Second-generation aspect-oriented programming

Apply advice dynamically with the new crop of AOP frameworks

Modularization makes programming possible. Throughout the history of computing, a parade of organizational devices—the high-level language, the subroutine, the object—has allowed us to write increasingly more expressive and powerful code. But, just as with computer hardware, when our abilities improve, we raise the bar again, and here in the twenty-first century, we still struggle to quickly and cheaply produce large programs. What is the next step, the new way to structure our programs that will take our abilities to the next level?

Aspect-oriented programming (AOP) is one attempt at an answer. Conceived at Xerox PARC (an auspicious pedigree!) in the late 1990s, AOP intends to modularize cross-cutting concerns: lines of code that would have to be repeated throughout an ordinary program. AOP gathers all of these cross-cutting concerns into a single place, an AOP construct similar to a class, known as advice.

AspectJ, also originally from Xerox PARC and now developed by the Eclipse Foundation, is an implementation of AOP for the Java platform. It is a mature and solid framework that has gone through several significant releases and is even supported by some third-party tools. Recently, however, application server designers have realized that while—just as AOP proponents have been saying for years—AOP seems a natural way to implement many kinds of application server functionality such as remoting, persistence, and transactions, AOP would be much easier to use in the dynamic environment of the Java platform if its implementation were equally dynamic.

For a thorough introduction to AOP concepts and the AspectJ implementation, see Ramnivas Laddad's three-part JavaWorld series, "I Want My AOP!". For this discussion, I assume you're up to speed on AOP basics and briefly present classic AOP examples so we can get on to the new stuff.

Old-school AOP with AspectJ

Here's an example of how aspect-oriented programming might be used in a middleware framework: Imagine that in our framework, a client accesses services via proxies. The services might be in another VM and might be reached by any remoting technology; hiding these things from the client is the framework's reason for being. One of our framework's features is its ability to propagate any context that a developer wishes from the client to the services it calls transparently to the client. For example, an application might log a user into a security service and put an authentication token in the context. From then on, any services called by that application would be able to retrieve the authentication token from the context—on the server side—and use it to control the functionality to which the client has access.

First, let's write a simple test to show that context is propagated:

public class ContextPassingTest extends TestCase {
   public void test() {
      ClientSideContext.instance().put("userID", "dave");
      ContextRetriever proxy = (ContextRetriever)
         ProxyFactory.instance().getProxy("ContextRetriever");
      assertEquals("dave", proxy.get("userID"));
   }
}

In our test, we first put an authentication token into the context. Next, we get a proxy to our service from a singleton ProxyFactory. (This is an example of the Service Locator pattern, in which a factory hides from the client the complexity of constructing a proxy to a remote service.) The service, an instance of ContextRetriever, simply returns the requested value from its context. In the test's last line, we ask for our authentication token back and test to see whether it has the value it should. That's it!

As with any nice compact example, a couple of comments are in order. First, note that while this test may seem rather pointless, in a real application, we would read from the context in a place different from where we would write to it and actually use context information on the server side instead of just sending it back.

Second, note that, although this example uses the Singleton pattern in several places because it is well known and succinct, if you ever find yourself writing a widely used framework, you should certainly not use the Singleton pattern in its API. Singleton requires the singleton instance to be a concrete class, whereas interesting classes should always be hidden behind interfaces to allow the implementations to be swapped without affecting clients. Furthermore, since the singleton reference is global, it proves difficult to make an object use a different instance when necessary, such as in testing. (See "Use Your Singletons Wisely" for more on this topic.) Having warned you about the Singleton pattern, I return to using it for brevity, but don't take it to heart.

Now let's look at the classes our test uses. ClientSideContext is simply a singleton wrapper around a HashMap, a place to store context until it's needed:

public class ClientSideContext {
   private static final ClientSideContext INSTANCE = new ClientSideContext();
   public static ClientSideContext instance() {
      return INSTANCE;
   }
   private final Map mContext = new HashMap();
   public void put(Object key, Object value) {
      mContext.put(key, value);
   }
   public Map getContext() {
      return mContext;
   }
}

The ContextRetriever interface (not shown; see Resources for complete source code) has a single method, get(Object). In this simple example, ProxyFactory (also not shown) just creates and returns an instance of the following ContextRetriever implementation:

public class ContextRetrieverImpl implements ContextRetriever {
   public Object get(Object key) {
      return ServerSideContext.instance().get(key);
   }
}

ContextRetrieverImpl delegates to a singleton instance of ServerSideContext, which is similar to ClientSideContext but is used on the server side:

public class ServerSideContext {
   private static final ServerSideContext INSTANCE = new ServerSideContext();
   public static ServerSideContext instance() {
      return INSTANCE;
   }
   private final Map mContext = new HashMap();
   public void setContext(Map context) {
      mContext.putAll(context);
   }
   public Object get(Object key) {
      return mContext.get(key);
   }
}

So, how does the context get from client to server? With the following aspect:

aspect ContextPasser {
   before(): execution(* ContextRetrieverImpl.*(..)) {
      ServerSideContext.instance().setContext(
         ClientSideContext.instance().getContext());
   }
}

This aspect contains a single piece of advice. The advice is before() advice, which runs before the method being advised. The execution() statement determines the methods before which the advice runs. In this case, the expression ContextRetrieverImpl.*(..), referred to as a pointcut, causes the advice to run before any method of the class ContextRetrieverImpl. In the advice's body, where the actual work is done, we finally see how our context is passed: the entire context is copied from ClientSideContext to ServerSideContext. In a real framework, of course, the server side might be in another VM and we'd have a bit more work to do, but our short version illustrates the point.

Writing our context-passing functionality in an aspect gives us some nice advantages over a conventional object-oriented design. It reduces the dependencies from the client and server sides to the context-related classes, without requiring the service to implement an interface (as an EJB (Enterprise JavaBean) component must do to receive a SessionContext). We've actually decoupled context passing from the rest of the framework; so, if for some reason we don't need context passing—perhaps, in some applications, we intend to use only services that don't need context—we can recompile the application without the ContextPasser aspect. The test above will fail, but everything will compile, and code that doesn't require context to be passed will work just fine without the overhead of passing unused context. This is exactly the sort of modularization that AOP was intended to provide. And we don't need to stop here: security enforcement code and even the proxying itself can be moved into aspects as well.

An AOP wish list

As powerful as the AspectJ implementation of AOP is, a demanding developer can always find something new to request. The most obvious is right in front of us: aspects aren't written in Java. That means learning not just a new design, but also new language syntax—and not only does the developer need to understand the AspectJ language, so do the tools that he or she uses. An all-Java way of writing aspects would be welcome.

The way AspectJ ties advice to methods—writing a pointcut expression that matches the names of the methods to be advised—changes the meaning of traditional Java programs in another way. In ordinary Java, a method name is simply an identifier. Far more important than any comment, a well-chosen name is the best way of making a method's purpose obvious to a reader. But in AspectJ, method names (and class, constructor, and field names—although I've given only an example of advising a method, AspectJ can also attach advice to other program constructs) have two purposes: as well as communicating, they must also serve as targets of matching expressions. Changing a method name in a program that includes aspects can cause the method to not be advised when it should be, or to be advised when it shouldn't. Adding and removing methods can cause unexpected effects with aspects as well. Some tools address these issues with AspectJ, but aspects aren't making our life as simple as we had hoped. And even if we manage to keep all of our pointcuts synchronized with our identifiers, we may be tempted to change our identifiers to match existing pointcuts or allow us to write shorter pointcuts, which might compromise our program's readability. Is there another way?

A different issue arises when considering more complex applications than our little example, particularly in distributed, multiuser systems. The advice shown in our example applies to every instance of ContextRetrieverImpl in the virtual machine. Instead, we might want to be able to have some instances of ContextRetrieverImpl that are advised and some that are not. For example, if advice attached to an object consumes significant memory or other resources, we probably don't want it attached to currently unused instances sitting in a cache. Advice with references to unserializable objects might prevent us from serializing an otherwise serializable object. AspectJ does not currently allow us to advise only some instances of a class. There are workarounds, but it would be nice to address the issue directly.

Finally, we might like another kind of flexibility: we might like to be able to add aspects to or subtract aspects from an already-compiled program, or to change the program constructs to which an aspect applies without recompiling. We can't with AspectJ because it weaves advice into advised classes at compile time. (Demonstrate this with the example by deleting the compiled aspect, ContextPasser.class, before running the test.) This behavior violates good object-oriented practices, which strive for designs whose behavior can be changed by configuration, without recompiling. The just-released AspectJ 1.2 does have limited support for weaving advice at load time rather than at compile time, but it addresses only some of the issues discussed here.

The recent trend in the enterprise software world towards lightweight, transparent middleware runs head-on into these issues. In the last year or so, numerous developer groups, looking for a way around the ever-increasing complexity of EJB, have poured their energy into new frameworks aiming to reduce the amount of boilerplate code and configuration that must be written (or generated) when creating a service. High-profile examples in the open source world include JBoss 4.0, Spring, PicoContainer, and HiveMind. Large application server vendors are interested as well, as evidenced, for example, by BEA's support of the Beehive project. Many Web services frameworks, both free and commercial, provide a simplified container as well as or instead of an EJB-based container. And finally, the trend reached a new level of importance in May when Sun's Linda DeMichiel revealed that EJB 3.0 will sweep away much of the old EJB API as well.

All of this activity has energized several interesting areas of investigation in application server design. The facet of interest to our AOP discussion is that middleware has finally been widely recognized as AOP's killer application. Much application-server functionality can be cleanly and logically expressed as aspects. Context-passing, remoting, security, and transactions can all be thought of as add-on functionality that happens "around" (before and/or after) a method call to an ordinary object. Aspect-oriented programming allows an application server designer to provide these features without requiring service developers to extend abstract classes or implement interfaces.

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