Spring @Configuration如何缓存对bean的引用

Question

How does Spring prevent a second call to bar() when using Java based configurations?

I'm wondering compile time annotation processing or by proxying the method?

@Configuration
public class AppConfig {

  @Bean
  public Foo foo() {
      return new Foo(bar());
  }

  @Bean
  public Foo foo2() {
      return new Foo(bar());
  }

  @Bean
  public Bar bar() {
      return new Bar();
  }
}  

Answer 1

Assuming you created your context a little something like

AnnotationConfigApplicationContext context =
    new AnnotationConfigApplicationContext(AppConfig.class);

Because of @Configuration , Spring will create a bean of type AppConfig and proxy it because it has @Bean methods. You should check out ConfigurationClassEnhancer for implementation details.

These methods aren't called on the object directly. Obviously they can't since they aren't known at compile time. They are called through reflection on the proxy.

So when you have

@Bean
public CustomBean foo() {
    return new CustomBean(bar());
}

which is equivalent to

@Bean
public CustomBean foo() {
    return new CustomBean(this.bar());
}

the this is referring to a proxy which caches the result of the method invocation and returns it immediately if it's called it before.

Answer 2

Spring does not "prevent" a call to bar() . Instead, at startup, spring generates a list of @Bean marked methods then calls each method one time. If you want, you can call bar() a hundred times. Spring, however; will only call it one time.