framework; they'd be somewhere off to the right side of the table.
"Transparency" indicates whether a framework operates without requiring changes to client code ("transparent"), or whether it requires clients to obtain advised objects from factories ("factory"). JBoss AOP offers a choice of either technique. Frameworks with "per-instance aspects" can attach advice to selected instances of an advised class and not to others. This differs from the ability to associate a different set of aspect state with each advised instance, which AspectJ and AspectWerkz do offer. Note that using per-instance aspects requires using a factory.
"Constructor, field, throw and cflow interception" shows whether a framework can advise program constructs other than methods. Note that all of these frameworks also support introductions or mixins, an important AOP feature that I didn't demonstrate. Most AOP frameworks are standalone, even those associated with application servers. Only Spring's AOP framework is not, or at least is not available separately. The "AOP Alliance" is a common API for aspect-oriented programming implemented by some frameworks.
Finally, the table lists the application server vendors or developers with which most are associated. The IBM-founded Eclipse Foundation maintains AspectJ, and IBM is expected to add AspectJ support to WebSphere. The originally independent AspectWerkz project is now sponsored by BEA and supports BEA's proprietary JRockit virtual machine. JBoss AOP comes from JBoss and will be used extensively in JBoss 4.0.
This table isn't exhaustive. Many more AOP implementations and related projects are available, some going strong, some just announced, and some fading away. The Aspect-Oriented Software Development conference Website is one place to keep watch.
Where do we go from here?
With so many options for aspect-oriented programming, which one should you use? Different projects will find their best fits in different frameworks. More complex projects, especially those involving distributed, multiuser systems, will probably want the dynamic features of the newer frameworks. I hope that understanding the issues outlined in this article will make it easier for you to choose the right framework for your own projects.
Although most of these frameworks are really standalone, it seems likely that many organizations that come to AOP through application server development will choose the AOP framework allied with their application server. The future of AOP in the Java world may be the future of the application server market. It's too early to tell whether the AOP Alliance or some other standards effort will allow developers to move between frameworks with ease, but it seems unlikely given the diversity in approaches among the different frameworks.
And will AOP ever take hold outside of middleware? Most likely. Most developers will work on AOP-enabled middleware without ever worrying about AOP most of the time. But every project needs to open the hood every now and then, and, having seen what AOP can do in middleware, more and more of us are likely to use it elsewhere in our own software systems. The growing ubiquity of middleware just may be AOP's big break.
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