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

java 复制代码

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 复制代码

<?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.xsd
        http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd
        http://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 复制代码

<?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.xsd
        http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd
        http://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。。。

java 复制代码

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：

text 复制代码

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

java 复制代码

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 bean
    NamedBeanHolder<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 bean
    BeanFactory 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 bean
        ObjectProvider<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：

text 复制代码

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

java 复制代码

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);
}