Java Spring中如何在依赖类构造函数之前调用依赖类构造函数

我使用Spring框架编写了以下代码.

CODE个

package com.spring.core.trialDI;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.context.ApplicationContext;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;
import org.springframework.stereotype.Component;

@Configuration
@ComponentScan("com.spring.core.trialDI")
class LegendConfig {

}

@Component
class A {
    public A() {
        System.out.println("I am A constructor!");
    }
}

@Component
class B {
    A a;

    public B() {
        System.out.println("I am B constructor!");
    }

    @Autowired
    public void setA(@Qualifier("a") A a) {
        System.out.println("I am setA of B!");
        this.a = a;
    }
}

@Component
class C {
    A a;

    public C() {
        System.out.println("I am C constructor!");
    }

    @Autowired
    public void setA(@Qualifier("a") A a) {
        System.out.println("I am setA of C!");
        this.a = a;
    }
}
public class TrialTheory {
    public static void main(String[] args) {
        ApplicationContext applicationContext = new AnnotationConfigApplicationContext(LegendConfig.class);
        B b = applicationContext.getBean(B.class);
        C c = applicationContext.getBean(C.class);
    }
}

OUTPUT个

I am A constructor!
I am B constructor!
I am setA of B!
I am C constructor!
I am setA of C!

现在和在main()方法中一样，我首先创建了一个B类型的Bean，所以class B的构造函数必须在setter之前先调用.(构造函数创建对象，其方法只能在创建后使用).

但是，为什么在任何人之前调用A构造函数呢？幕后发生了什么？