How does ApplicationContextAware work in Spring?

When spring instantiates beans, it looks for a couple of interfaces like ApplicationContextAware and InitializingBean. If they are found, the methods are invoked. E.g. (very simplified)

Class<?> beanClass = beanDefinition.getClass();
Object bean = beanClass.newInstance();
if (bean instanceof ApplicationContextAware) {
    ((ApplicationContextAware) bean).setApplicationContext(ctx);
}

Note that in newer version it may be better to use annotations, rather than implementing spring-specific interfaces. Now you can simply use:

@Inject // or @Autowired
private ApplicationContext ctx;