什么是Bean的循环依赖
A对象中有B属性。B对象中有A属性。这就是循环依赖。我依赖你，你也依赖我。
比如：丈夫类Husband，妻子类Wife。Husband中有Wife的引用。Wife中有Husband的引用。
singleton下的set注入产生的循环依赖
我们来编写程序，测试一下在singleton+setter的模式下产生的循环依赖，Spring是否能够解决？
package com.powernode.spring6.bean;
/**
* @author 动力节点
* @version 1.0
* @className Husband
* @since 1.0
**/
public class Husband {
private String name;
private Wife wife;
public void setName(String name) {
this.name = name;
}
public String getName() {
return name;
}
public void setWife(Wife wife) {
this.wife = wife;
}
// toString()方法重写时需要注意：不能直接输出wife，输出wife.getName()。要不然会出现递归导致的栈内存溢出错误。
@Override
public String toString() {
return "Husband{" +
"name='" + name + '\'' +
", wife=" + wife.getName() +
'}';
}
}
package com.powernode.spring6.bean;
/**
* @author 动力节点
* @version 1.0
* @className Wife
* @since 1.0
**/
public class Wife {
private String name;
private Husband husband;
public void setName(String name) {
this.name = name;
}
public String getName() {
return name;
}
public void setHusband(Husband husband) {
this.husband = husband;
}
// toString()方法重写时需要注意：不能直接输出husband，输出husband.getName()。要不然会出现递归导致的栈内存溢出错误。
@Override
public String toString() {
return "Wife{" +
"name='" + name + '\'' +
", husband=" + husband.getName() +
'}';
}
}
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">
<bean id="husbandBean" class="com.powernode.spring6.bean.Husband" scope="singleton">
<property name="name" value="张三"/>
<property name="wife" ref="wifeBean"/>
</bean>
<bean id="wifeBean" class="com.powernode.spring6.bean.Wife" scope="singleton">
<property name="name" value="小花"/>
<property name="husband" ref="husbandBean"/>
</bean>
</beans>
测试程序
package com.powernode.spring6.test;
import com.powernode.spring6.bean.Husband;
import com.powernode.spring6.bean.Wife;
import org.junit.Test;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
/**
* @author 动力节点
* @version 1.0
* @className CircularDependencyTest
* @since 1.0
**/
public class CircularDependencyTest {
@Test
public void testSingletonAndSet(){
ApplicationContext applicationContext = new ClassPathXmlApplicationContext("spring.xml");
Husband husbandBean = applicationContext.getBean("husbandBean", Husband.class);
Wife wifeBean = applicationContext.getBean("wifeBean", Wife.class);
System.out.println(husbandBean);
System.out.println(wifeBean);
}
}
通过测试得知：在singleton + set注入的情况下，循环依赖是没有问题的。Spring可以解决这个问题。
prototype下的set注入产生的循环依赖
我们再来测试一下：prototype+set注入的方式下，循环依赖会不会出现问题？
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">
<bean id="husbandBean" class="com.powernode.spring6.bean.Husband" scope="prototype">
<property name="name" value="张三"/>
<property name="wife" ref="wifeBean"/>
</bean>
<bean id="wifeBean" class="com.powernode.spring6.bean.Wife" scope="prototype">
<property name="name" value="小花"/>
<property name="husband" ref="husbandBean"/>
</bean>
</beans>
执行测试程序：发生了异常，异常信息如下：
Caused by: org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name ‘husbandBean’: Requested bean is currently in creation: Is there an unresolvable circular reference?
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:265)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199)
at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:325)
… 44 more
翻译为：创建名为“husbandBean”的bean时出错：请求的bean当前正在创建中：是否存在无法解析的循环引用？
通过测试得知，当循环依赖的所有Bean的scope="prototype"的时候，产生的循环依赖，Spring是无法解决的，会出现BeanCurrentlyInCreationException异常。
大家可以测试一下，以上两个Bean，如果其中一个是singleton，另一个是prototype，是没有问题的。
为什么两个Bean都是prototype时会出错呢？
singleton下的构造注入产生的循环依赖
我们再来测试一下singleton + 构造注入的方式下，spring是否能够解决这种循环依赖。
package com.powernode.spring6.bean2;
/**
* @author 动力节点
* @version 1.0
* @className Husband
* @since 1.0
**/
public class Husband {
private String name;
private Wife wife;
public Husband(String name, Wife wife) {
this.name = name;
this.wife = wife;
}
// -----------------------分割线--------------------------------
public String getName() {
return name;
}
@Override
public String toString() {
return "Husband{" +
"name='" + name + '\'' +
", wife=" + wife +
'}';
}
}
package com.powernode.spring6.bean2;
/**
* @author 动力节点
* @version 1.0
* @className Wife
* @since 1.0
**/
public class Wife {
private String name;
private Husband husband;
public Wife(String name, Husband husband) {
this.name = name;
this.husband = husband;
}
// -------------------------分割线--------------------------------
public String getName() {
return name;
}
@Override
public String toString() {
return "Wife{" +
"name='" + name + '\'' +
", husband=" + husband +
'}';
}
}
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">
<bean id="hBean" class="com.powernode.spring6.bean2.Husband" scope="singleton">
<constructor-arg name="name" value="张三"/>
<constructor-arg name="wife" ref="wBean"/>
</bean>
<bean id="wBean" class="com.powernode.spring6.bean2.Wife" scope="singleton">
<constructor-arg name="name" value="小花"/>
<constructor-arg name="husband" ref="hBean"/>
</bean>
</beans>
测试程序
@Test
public void testSingletonAndConstructor(){
ApplicationContext applicationContext = new ClassPathXmlApplicationContext("spring2.xml");
Husband hBean = applicationContext.getBean("hBean", Husband.class);
Wife wBean = applicationContext.getBean("wBean", Wife.class);
System.out.println(hBean);
System.out.println(wBean);
}
执行结果：发生了异常，信息如下：
Caused by: org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name ‘hBean’: Requested bean is currently in creation: Is there an unresolvable circular reference?
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.beforeSingletonCreation(DefaultSingletonBeanRegistry.java:355)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:227)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:324)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199)
at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:325)
… 56 more
和上一个测试结果相同，都是提示产生了循环依赖，并且Spring是无法解决这种循环依赖的。
主要原因是因为通过构造方法注入导致的：因为构造方法注入会导致实例化对象的过程和对象属性赋值的过程没有分离开，必须在一起完成导致的。
Spring解决循环依赖的机理
Spring为什么可以解决set + singleton模式下循环依赖？
根本的原因在于：这种方式可以做到将“实例化Bean”和“给Bean属性赋值”这两个动作分开去完成。
实例化Bean的时候：调用无参数构造方法来完成。此时可以先不给属性赋值，可以提前将该Bean对象“曝光”给外界。
给Bean属性赋值的时候：调用setter方法来完成。
两个步骤是完全可以分离开去完成的，并且这两步不要求在同一个时间点上完成。
也就是说，Bean都是单例的，我们可以先把所有的单例Bean实例化出来，放到一个集合当中（我们可以称之为缓存），所有的单例Bean全部实例化完成之后，以后我们再慢慢的调用setter方法给属性赋值。这样就解决了循环依赖的问题。
总结：
Spring只能解决setter方法注入的单例bean之间的循环依赖。ClassA依赖ClassB，ClassB又依赖ClassA，形成依赖闭环。Spring在创建ClassA对象后，不需要等给属性赋值，直接将其曝光到bean缓存当中。在解析ClassA的属性时，又发现依赖于ClassB，再次去获取ClassB，当解析ClassB的属性时，又发现需要ClassA的属性，但此时的ClassA已经被提前曝光加入了正在创建的bean的缓存中，则无需创建新的的ClassA的实例，直接从缓存中获取即可。从而解决循环依赖问题。