DefaultListableBeanFactory 相关机制探索
DefaultListableBeanFactory 相关机制探索
本文的分析以 DefaultListableBeanFactory 为对象。
spring-beans依赖的版本为 5.0.9.RELEASE。
类图
关键属性
-
DefaultSingletonBeanRegistryprivate final Map: 存放完全实例化并且属性赋值完成的Bean，可以直接使用（一级缓存）；private final Map: 存放早期的单例的bean的引用，尚未配置属性（二级缓存）；private final Map: 存放单例的工厂 （三级缓存）；private final Set: 当前正在被创建的singletonsCurrentlyInCreation bean的缓存，创建前（createBean）放入，创建后移除；private boolean singletonsCurrentlyInDestruction: 当前是否在销毁singleton对象，如果是的话，此时创建单例对象会报异常；private final Map: 需要进行销毁操作的bean；
-
AbstractBeanFactory:private final Set: 记录那些被创建了至少一次的alreadyCreated bean名称；private final ThreadLocal: 当前正在被创建的原型bean，泛型中可以是字符串类型，存bean名称，也可以是Set类型存多个原型的bean的名称；
关键方法
DefaultListableBeanFactory#preInstantiateSingletons：实例化所有非抽象的，非延迟加载的单例Bean- 循环所有的
bean名称列表，对每个名称，用getBean(String)方法获取bean - 循环所有的单例
bean，如果是SmartInitializingSingleton类型就执行回调方法
- 循环所有的
AbstractBeanFactory#doGetBean: 获取bean, 如果bean不存在还会进行创建DefaultSingletonBeanRegistry#getSingleton(java.lang.String, boolean): 从三级缓存中获取单例beanAbstractAutowireCapableBeanFactory#createBeanInstance: 创建bean实例AbstractAutowireCapableBeanFactory#populateBean: 为bean注入属性DefaultListableBeanFactory#destroySingletons: 销毁所有的单例bean
BeanFactory doGetBean过程
-
doGetBean流程概览:
-
getSingleton(String beanName, ObjectFactory singletonFactory)流程概览：
-
polulateBean方法执行流程：
BeanFactory#destroySingletons 销毁过程
- 设置
singletonsCurrentlyInDestruction这个属性为true； - 遍历这个属性
disposableBeans，调用其中的每个bean的销毁方法； - 销毁每个
bean之前，先销毁其依赖，如果有循环依赖的情况，不影响（比如 A 依赖 B，B 又依赖 A，A先开始进入销毁方法，结果是 B销毁，A再销毁）；
属性注入循环依赖
两个相互依赖的类：
@Setter
@Getter
public class Man {
private String name;
private Woman wife;
@Override
public String toString() {
return "Man{" + "name='" + name + '\'' + ", wife=" + wife.getName() + '}';
}
}
@Setter
@Getter
public class Woman {
private String name;
private Man husband;
@Override
public String toString() {
return "Woman{" + "name='" + name + '\'' + ", husband=" + husband.getName() + '}';
}
}
手动注册 BeanDefinition 到容器中:
public class CycleDependencyByPropertyDemo {
@Test
public void autowireDemo(){
DefaultListableBeanFactory beanFactory = new DefaultListableBeanFactory();
// 通过代码注册bean
bindBeansByCode(beanFactory);
// 获取bean
Man man = (Man) beanFactory.getBean("man");
System.out.println(man);
}
private static void bindBeansByCode(BeanDefinitionRegistry beanDefinitionRegistry) {
// 注册man
GenericBeanDefinition manDefinition = (GenericBeanDefinition) BeanDefinitionBuilder
.genericBeanDefinition(Man.class)
.addPropertyReference("wife", "woman")
.addPropertyValue("name", "Jack")
.getBeanDefinition();
// 注册woman
GenericBeanDefinition womanDefinition = (GenericBeanDefinition) BeanDefinitionBuilder
.genericBeanDefinition(Woman.class)
.addPropertyReference("husband", "man")
.addPropertyValue("name", "Alice")
.getBeanDefinition();
beanDefinitionRegistry.registerBeanDefinition("man", manDefinition);
beanDefinitionRegistry.registerBeanDefinition("woman", womanDefinition);
}
}
执行结果：
Man{name='Jack', wife=Alice}
Process finished with exit code 0
结论：通过属性注入是没有循环依赖发生的。
解决这类循环依赖的关键在于：
- 将刚刚实例化的
bean以ObjectFactory的方式提供在private final Map（第三级缓存）属性中； getSingleton(String beanName, boolean allowEarlyReference)中从第三级缓存中和第二级缓存中取还未创建完全的bean；
带有DependsOn关系的属性注入
如果基于上述的例子加入 dependsOn 关系，就会导致异常，改造后代码如下
private static void bindBeansByCode(BeanDefinitionRegistry beanDefinitionRegistry) {
// 注册man
GenericBeanDefinition manDefinition = (GenericBeanDefinition) BeanDefinitionBuilder
.genericBeanDefinition(Man.class)
.addPropertyReference("wife", "woman")
.addPropertyValue("name", "Jack")
+ .addDependsOn("woman")
.getBeanDefinition();
// 注册woman
GenericBeanDefinition womanDefinition = (GenericBeanDefinition) BeanDefinitionBuilder
.genericBeanDefinition(Woman.class)
.addPropertyReference("husband", "man")
.addPropertyValue("name", "Alice")
+ .addDependsOn("man")
.getBeanDefinition();
beanDefinitionRegistry.registerBeanDefinition("man", manDefinition);
beanDefinitionRegistry.registerBeanDefinition("woman", womanDefinition);
}
输出如下：
org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'woman': Circular depends-on relationship between 'woman' and 'man'
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:299)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:304)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199)
at com.zmf.spring.bean.cycle.CycleDependencyByPropertyWithDependsOnDemo.autowireDemo(CycleDependencyByPropertyWithDependsOnDemo.java:21)
结论：带有DependsOn关系就不能解决循环依赖
构造器注入循环依赖
两个相互依赖的类如下：
@Setter
@Getter
public class ConMan {
private String name;
private ConWoman wife;
public ConMan(String name, ConWoman wife) {
this.name = name;
this.wife = wife;
}
@Override
public String toString() {
return "ConMan{" + "name='" + name + '\'' + ", wife=" + wife.getName() + '}';
}
}
@Setter
@Getter
public class ConWoman {
private String name;
private ConMan husband;
public ConWoman(String name, ConMan husband) {
this.name = name;
this.husband = husband;
}
@Override
public String toString() {
return "ConWoman{" + "name='" + name + '\'' + ", husband=" + husband.getName() + '}';
}
}
测试类 CycleDependencyByConstructorDemo 以构造器相互注入的形式实例化 bean，代码如下：
public class CycleDependencyByConstructorDemo {
@Test
public void constructDemo(){
DefaultListableBeanFactory beanFactory = new DefaultListableBeanFactory();
// 通过代码注册bean
bindBeansByCode(beanFactory);
// 获取bean
ConMan man = (ConMan) beanFactory.getBean("man");
System.out.println(man);
}
private static void bindBeansByCode(BeanDefinitionRegistry beanDefinitionRegistry) {
// 注册man
GenericBeanDefinition manDefinition = (GenericBeanDefinition) BeanDefinitionBuilder
.genericBeanDefinition(ConMan.class)
.addPropertyValue("name", "Jack")
.addConstructorArgValue("Jack")
.addConstructorArgReference("woman")
.getBeanDefinition();
// 注册woman
GenericBeanDefinition womanDefinition = (GenericBeanDefinition) BeanDefinitionBuilder
.genericBeanDefinition(ConWoman.class)
.addConstructorArgValue("Alice")
.addConstructorArgReference("man")
.getBeanDefinition();
beanDefinitionRegistry.registerBeanDefinition("man", manDefinition);
beanDefinitionRegistry.registerBeanDefinition("woman", womanDefinition);
}
}
执行结果如下：
Caused by: org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'man': Requested bean is currently in creation: Is there an unresolvable circular reference?
结论：构造器注入的循环依赖不能自动解决
Bean 生命周期
参考资料
手动注册BeanDefinition
spring中的三级缓存