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I've been using spring for some time, but I always wondered how does it work, more specifically, how do they load and weave beans/classes marked only with an interface or @annotation.
For the xml declarations, it's easy to see how spring preprocesses my beans (they are declared in the xml context that spring reads), but for the classes marked only with annotations, I can't see how that works, since I don't pass any agent to the jvm or so.
I believe there is some Java/JVM hook that allows you to preprocess classes by some sort of criteria, but I wasn't able to found out anything on the docs.
Can someone point me to some docs? Is this related to the java.lang.instrument.ClassFileTransformer API?
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There are basically two types of IOC Containers in Spring: BeanFactory: BeanFactory is like a factory class that contains a collection of beans. It instantiates the bean whenever asked for by clients. ApplicationContext: The ApplicationContext interface is built on top of the BeanFactory interface.
Spring IoC Container is the core of Spring Framework. It creates the objects, configures and assembles their dependencies, manages their entire life cycle. The Container uses Dependency Injection(DI) to manage the components that make up the application.
An IoC container is a common characteristic of frameworks that implement IoC. In the Spring framework, the interface ApplicationContext represents the IoC container. The Spring container is responsible for instantiating, configuring and assembling objects known as beans, as well as managing their life cycles.
The Spring container is at the core of the Spring Framework. The container will create the objects, wire them together, configure them, and manage their complete life cycle from creation till destruction. The Spring container uses DI to manage the components that make up an application.
Actually by default Spring does not do any bytecode postprocessing neither for XML-, nor annotation-configured beans. Instead relevant beans are wrapped into dynamic proxies (see e.g. java.lang.reflect.Proxy in the Java SDK). Dynamic proxies wrap the actual objects you use and intercept method calls, allowing to apply AOP advices. The difference is that proxies are essentially new artificial classes created by the framework, whereas weaving/bytecode postprocessing changes the existing ones. The latter is impossible without using the Instrumentation API you mentioned.
As for the annotations, the implementation of <context:component-scan> tag will scan the classpath for all classes with the Spring annotations and create Spring metadata placeholders for them. After that they are treated as if they were configured via XML (or to be more specific both are treated the same).
Although Spring doesn't do bytecode postprocessing itself you can configure the AspectJ weaving agent that should work just fine with Spring, if proxies do not satisfy you.
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