Spring父子容器

问题呈现

一个容器可以存在父容器，比如定义一个 Book 类

public class Book {private String name;private Double price;private Integer id;// set get toString...
}

定义两个 XML 文件，一个是父容器的，一个是子容器的：

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xmlns:context="http://www.springframework.org/schema/context"xmlns:aop="http://www.springframework.org/schema/aop"xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsdhttp://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsdhttp://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop.xsd"><bean class="org.example.demo.Book" id="book" ><property name="id" value="111" /><property name="name" value="刘和广" /><property name="price" value="77d" /></bean>
</beans>

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xmlns:context="http://www.springframework.org/schema/context"xmlns:aop="http://www.springframework.org/schema/aop"xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsdhttp://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsdhttp://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop.xsd"><bean class="org.example.demo.Book" id="book" ><property name="id" value="222" /><property name="name" value="刘和广之子" /><property name="price" value="77d" /></bean>
</beans>

现在去加载两个容器，注意必须手动 setConfigLocations，配置文件不能放在 ClassPathXmlApplicationContext 构造方法

当从 child 中查找 bean 的时候，首先去 child 容器中查找，这个容器中如果不存在该 bean，那么就去父容器中查找，父容器中不存在，就去爷爷容器中查找。。。

public class App {public static void main(String[] args) {// 父子容器ClassPathXmlApplicationContext parent = new ClassPathXmlApplicationContext();parent.setConfigLocations("parent.xml");parent.refresh();ClassPathXmlApplicationContext child = new ClassPathXmlApplicationContext();child.setConfigLocations("child.xml");child.setParent(parent);child.refresh();Book book = child.getBean("book", Book.class);System.out.println("book = " + book);}
}

原理分析

要实现父子容器的效果，有一个前提，就是我们的容器本身要支持父子容器，而我们日常开发中最常用的 DefaultListableBeanFactory 这个容器，就是支持父子容器的：

HierarchicalBeanFactory 接口就表示带有层级关系的父子容器了，实现了该接口，就表示这个容器带有层级关系，即具备父子容器的功能。那么接下来，在 Bean 查找的过程中，首先就在当前容器中查找，如果找不到，就去父容器中查找。

根据类型查找，下面这个 getBean 方法最终会调用到 org.springframework.beans.factory.support.DefaultListableBeanFactory#resolveBean 方法：

Book book = child.getBean( Book.class);

private <T> T resolveBean(ResolvableType requiredType, @Nullable Object[] args, boolean nonUniqueAsNull) {//这里就是尝试在当前容器中去查找这个目标 beanNamedBeanHolder<T> namedBean = resolveNamedBean(requiredType, args, nonUniqueAsNull);if (namedBean != null) {return namedBean.getBeanInstance();}//如果当前容器中没有找到 Bean，则去查找到父容器，然后去父容器中查找 beanBeanFactory parent = getParentBeanFactory();if (parent instanceof DefaultListableBeanFactory dlfb) {//父容器存在，并且是 DefaultListableBeanFactory 类型的，那么则调用 resolveBean 方法去处理 bean，这其实相当于递归return dlfb.resolveBean(requiredType, args, nonUniqueAsNull);} else if (parent != null) {//父容器不是 DefaultListableBeanFactory 类型，但是父容器又存在，那么尝试调用父容器中的 getBeanProvider 方法去获取 beanObjectProvider<T> parentProvider = parent.getBeanProvider(requiredType);if (args != null) {return parentProvider.getObject(args);} else {return (nonUniqueAsNull ? parentProvider.getIfUnique() : parentProvider.getIfAvailable());}}return null;
}

根据名字去查找 bean，最终会来到 org.springframework.beans.factory.support.AbstractBeanFactory#doGetBean 方法中：

Book book = child.getBean("book", Book.class);

protected <T> T doGetBean(String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)throws BeansException {...else{// Fail if we're already creating this bean instance:// We're assumably within a circular reference.if (isPrototypeCurrentlyInCreation(beanName)) {throw new BeanCurrentlyInCreationException(beanName);}// 子容器不存在，去父容器查找BeanFactory parentBeanFactory = getParentBeanFactory();if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {// Not found -> check parent.String nameToLookup = originalBeanName(name);if (parentBeanFactory instanceof AbstractBeanFactory abf) {return abf.doGetBean(nameToLookup, requiredType, args, typeCheckOnly);} else if (args != null) {// Delegation to parent with explicit args.return (T) parentBeanFactory.getBean(nameToLookup, args);} else if (requiredType != null) {// No args -> delegate to standard getBean method.return parentBeanFactory.getBean(nameToLookup, requiredType);} else {return (T) parentBeanFactory.getBean(nameToLookup);}}}return adaptBeanInstance(name, beanInstance, requiredType);
}