Spring AOP with AspectJ: Load time weaving
If I'm using AspectJ based Spring AOP, am I then tied to configuring my aspects to use load time weaving? Or does Spring AOP also support run time/compile time weaving when using the AspectJ based approach?
Answer
I think we should be careful not to mix up Spring AOP vs. AspectJ.
- Like singh101 said, Spring AOP is proxy-based, more exactly based on Java SE dynamic proxies (for interfaces) or CGLIB proxies (for classes). It uses a subset of AspectJ syntax and is a kind of "AOP lite" approach basically limited to method execution pointcuts, missing many AspectJ pointcut types like method call, class member set/get, constructor call/execution and others. Technologically it is very much different from AspectJ and always incurs a runtime overhead due to the proxy approach (call indirection). Furthermore, it is limited to Spring Bean methods being called from outside the bean class, i.e. it does not work if a bean calls one of its own methods (because it does not go through the corresponding proxy) and it also does not work for non-Spring Bean classes (normal POJOs).
- AspectJ on the other hand is a full-fledged AOP framework which does not rely on either proxies or the Spring framework. It can be easily included into Spring applications, though. It works by generating byte code directly via its own compiler (which is a superset of the Java compiler) or instrumenting existing byte code. AspectJ can be used during compile time (no runtime overhead) or during classloading (load time weaving, LTW). While LTW has a little overhead during application start-up time (but the same applies to Spring AOP), both AspectJ weaving approaches have no runtime overhead due to call indirection because there are no proxies involved.
- The Spring manual chapter on AOP explains nicely how to integrate full AspectJ into Spring when Spring AOP is not powerful enough or simply too slow.