spring源码分析 循环依赖

首先出现循环依赖肯定是自己的代码设计出现了问题，但是保不准真有这种情况发生啊。
spring对原型模式的循环依赖的处理很简单,直接就抛出异常。创建失败

protected  T doGetBean(
            final String name, final Class requiredType, final Object[] args, boolean typeCheckOnly)
            throws BeansException {

        ......
            if (isPrototypeCurrentlyInCreation(beanName)) {
                throw new BeanCurrentlyInCreationException(beanName);
            }
        ......

下面讲一下spring能够处理循环依赖的注入方式 setter 方式。

先看一下简单例子

public class ServiceA {
    private String name;

    private ServiceB serviceB;

    public String getName() {
        return name;
    }

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

    public ServiceB getServiceB() {
        return serviceB;
    }

    public void setServiceB(ServiceB serviceB) {
        this.serviceB = serviceB;
    }
}


public class ServiceA {
    private String name;
    private ServiceB serviceB;

    public String getName() {
        return name;
    }

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

    public ServiceB getServiceB() {
        return serviceB;
    }

    public void setServiceB(ServiceB serviceB) {
        this.serviceB = serviceB;
    }
}


public class ServiceA {
    private String name;
    private ServiceB serviceB;

    public String getName() {
        return name;
    }

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

    public ServiceB getServiceB() {
        return serviceB;
    }

    public void setServiceB(ServiceB serviceB) {
        this.serviceB = serviceB;
    ｝
}

/**
 * 测试类
 * @author hepei
 * @date 2017年9月24日 下午11:37:43
 */
public class Test {
    @SuppressWarnings("deprecation")
    public static void main(String[] args) {
        XmlBeanFactory f=new XmlBeanFactory(new ClassPathResource("spring-context.xml"));
        ServiceA serviceA=(ServiceA) f.getBean("serviceA");
        System.out.println(serviceA.getName());
        System.out.println(serviceA.getServiceB().getServiceC().getPhone());
    }

}

spring-context.xml

<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-4.0.xsd">

    <bean id="serviceA" class="com.eroadsf.springdemo.ServiceA">
        <property name="name" value="何">property>
        <property name="serviceB" ref="serviceB">property>
    bean>

    <bean id="serviceB" class=" com.eroadsf.springdemo.ServiceB">
        <property name="address" value="玄武">property>
        <property name="serviceC" ref="serviceC">property>
    bean>
    <bean id="serviceC" class="com.eroadsf.springdemo.ServiceC">
        <property name="phone" value="138">property>
        <property name="serviceA" ref="serviceA">property>
    bean>
beans>

运行结果
何
138

下面通过源码分析一下spring是如何处理setter方式的循环依赖的。

Spring通过三级缓存加上“提前曝光”机制，配合Java的对象引用原理，比较完美地解决了某些情况下的循环依赖问题！

单例在Spring的同一个容器内只会被创建一次，后续在获取bean,就直接从单例缓存中获取。首先尝试从缓存中加载,如果加载不成功的话再尝试从singletonFactories中加载。因为在创建单例bean的时候会存在依赖注入的情况,而在创建依赖的时候为了避免循环依赖,在spring中创建bean的原则是不等bean创建完成就会将创建bean的ObjectFactory提早曝光加入到缓存中,一旦下一个bean创建时候需要依赖上一个bean则直接使用ObjectFactory.