As we mentioned earlier, there are two distinct types of AOP: static and dynamic. The difference between them is really the point at which the weaving process occurs and how this process is achieved.
Many of the first AOP implementations were static. In static AOP, the weaving process forms another step in the build process for an application. In Java terms, you achieve the weaving process in a static AOP implementation by modifying the actual bytecode of your application, changing and extending the application code as necessary. Clearly, this is a well-performing way of achieving the weaving process because the end result is just Java bytecode, and you do not perform any special tricks at runtime to determine when advice should be executed.
The drawback of this mechanism is that any modifications you make to the aspects, even if you simply want to add another joinpoint, require you to recompile the entire application. AspectJ is an excellent example of a static AOP implementation.
Dynamic AOP implementations, like Spring AOP, differ from static AOP implementations in that the weaving process is performed dynamically at runtime. How this is achieved is implementation-dependent, but as you will see, Spring's adopted approach is to create proxies for all advised objects, allowing for advice to be invoked as required. The slight drawback of dynamic AOP is that, typically, it does not perform as well as static AOP, but the performance is steadily increasing. The major benefit of dynamic AOP implementations is the ease with which you can modify the entire aspect set of an application without needing to recompile the main application code.
Choosing whether to use static or dynamic AOP is actually quite a hard decision. There is no reason for you to choose a single implementation exclusively, because both have their benefits. Indeed, Spring 1.1 introduced features that allow Spring to integrate with AspectJ, allowing you to use both types of AOP with ease. We cover this feature in more detail in Chapter 7. In general, the static AOP implementations have been around longer, and they tend to have more feature- rich implementations, with a greater number of available joinpoints. Indeed, Spring supports only a subset of the features available with AspectJ. Typically, if performance is absolutely critical or you need an AOP feature that is not implemented in Spring, then you will want to use AspectJ. In most other cases, Spring AOP is ideal for what you are trying to achieve. Make sure you are aware that many AOP-based features are already available in Spring, such as declarative transaction management. Reimplementing these using AspectJ is a waste of time and effort, especially since Spring has tried-and-tested implementations ready for you to use.
Most importantly, let the requirements of your application drive your choice of AOP implementation, and don't restrict yourself to a single implementation if a combination of implementations would better suit your application. In general, we have found that Spring AOP is less complex than AspectJ, so it tends to be our first choice. If we find that Spring AOP won't do what we want it to do, or we discover during application tuning that performance is poor, then we move to AspectJ instead.