Question

I am reading the Pro Spring 2.5 book and in chapter 4 they talk about life cycle callbacks. How is Spring able to notify when an instance is destroyed (after what I read it is only available on singletons)? What mechanism is used?

Answer 1

No magic here, destroy callbacks are called when the BeanFactory/ApplicationContext is destroyed (close() method is called). In desktop applications this has to be done manually, in servlet environment the same mechanism that was used to start the application context (typically ContextLoaderListener) handled destroying for you.

Two notes:

• destroying is important for objects requiring explicit clean-up like database connection pools (DataSource) or threads.

• Beans are destroyed in the opposite order compared to creation order. This is understandable: when bean A depends on B, B has to be created first. When bean A is destroyed, B has to be destroyed afterwards, otherwise A would not be able to access its dependencies during destroy phase.

Also Spring allows you to automatically register shutdown hook to handle shutdown automatically. I wouldn’t advice that, but just for the record:

ConfigurableApplicationContext ctx = new ClassPathXmlApplicationContext(...)
ctx.registerShutdownHook();