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> T doGetBean(
final String name, final Class<T> 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

<?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-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.