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Spring 源码学习~ 循环依赖(面试必问系列,学 java 基础的心得体会

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package com.example;


import com.example.config.AppConfiguration;


import org.springframework.context.annotation.AnnotationConfigApplicationContext;


import com.example.bean.A;


/**


  • <pre>

  • </pre>

  • <pre>

  • @author mazq

  • 修改记录

  • 修改后版本: 修改人: 修改日期: 2020/11/05 10:22 修改内容:

  • </pre>


*/


public class TestApplication {


public static void testCircularReferences() {


AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext();


context.register(AppConfiguration.class);


//context.setAllowCircularReferences(false);


context.refresh();


A bean = context.getBean(A.class);


System.out.println(bean);


}


public static void main(String[] args) {


// 测试 Sprin 循环依赖


testCircularReferences();


}


}


经过测试,一直在循环调用:



4、循环依赖解决方法


==============


对于这种情况,Spring 有处理方法?答案是有的,方法就是通过 @Autowired 注解,当然 bean 要是单例的,多例的情况不支持,原因后面分析


@Component


public class A {


@Autowired


B b;


public A() {


System.out.println("A class is create");


}


}



补充:除了 @Autowired 方法,我们还可以通过 set 方法处理循环依赖问题,当然也是仅支持单例 bean,多例的情况不支持


5、关闭 Spring 循环依赖


==================


有个疑问?Spring 的循环依赖支持,默认情况是开启?是否有什么开关控制?通过源码学习,可以通过 setAllowCircularReferences 设置


AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext();


context.register(AppConfiguration.class);


// 关闭 Spring 循环依赖支持


context.setAllowCircularReferences(false);


context.refresh();


通过测试,设置不开启这个属性的时候,即使加上 @Autowired,代码还是抛异常了


6、prototype(多例)循环依赖


=======================


在多例的情况,Spring 能支持循环依赖?加上 @Scope("prototype"),将 bean 变成多例的



经过测试:多例的情况会抛出异常,即使加上了 @Autowired,原因请看下文


7、Spring 循环依赖特征


==================


ok,经过前面例子的验证,到这来,可以对 Spring 的循环依赖特点进行归纳:


  • Spring 中的循环依赖场景构造器的循环依赖,通过构造函数 Field 属性的循环依赖,通过 set 方法

  • Spring 的循环依赖是默认开启的(setAllowCircularReferences)

  • Spring 对单例和多例 Bean 的支持单例 Bean(singleton) :只能通过 @Autowired 和 set 方法支持多例 Bean(prototype):默认不支持,直接抛异常 BeanCurrentlyInCreationException


8、Spring 循环依赖原理


==================


我们通过实验进行了验证,也归纳出了 Spring 循环依赖的特点,然后具体原因是什么?我们只能通过源码学习得到答案


在上一章的学习中,我们对 Bean 的创建有了一个粗略的了解,所以,顺着这条路线,跟下源码:


在前面的学习,我们知道了{@link org.springframework.beans.factory.support.AbstractBeanFactory#doGetBean}这个方法就是 Spring Bean 创建的真正执行方法


protected <T> T doGetBean(


String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)


throws BeansException {


// 处理 BeanName,前面说的 FactoryBean 带‘&’符号,要在这里进行转换


String beanName = transformedBeanName(name);


Object bean;


// Eagerly check singleton cache for manually registered singletons.


// 从 map(singletonObjects)里获取单例 bean,确定是否已经存在对应实例


Object sharedInstance = getSingleton(beanName);


if (sharedInstance != null && args == null) {


if (logger.isDebugEnabled()) {


if (isSingletonCurrentlyInCreation(beanName)) {


logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +


"' that is not fully initialized yet - a consequence of a circular reference");


}


else {


logger.debug("Returning cached instance of singleton bean '" + beanName + "'");


}


}


// 两种情况:普通的 bean,直接从 singletonObjects 返回 sharedInstance


//如果是 FactoryBean,返回其创建的对象实例


bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);


}


else {


// Fail if we're already creating this bean instance:


// We're assumably within a circular reference.


// 校验是否是多例(Prototype)的 Bean,多例的 bean 是不支持循环依赖的


// 为了避免循环依赖,遇到这种情况,直接抛出异常


if (isPrototypeCurrentlyInCreation(beanName)) {


throw new BeanCurrentlyInCreationException(beanName);


}


// Check if bean definition exists in this factory.


// 检查 BeanFactory 是否存在这个 BeanDefinition


BeanFactory parentBeanFactory = getParentBeanFactory();


if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {


// Not found -> check parent.


// 当前容器找不到 BeanDefinition,去 parent 容器查询


String nameToLookup = originalBeanName(name);


if (parentBeanFactory instanceof AbstractBeanFactory) {


return ((AbstractBeanFactory) parentBeanFactory).doGetBean(


nameToLookup, requiredType, args, typeCheckOnly);


}


else if (args != null) {


// Delegation to parent with explicit args.


// 返回 parent 容器的查询结果


return (T) parentBeanFactory.getBean(nameToLookup, args);


}


else {


// No args -> delegate to standard getBean method.


return parentBeanFactory.getBean(nameToLookup, requiredType);


}


}


if (!typeCheckOnly) {


//typeCheckOnly 为 false 的情况,将 beanName 放在一个 alreadyCreated 的集合


markBeanAsCreated(beanName);


}


try {


RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);


checkMergedBeanDefinition(mbd, beanName, args);


// Guarantee initialization of beans that the current bean depends on.


// 校验是否配置了 depends-on


String[] dependsOn = mbd.getDependsOn();


if (dependsOn != null) {


for (String dep : dependsOn) {


// 存在循环引用的情况,要抛出异常


if (isDependent(beanName, dep)) {


throw new BeanCreationException(mbd.getResourceDescription(), beanName,


"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");


}


// 正常情况,注册依赖关系


registerDependentBean(dep, beanName);


try {


// 初始化被依赖项


getBean(dep);


}


catch (NoSuchBeanDefinitionException ex) {


throw new BeanCreationException(mbd.getResourceDescription(), beanName,


"'" + beanName + "' depends on missing bean '" + dep + "'", ex);


}


}


}


// Create bean instance.


// 单例的 Bean


if (mbd.isSingleton()) {


sharedInstance = getSingleton(beanName, () -> {


try {


// 创建单例 bean


return createBean(beanName, mbd, args);


}


catch (BeansException ex) {


// Explicitly remove instance from singleton cache: It might have been put there


// eagerly by the cre


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ation process, to allow for circular reference resolution.


// Also remove any beans that received a temporary reference to the bean.


destroySingleton(beanName);


throw ex;


}


});


bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);


}


// 多例的 Bean,scope = protoType


else if (mbd.isPrototype()) {


// It's a prototype -> create a new instance.


Object prototypeInstance = null;


try {


// 多例的情况,创建 bean 之前添加标记(用于循环依赖校验)


beforePrototypeCreation(beanName);


// 执行多例 Bean 创建


prototypeInstance = createBean(beanName, mbd, args);


}


finally {


// 创建原型(多例)bean 之后擦除标记


afterPrototypeCreation(beanName);


}


bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);


}


// 如果不是单例 bean 也不是多例的 bean,委托给对应的实现类


else {


String scopeName = mbd.getScope();


if (!StringUtils.hasLength(scopeName)) {


throw new IllegalStateException("No scope name defined for bean ′" + beanName + "'");


}


Scope scope = this.scopes.get(scopeName);


if (scope == null) {


throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");


}


try {


Object scopedInstance = scope.get(beanName, () -> {


beforePrototypeCreation(beanName);


try {


// 执行 bean 创建


return createBean(beanName, mbd, args);


}


finally {


afterPrototypeCreation(beanName);


}


});


bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);


}


catch (IllegalStateException ex) {


throw new BeanCreationException(beanName,


"Scope '" + scopeName + "' is not active for the current thread; consider " +


"defining a scoped proxy for this bean if you intend to refer to it from a singleton",


ex);


}


}


}


catch (BeansException ex) {


cleanupAfterBeanCreationFailure(beanName);


throw ex;


}


}


// Check if required type matches the type of the actual bean instance.


// 检查一下类型是否正确,不正确抛出异常,正确返回实例


if (requiredType != null && !requiredType.isInstance(bean)) {


try {


T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);


if (convertedBean == null) {


throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());


}


return convertedBean;


}


catch (TypeMismatchException ex) {


if (logger.isDebugEnabled()) {


logger.debug("Failed to convert bean '" + name + "' to required type '" +


ClassUtils.getQualifiedName(requiredType) + "'", ex);


}


throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());


}


}


return (T) bean;


}


  • 源码比较复杂,所以可以带着疑问来跟,首先以单例 Bean 的情况:#doGetBean.getSingleton



protected Object getSingleton(String beanName, boolean allowEarlyReference) {


// 一级缓存:singletonObjects (单例池)


Object singletonObject = this.singletonObjects.get(beanName);


if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {


synchronized (this.singletonObjects) {


// 二级缓存:earlySingletonObjects(BeanDefinition 还没进行属性填充)


singletonObject = this.earlySingletonObjects.get(beanName);


if (singletonObject == null && allowEarlyReference) {


// 三级缓存:singletonFactories


ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);


if (singletonFactory != null) {


singletonObject = singletonFactory.getObject();


this.earlySingletonObjects.put(beanName, singletonObject);


this.singletonFactories.remove(beanName);


}


}


}


}


return singletonObject;


}


在某些情况,循环依赖会造成循环调用,所以需要怎么解决?



Spring 框架的方法是使用了三级缓存,其实最关键的是 earlySingletonObjects


  • 一级缓存:singletonObjects,这是 Spring BeanDefinition 的单例池,首先只保存单例 Bean 的 BeanDefinition,而且这个 Bean 是一个真正的 bean,也就是进行过属性填充的

  • 二级缓存:earlySingletonObjects,early 从单词意思来说,这个缓存是在 singletonObjects 之前的,也就是 BeanDefinition 还没进行属性填充等等操作,Spring 引入这个缓存的目的就是为了处理单例 bean 的循环依赖问题

  • 三级缓存:singletonFactories,缓存的是 ObjectFactory,表示对象工厂,为什么要加上这个缓存?原因比较复杂,涉及到 AOP 等等原因,因为我还没理解清楚,所以本文不说明


加上了 earlySingletonObjects 缓存之后,Spring 就能支持单例 bean 的循环依赖,参考语雀某大佬的笔记,画图表示:



  • 带着疑问来跟一下多例 Bean 的情况:


Spring 框架是不支持多例 bean 的循环依赖的,原因跟下代码:#doGetBean


// Fail if we're already creating this bean instance:


// We're assumably within a circular reference.


// 校验是否是多例(Prototype)的 Bean,多例的 bean 是不支持循环依赖的


// 为了避免循环依赖,遇到这种情况,直接抛出异常


if (isPrototypeCurrentlyInCreation(beanName)) {


throw new BeanCurrentlyInCreationException(beanName);

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Spring源码学习~循环依赖(面试必问系列,学java基础的心得体会