@Bean简单理解

To declare a bean in JavaConfig, you write a method that creates an instance of the
desired type and annotate it with  @Bean . For example, the following method declares
the  CompactDisc bean:
@Bean
public CompactDisc sgtPeppers() {
return new SgtPeppers();
}
The @Bean annotation tells Spring that this method will return an object that should
be registered as a bean in the Spring application context. The body of the method
contains logic that ultimately results in the creation of the bean instance.
By default, the bean will be given an ID that is the same as the  @Bean -annotated
method’s name. In this case, the bean will be named  compactDisc . If you’d rather it
have a different name, you can either rename the method or prescribe a different
name with the  name attribute:
@Bean(name="lonelyHeartsClubBand")
public CompactDisc sgtPeppers() {
return new SgtPeppers();
}
No matter how you name the bean, this bean declaration is about as simple as they
come. The body of the method returns a new instance of  SgtPeppers . But because it’s
expressed in Java, it has every capability afforded it by the Java language to do almost
anything to arrive at the  CompactDisc that is returned.
Unleashing your imagination a bit, you might do something crazy like randomly
selecting a  CompactDisc from a selection of choices:
@Bean
public CompactDisc randomBeatlesCD() {
int choice = (int) Math.floor(Math.random() * 4);
if (choice == 0) {
return new SgtPeppers();
} else if (choice == 1) {
return new WhiteAlbum();
} else if (choice == 2) {
return new HardDaysNight();
} else {
return new Revolver();
}
}
I’ll let you daydream a bit about all the ways you can exploit the power of Java to pro-
duce a bean from an  @Bean -annotated method. When you’re done, we’ll pick it back up
and look at how you can inject the  CompactDisc bean into the  CDPlayer in JavaConfig.