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allowBeanDefinitionOverriding属性含义

设置是否允许通过注册具有相同名称的不同定义来覆盖bean定义，并自动替换前者。否则，将引发异常。默认值为“true”。

allowCircularReferences属性含义

设置是否允许bean之间的循环引用并自动尝试解析它们。

默认值为“true”。关闭此选项可在遇到循环引用时引发异常，完全不允许循环引用。

循环依赖应该都比较了解，主要看看allowBeanDefinitionOverriding。

public class TestSpring {
       public static void main(String[] args) {
           ClassPathXmlApplicationContext applicationContext = new ClassPathXmlApplicationContext("bean1.xml", "bean2.xml");        User user = (User) applicationContext.getBean("user");        System.out.println("name: " + user.getName());    }}

加载了两个配置文件，并且都配置了id为user的bean对象。

bean1.xml中，name为lisi

bean2.xml中，name为wangwu

根据allowBeanDefinitionOverriding属性的定义，默认值为true，并且相同名称的bean会进行覆盖，所以按照xml文件的加载顺序，bean2会覆盖bean1的配置。

最终输出结果为：name: wangwu

如果交换配置文件的加载顺序

ClassPathXmlApplicationContext applicationContext = new ClassPathXmlApplicationContext("bean2.xml", "bean.xml");

最终输出结果为：name: lisi

如何修改属性值

方式一：

自定义一个applicationContext，并继承ClassPathXmlApplicationContext，重写customizeBeanFactory方法。

public class MyClassPathXmlApplicationContext extends ClassPathXmlApplicationContext {
       public MyClassPathXmlApplicationContext(String... configLocations) {
           super(configLocations);    }    @Override    protected void customizeBeanFactory(DefaultListableBeanFactory beanFactory) {
           super.setAllowBeanDefinitionOverriding(false);        super.setAllowCircularReferences(false);        super.customizeBeanFactory(beanFactory);    }}

public class TestSpring {
       public static void main(String[] args) {
           MyClassPathXmlApplicationContext applicationContext = new MyClassPathXmlApplicationContext("bean2.xml", "bean1.xml");        User user = (User) applicationContext.getBean("user");        System.out.println("name: " + user.getName());    }}

报错如下，不允许出现定义同名的bean了。

Exception in thread "main" org.springframework.beans.factory.parsing.BeanDefinitionParsingException: Configuration problem: Failed to register bean definition with name 'user'Offending resource: class path resource [bean1.xml]; nested exception is org.springframework.beans.factory.support.BeanDefinitionOverrideException: Invalid bean definition with name 'user' defined in class path resource [bean1.xml]: Cannot register bean definition [Generic bean: class [com.wyl.learn.User]; scope=; abstract=false; lazyInit=false; autowireMode=0; dependencyCheck=0; autowireCandidate=true; primary=false; factoryBeanName=null; factoryMethodName=null; initMethodName=null; destroyMethodName=null; defined in class path resource [bean1.xml]] for bean 'user': There is already [Generic bean: class [com.wyl.learn.User]; scope=; abstract=false; lazyInit=false; autowireMode=0; dependencyCheck=0; autowireCandidate=true; primary=false; factoryBeanName=null; factoryMethodName=null; initMethodName=null; destroyMethodName=null; defined in class path resource [bean2.xml]] bound.	at org.springframework.beans.factory.parsing.FailFastProblemReporter.error(FailFastProblemReporter.java:72)	at org.springframework.beans.factory.parsing.ReaderContext.error(ReaderContext.java:119)	at org.springframework.beans.factory.parsing.ReaderContext.error(ReaderContext.java:104)	at org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader.processBeanDefinition(DefaultBeanDefinitionDocumentReader.java:314)	at org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader.parseDefaultElement(DefaultBeanDefinitionDocumentReader.java:197)	at org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader.parseBeanDefinitions(DefaultBeanDefinitionDocumentReader.java:176)	at org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader.doRegisterBeanDefinitions(DefaultBeanDefinitionDocumentReader.java:149)	at org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader.registerBeanDefinitions(DefaultBeanDefinitionDocumentReader.java:96)	at org.springframework.beans.factory.xml.XmlBeanDefinitionReader.registerBeanDefinitions(XmlBeanDefinitionReader.java:514)	at org.springframework.beans.factory.xml.XmlBeanDefinitionReader.doLoadBeanDefinitions(XmlBeanDefinitionReader.java:394)	at org.springframework.beans.factory.xml.XmlBeanDefinitionReader.loadBeanDefinitions(XmlBeanDefinitionReader.java:337)	at org.springframework.beans.factory.xml.XmlBeanDefinitionReader.loadBeanDefinitions(XmlBeanDefinitionReader.java:305)	at org.springframework.beans.factory.support.AbstractBeanDefinitionReader.loadBeanDefinitions(AbstractBeanDefinitionReader.java:188)	at org.springframework.beans.factory.support.AbstractBeanDefinitionReader.loadBeanDefinitions(AbstractBeanDefinitionReader.java:224)	at org.springframework.beans.factory.support.AbstractBeanDefinitionReader.loadBeanDefinitions(AbstractBeanDefinitionReader.java:195)	at org.springframework.beans.factory.support.AbstractBeanDefinitionReader.loadBeanDefinitions(AbstractBeanDefinitionReader.java:257)	at org.springframework.context.support.AbstractXmlApplicationContext.loadBeanDefinitions(AbstractXmlApplicationContext.java:128)	at org.springframework.context.support.AbstractXmlApplicationContext.loadBeanDefinitions(AbstractXmlApplicationContext.java:94)	at org.springframework.context.support.AbstractRefreshableApplicationContext.refreshBeanFactory(AbstractRefreshableApplicationContext.java:133)	at org.springframework.context.support.AbstractApplicationContext.obtainFreshBeanFactory(AbstractApplicationContext.java:637)	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:522)	at org.springframework.context.support.ClassPathXmlApplicationContext.
   
    (ClassPathXmlApplicationContext.java:144)	at org.springframework.context.support.ClassPathXmlApplicationContext.
    
     (ClassPathXmlApplicationContext.java:95)	at com.wyl.learn.MyClassPathXmlApplicationContext.
     
      (MyClassPathXmlApplicationContext.java:9)	at com.wyl.learn.TestSpring.main(TestSpring.java:5)Caused by: org.springframework.beans.factory.support.BeanDefinitionOverrideException: Invalid bean definition with name 'user' defined in class path resource [bean1.xml]: Cannot register bean definition [Generic bean: class [com.wyl.learn.User]; scope=; abstract=false; lazyInit=false; autowireMode=0; dependencyCheck=0; autowireCandidate=true; primary=false; factoryBeanName=null; factoryMethodName=null; initMethodName=null; destroyMethodName=null; defined in class path resource [bean1.xml]] for bean 'user': There is already [Generic bean: class [com.wyl.learn.User]; scope=; abstract=false; lazyInit=false; autowireMode=0; dependencyCheck=0; autowireCandidate=true; primary=false; factoryBeanName=null; factoryMethodName=null; initMethodName=null; destroyMethodName=null; defined in class path resource [bean2.xml]] bound.	at org.springframework.beans.factory.support.DefaultListableBeanFactory.registerBeanDefinition(DefaultListableBeanFactory.java:927)	at org.springframework.beans.factory.support.BeanDefinitionReaderUtils.registerBeanDefinition(BeanDefinitionReaderUtils.java:166)	at org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader.processBeanDefinition(DefaultBeanDefinitionDocumentReader.java:311)	... 21 more
     
    
   

方式二：

直接调用set方法，并重新调用refresh()方法让属性生效，直接会出出现和方式一同样的报错信息。

public class TestSpring {
       public static void main(String[] args) {
           ClassPathXmlApplicationContext applicationContext = new ClassPathXmlApplicationContext("bean2.xml", "bean1.xml");        applicationContext.setAllowBeanDefinitionOverriding(false);        applicationContext.setAllowCircularReferences(false);        applicationContext.refresh();        User user = (User) applicationContext.getBean("user");        System.out.println("name: " + user.getName());    }}

原理分析

两种方式的关键都在于customizeBeanFactory方法

protected void customizeBeanFactory(DefaultListableBeanFactory beanFactory) {
   		if (this.allowBeanDefinitionOverriding != null) {
   			beanFactory.setAllowBeanDefinitionOverriding(this.allowBeanDefinitionOverriding);		}		if (this.allowCircularReferences != null) {
   			beanFactory.setAllowCircularReferences(this.allowCircularReferences);		}	}

spirng在加载过程中，会执行到这段代码，首先判断allowBeanDefinitionOverriding属性是否为null，而这两个属性默认情况下是为null。（包装类型定义的，所以默认为null），所以beanFactory.setAllowBeanDefinitionOverriding(this.allowBeanDefinitionOverriding);就不会被执行，

那么在DefaultListableBeanFactory中这两个属性的地方就会取默认值true。（真正使用时是通过DefaultListableBeanFactory类中定义的这个两个属性值获取的）

AbstractRefreshableApplicationContext提供了修改属性的方法，如果重写了方法，那么修改的是DefaultListableBeanFactory中的属性。

DefaultListableBeanFactory中的属性，默认为true。

所以第二种方式的原理就是，先通过set方式，让两个属性值不为null，那么就会修改DefaultListableBeanFactory中的属性了。

protected void customizeBeanFactory(DefaultListableBeanFactory beanFactory) {
   		//不为null，成立，进入if条件修改beanFactory.setAllowBeanDefinitionOverriding(true)		if (this.allowBeanDefinitionOverriding != null) {
   			beanFactory.setAllowBeanDefinitionOverriding(this.allowBeanDefinitionOverriding);		}		if (this.allowCircularReferences != null) {
   			beanFactory.setAllowCircularReferences(this.allowCircularReferences);		}	}

而第一种方式避免了重新调用refresh的过程，是直接在customizeBeanFactory中进行属性赋值，赋值完成后再调用父类的customizeBeanFactory方法继续完成之后的工作，有点类似静态代理模式的运用，重写父类方法，完成自己的业务逻辑之后再调用父类原本的方法。

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