Spring: Configuration

Question

In Spring Java configuration, suppose I want to re-use a @Bean in another @Bean definition. I can do this either in one file:

@Bean
public A buildA() {
    return new A();
}

@Bean
public B buildB() {
    return new B(buildA());
}

or I can configure A in one file and autowire it in another file like (field injection for brevity):

@Autowired
private A a;

@Bean
public B buildB() {
    return new B(a);
}

I wonder, if the two possibilities are exactly the same? For me it looks as if, the first version might instatiate A twice, while the second doesn't.

I am asking this, since in my special use case, A is establishing a connection to a messaging broker and I have several Bs that consume the stream (I use .toReactivePublisher() from spring integration in A), and I don't want to connect twice or more to the broker.

First version does not instantiate A twice, that construction is very common. Spring beans are singleton by default. — Victor
– Victor, Commented May 8, 2019 at 13:35
Does this still apply, if I have more than one @Bean of type A? — PeMa
– PeMa, Commented May 8, 2019 at 13:39
Another option is passing A as a parameter to buildB: @Bean public B buildB(A a) { return new B(a); } — Jesper
– Jesper, Commented May 8, 2019 at 13:54
@Jesper Ok I didn't know that. So I suppose I can use @Qualifier() for the variable and thus specifiy the bean explicitely? — PeMa
– PeMa, Commented May 8, 2019 at 14:01

Sync · Accepted Answer · 2019-05-08 13:34:27Z

5

Yes, they're exactly the same. Multiple calls to a @Bean annotated method will not create multiple instances of the same bean.

For an explanation on why it doesn't happen, please see this answer.

answered May 8, 2019 at 13:34

Sync

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