Spring Bean三级缓存机制的技术指南

private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);

private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);

private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);

import org.springframework.beans.factory.config.BeanDefinition;
import org.springframework.beans.factory.support.DefaultSingletonBeanRegistry;
import org.springframework.beans.factory.support.SingletonBeanRegistry;

public class CustomSingletonBeanRegistry extends DefaultSingletonBeanRegistry {

    @Override
    protected Object getSingleton(String beanName, boolean allowEarlyReference) {
        // 获取 Bean 时，先尝试从一级缓存中获取
        Object singleton = super.getSingleton(beanName);
        if (singleton != null) {
            return singleton;
        }

        // 若一级缓存中没有，则检查是否需要从二级缓存中获取
        if (allowEarlyReference) {
            singleton = earlySingletonObjects.get(beanName);
            if (singleton != null) {
                return singleton;
            }
        }

        // 若二级缓存中也没有，则从三级缓存（即singletonFactories）中获取
        singleton = singletonFactories.get(beanName);
        if (singleton != null) {
            return singleton;
        }

        // 如果三级缓存中也没有，就执行标准的实例化过程
        return createBean(beanName);
    }
}

public class BeanLifecycleExample {

    public static void main(String[] args) {
        SingletonBeanRegistry beanRegistry = new CustomSingletonBeanRegistry();
        
        // 模拟一个循环依赖的情况：A依赖B，B依赖A
        beanRegistry.registerSingleton("A", new A(beanRegistry));
        beanRegistry.registerSingleton("B", new B(beanRegistry));

        A aBean = (A) beanRegistry.getSingleton("A", true);
        System.out.println(aBean);
    }
}

class A {
    private final SingletonBeanRegistry registry;
    
    public A(SingletonBeanRegistry registry) {
        this.registry = registry;
        System.out.println("A bean instantiated");
    }

    public void setB(B b) {
        System.out.println("B injected into A");
    }
}

class B {
    private final SingletonBeanRegistry registry;
    
    public B(SingletonBeanRegistry registry) {
        this.registry = registry;
        System.out.println("B bean instantiated");
    }

    public void setA(A a) {
        System.out.println("A injected into B");
    }
}