5-Bean的循环依赖问题

一、Sigle+Set模式的 循环依赖问题。

• 只有 socle 为 single 才有这个曝光的机制。

-

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

    <bean id="husbandBean" class="com.jhfuture.spring6.Husband" scope="singleton">
        <property name="name" value="张三">

        </property>
        <property name="wife" ref="wifeBean"/>
    </bean>
    <bean id="wifeBean" class="com.jhfuture.spring6.Wife" scope="singleton">
        <property name="name" value="梨花"/>
        <property name="husband" ref="husbandBean"></property>
    </bean>
</beans>

java src

Husband
package com.jhfuture.spring6;

public class Husband {
    private String name;
    private Wife wife;

    public void setName(String name) {
        this.name = name;
    }

    public void setWife(Wife wife) {
        this.wife = wife;
    }

    public String getName() {
        return name;
    }

    @Override
    public String toString() {
        return "Husband{" +
                "name='" + name + '\'' +
                ", wife=" + wife.getName() +
                '}';
    }
}

Wife

package com.jhfuture.spring6;

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;
    }

    @Override
    public String toString() {
        return "Wife{" +
                "name='" + name + '\'' +
                ", husband=" + husband.getName() +
                '}';
    }
}

二、prototype +Set 模式 —>报错

直接报错

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'husbandBean' defined in class path resource [circle.xml]: Cannot resolve reference to bean 'wifeBean' while setting bean property 'wife'

    
    
    

XML文件

<?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.jhfuture.spring6.Husband" scope="prototype">
        <property name="name" value="张三">

        </property>
        <property name="wife" ref="wifeBean"/>
    </bean>
    <bean id="wifeBean" class="com.jhfuture.spring6.Wife" scope="prototype">
        <property name="name" value="梨花"/>
        <property name="husband" ref="husbandBean"></property>
    </bean>
</beans>

三、single + prototype +Set模式 —>无问题

<?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.jhfuture.spring6.Husband" scope="prototype">
        <property name="name" value="张三">

        </property>
        <property name="wife" ref="wifeBean"/>
    </bean>
    <bean id="wifeBean" class="com.jhfuture.spring6.Wife" scope="single">
        <property name="name" value="梨花"/>
        <property name="husband" ref="husbandBean"></property>
    </bean>
</beans>

五、基于构造依赖下产生的循环依赖—–>无法解决

总结 Spring解决循环依赖的机理

核心–>三层缓存

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。