前言介绍

本文浅谈Spring循环依赖问题,这是一个面试比较常见的问题

1、什么是循环依赖?

所谓的循环依赖就是两个以及两个以上的类互相调用依赖,形成闭环

// 类A依赖于B
class A{
    public B b;
}

// 类B依赖了C
class B{
    public C c;
}

// 类C依赖了A
class C{
    public A a;
}

 


然后?看起来是很正常的,我们随便new一个类,循环依赖的类都是能正常调用的

A a = new A();
System.out.println(a);

为什么?因为这种情况,A.java->A.class,我们new就能获取到实例的对象,这个通过jvm支持的,jdk是能支持这种情况的,不过本文不详细说明,本文要讨论的Spring中的bean,循环依赖在Spring中就是一个问题了

为什么?首先回顾一下之前的知识点,首先在Spring框架中类的创建都是给Spring IOC容器创建的,如图:

 


然后?真的出现这种情况,会怎么样?

 


Spring框架检测到这种场景会抛 BeanCurrentlyInCreationException,提前暴露对象的方法,因为Spring创建bean的过程是一个很复杂的过程,首先是xml解析为document对象,document对象再转成BeanDefinition,然后进行bean的生命周期,才算得上是一个真正的spring bean,接着进行后置处理器加工,假如出现这种,设想一下会怎么样?spring容器就会一直循环调用,当然是在特定的条件,为什么说是特定情况?请看下文

2、实验环境准备

实验环境:

  • SpringFramework版本Springframework5.0.x
  • 开发环境JAR管理:gradle 4.9/ Maven3.+开发IDE:IntelliJ IDEA 2018.2.5JDK:jdk1.8.0_31Git Server:Git fro window 2.8.3Git Client:SmartGit18.1.5(可选)

3、循环依赖问题

我们可以通过例子进行验证,创建类A:

package com.example.bean;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;

/**
 * <pre>
 *      A class
 * </pre>
 *
 * <pre>
 * @author mazq
 * 修改记录
 *    修改后版本:     修改人:  修改日期: 2020/11/05 10:31  修改内容:
 * </pre>
 */
@Component
public class A {

    //@Autowired
    B b;

    public A() {
    	b = new B();
        System.out.println("A class is create");
    }
}

类B:

package com.example.bean;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;

/**
 * <pre>
 *      B class
 * </pre>
 *
 * <pre>
 * @author mazq
 * 修改记录
 *    修改后版本:     修改人:  修改日期: 2020/11/16 14:03  修改内容:
 * </pre>
 */
@Component
public class B {

    //@Autowired
    A a;

    public B() {
    	a = new A();
        System.out.println("B class is create");
    }
}

注册类A、B

package com.example.config;

import com.example.bean.B;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import com.example.bean.A;

/**
 * <pre>
 *      AppConfiguration
 * </pre>
 *
 * <pre>
 * @author mazq
 * 修改记录
 *    修改后版本:     修改人:  修改日期: 2020/11/05 10:26  修改内容:
 * </pre>
 */
@Configuration
public class AppConfiguration {

    @Bean
    public A a(){
        return new A();
    }

    @Bean
    public B b() {
        return new B();
    }

}

package com.example;

import com.example.config.AppConfiguration;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import com.example.bean.A;

/**
 * <pre>
 *      TestController
 * </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 creation 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);
}

多例的情况:看代码是通过prototypesCurrentlyInCreation里的数据校验的,prototypesCurrentlyInCreation是一个ThreadLocal对象

protected boolean isPrototypeCurrentlyInCreation(String beanName) {
	Object curVal = this.prototypesCurrentlyInCreation.get();
	return (curVal != null &&
			(curVal.equals(beanName) || (curVal instanceof Set && ((Set<?>) curVal).contains(beanName))));
}

继续找代码,找到beforePrototypeCreation:

protected void beforePrototypeCreation(String beanName) {
	Object curVal = this.prototypesCurrentlyInCreation.get();
	if (curVal == null) {
		this.prototypesCurrentlyInCreation.set(beanName);
	}
	else if (curVal instanceof String) {
		Set<String> beanNameSet = new HashSet<>(2);
		beanNameSet.add((String) curVal);
		beanNameSet.add(beanName);
		this.prototypesCurrentlyInCreation.set(beanNameSet);
	}
	else {
		Set<String> beanNameSet = (Set<String>) curVal;
		beanNameSet.add(beanName);
	}
}

Ctrl+Alt+H,查看这个方法的调用栈:其实就是在#doGetBean就调用了,也就是bean创建之前

 

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);

知识点归纳

  • 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
  • Spring支持单例bean的循环依赖原因:使用了三级缓存

原文链接:https://www.cnblogs.com/mzq123/p/13993740.html

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