Question

As we all know, AOP is provided by CGLIB / JDK Proxy in Spring. Does the weaving happen at runtime, or is the bytecode modified at compiletime to include the advice?

Answer 1

According to Spring documentation, it is done at runtime:

Spring AOP defaults to using standard J2SE dynamic proxies for AOP proxies.

Since Java’s dynamic proxies work only with interfaces:

Spring AOP can also use CGLIB proxies. This is necessary to proxy classes, rather than interfaces.

The Spring documentation has a good explanation of how the proxying works.

Compare this to AspectJ, where the weaving happens on compile, which interestingly enough can happen at various points:

The weaving process itself can take place at one of three different times: compile-time, post-compile time, and load-time.

In AspectJ:

• Compile-time weaving happens by using the ajc compiler to compile your sources.
• Post-compile time weaving happens by using the ajc compiler to add aspects to existing classes/jars.
• Load-time weaving allows you to configure your aspects without recompiling, but requires special support by the class loader (load-time weaving performs weaving when the class loads). It is implemented usually via Java agent instrumentation, and performs worse than the other weaving times.