spring 如何解决bean循环依赖 spring怎么解决的循环依赖

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goody 2024-02-26 16:13:53

文章标签 spring 如何解决bean循环依赖 spring java 面试三级缓存 文章分类 架构后端开发

文章目录

什么是循环依赖？
产生原因
Spring的解决办法
循环依赖解决的过程
源码剖析
总结
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之前简单讲过Spring循环依赖的解决办法，但是没有深入源码分析，今天源码相关分析来了。

什么是循环依赖？

循环依赖问题就是A->B->A，spring在创建A的时候,发现需要依赖B,因为去创建B实例,发现B又依赖于A,又去创建A,因为形成一个闭环，无法停止下来就可能会导致cpu计算飙升

public class A {
	private B b;
}
public class B {
	private A a;
}

产生原因

如图所示

spring 如何解决bean循环依赖 spring怎么解决的循环依赖_spring 如何解决bean循环依赖

Spring的解决办法

为了解决此处闭环，重复循环创建依赖对象，添加三级缓存进行提前暴露对象
spring解决这个问题主要靠巧妙的三层缓存，所谓的缓存主要是指这三个map

singletonObjects主要存放的是单例对象，属于第一级缓存，可以说成品对象；

// Cache of singleton objects: bean name to bean instance. 

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

earlySingletonObjects属于第二级缓存，作用是防止重复创建代理获取提前引用，并且防止循环依赖，可以说半成品对象；

/** Cache of early singleton objects: bean name to bean instance. */

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

singletonFactories属于单例工厂对象，属于第三级缓存

/** Cache of singleton factories: bean name to ObjectFactory. */

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

循环依赖解决的过程

首先画一个获取Bean简单流程

spring 如何解决bean循环依赖 spring怎么解决的循环依赖_spring_02

为什么先展示这个流程呢，因为在你去查看 Spring 解决循环依赖的整个源码的时候，你会发现会多次出现这个流程。

spring 如何解决bean循环依赖 spring怎么解决的循环依赖_面试_03

源码剖析

创建bean.xml

<?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"
       xsi:schemaLocation="
	http://www.springframework.org/schema/beans
	http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
	http://www.springframework.org/schema/context
	http://www.springframework.org/schema/context/spring-context-3.0.xsd">

    <!-- 定义一个Bean -->
    <bean id="a" class="com.demo.test.A">
        <property name="b" ref="b"/>
    </bean>
    <bean id="b" class="com.demo.test.B">
        <property name="a" ref="a"/>
    </bean>
</beans>

测试类

package com.demo.test;
import org.junit.Test;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
/**
 * @Author cuizb
 * @Date 2022/1/18 13:50:43
 * @Desc ****
 */
@SpringBootTest
public class TestDemo {
    @Test
    public void test() {
        ApplicationContext context = new ClassPathXmlApplicationContext("bean.xml");
        A a = (A) context.getBean("a");
    }
}

在获取xml文件处打上断点进行debug，此处是获取解析xml文件

@Override
	protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
		// Create a new XmlBeanDefinitionReader for the given BeanFactory.
		XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);

		// Configure the bean definition reader with this context's
		// resource loading environment.
		beanDefinitionReader.setEnvironment(this.getEnvironment());
		beanDefinitionReader.setResourceLoader(this);
		beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));

		// Allow a subclass to provide custom initialization of the reader,
		// then proceed with actually loading the bean definitions.
		initBeanDefinitionReader(beanDefinitionReader);
		loadBeanDefinitions(beanDefinitionReader);
	}

解析xml获的两个beanName，a和b

spring 如何解决bean循环依赖 spring怎么解决的循环依赖_spring_04

getBean()

for (String beanName : beanNames) {
			RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
			if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
				if (isFactoryBean(beanName)) {
					... ...
				} else {
					getBean(beanName);
				}
			}
		}

doGetBean()

@Override
public Object getBean(String name) throws BeansException {
	return doGetBean(name, null, null, false);
}

spring 如何解决bean循环依赖 spring怎么解决的循环依赖_三级缓存_05

getSingleton(String beanName, boolean allowEarlyReference)方法就是从三级缓存中获取对象，第一次获取A对象时，容器中肯定没有，所以如上图所示，当前对象的值为null，那么接下来就是进行创建A对象。

/**
	 * Return the (raw) singleton object registered under the given name.
	 * <p>Checks already instantiated singletons and also allows for an early
	 * reference to a currently created singleton (resolving a circular reference).
	 * @param beanName the name of the bean to look for
	 * @param allowEarlyReference whether early references should be created or not
	 * @return the registered singleton object, or {@code null} if none found
	 */
	@Nullable
	protected Object getSingleton(String beanName, boolean allowEarlyReference) {
		Object singletonObject = this.singletonObjects.get(beanName);
		if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
			synchronized (this.singletonObjects) {
				singletonObject = this.earlySingletonObjects.get(beanName);
				if (singletonObject == null && allowEarlyReference) {
					ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
					if (singletonFactory != null) {
						singletonObject = singletonFactory.getObject();
						this.earlySingletonObjects.put(beanName, singletonObject);
						this.singletonFactories.remove(beanName);
					}
				}
			}
		}
		return singletonObject;
	}

再次调用getBean的重载方法getSingleton(String beanName, ObjectFactory<?> singletonFactory)进行将A对象工厂，放到三级缓存中，也就是map中，key=beanName，value=lambda表达式（createBean()）

sharedInstance = getSingleton(beanName, () -> {
		try {
			return createBean(beanName, mbd, args);
		}
		catch (BeansException ex) {
			// Explicitly remove instance from singleton cache: It might have been put there
			// eagerly by the creation process, to allow for circular reference resolution.
			// Also remove any beans that received a temporary reference to the bean.
			destroySingleton(beanName);
			throw ex;
		}
	});

之后会对A对象填充b属性值，然后从容器中寻找B，查不到会再次走一遍上述流程，然后存储B对象工厂到三级缓存中，在填充B对象a属性值时，会从容器中查找A对象，会将实例化的A对象从三级缓存删除放在二级缓冲区，key=beanName，value=A对象的地址（A@1554），此时A对象在二级缓存中查到，然后对B对象中a属性赋值A对象的地址，此时B对象生成一个地址B@1558，然后进行初始化B对象，删除三级缓存和二级缓存，放到一级缓存，然后返回到A对象，将B对象的地址赋值给A对象中b属性，然后删除二级缓存，放到一级缓存中去，此时A对象已经完成实例化并且初始化对象。

在对B对象进行实例化时，会发现，B对象已经存在容器中一级缓存，直接获取到B对象。

spring 如何解决bean循环依赖 spring怎么解决的循环依赖_面试_06