What is the purpose of the @Configuration annotation?

@Configuration marks a class as a source of bean definitions written in plain Java rather than XML. Spring processes the class specially so that any @Bean methods inside it are only ever invoked once per container, even if one @Bean method calls another directly - the container intercepts the call and returns the existing singleton instance.

@Configuration
public class AppConfig {
    @Bean
    public Clock systemClock() {
        return Clock.systemUTC();
    }
}

This behavior relies on CGLIB subclassing the configuration class at startup, which is why @Configuration classes cannot be declared final. It's the modern, type-safe replacement for the XML beans file used in older Spring applications.

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