http://narcissusoyf.iteye.com/blog/705511
在使用Ibatis的時候，如果某個sql的定義出現在引用sql的定義之后的話，笨笨的ibatis是會報錯的。。這讓用慣了spring的人會感到煩躁，為什么ibatis不能和spring一樣，做到xml定義的時候與順序無關。。。但是 spring 真的能夠做到完全與bean定義的順序無關么？下面的代碼，會讓我們警醒下：
Xml代碼 ?
<? xml version "1.0" encoding "UTF-8" ?> < beans xmlns "http://www.springframework.org/schema/beans" ?????????xmlns:xsi "http://www.w3.org/2001/XMLSchema-instance" ?????????xmlns:aop "http://www.springframework.org/schema/aop" ?????????xmlns:tx "http://www.springframework.org/schema/tx" ?????????xsi:schemaLocation ???????????http://www.springframework.org/schema/aop?http://www.springframework.org/schema/aop/spring-aop-2.0.xsd??? ?? ???????????http://www.springframework.org/schema/tx?http://www.springframework.org/schema/tx/spring-tx-2.0.xsd" ?? ???????????default-autowire "byName" > ???< bean id "a" class "org.springframework.aop.framework.ProxyFactoryBean" > ????????< property name "target" ref "targetA" /> ????????< property name "interceptorNames" > ????????????< list > ????????????????< value > </ value > ????????????</ list > ????????</ property > ????</ bean > ????< bean id "targetA" class "reference.A" > </ bean > ????< bean id "targetB" class "reference.B" > </ bean > ????< bean id "beforeAdvice" class "reference.BeforeAdvice" /> ????< bean id "beforeAdvisor" class "reference.BeforeAdvisor" > ????????< property name "advice" ref "beforeAdvice" /> ????</ bean > ????< bean id "b" class "org.springframework.aop.framework.ProxyFactoryBean" > ????????< property name "target" ref "targetB" /> ????????< property name "interceptorNames" > ????????????< list > ????????????????< value > </ value > ????????????</ list > ????????</ property > ????</ bean > </ beans >
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop" xmlns:tx="http://www.springframework.org/schema/tx" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.0.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.0.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-2.0.xsd"default-autowire="byName"> <bean id="a" class="org.springframework.aop.framework.ProxyFactoryBean"><property name="target" ref="targetA" /> <property name="interceptorNames"><list><value>beforeAdvisor</value></list></property></bean> <bean id="targetA" class="reference.A"></bean> <bean id="targetB" class="reference.B"></bean> <bean id="beforeAdvice" class="reference.BeforeAdvice" /><bean id="beforeAdvisor" class="reference.BeforeAdvisor"><property name="advice" ref="beforeAdvice"/></bean><bean id="b" class="org.springframework.aop.framework.ProxyFactoryBean"><property name="target" ref="targetB" /> <property name="interceptorNames"><list><value>beforeAdvisor</value></list></property></bean>
</beans>
?一個簡單的循環引用 + ProxyFactoryBean 定義。
啟動spring的代碼：
Java代碼 ?
public ?class ?Main? ??{ ?? ????public ?static ?void ?main(String[]?args)?throws ?IOException ?? ????{ ?? ?? ????????ApplicationContext?ac?=?new ?ClassPathXmlApplicationContext("/files/reference.xml" ); ?? ????????InterfaceA?a?=?(InterfaceA)?ac.getBean("a" ); ?? ????????a.ok(); ?? ????} ?? }??
public class Main
{public static void main(String[] args) throws IOException{ApplicationContext ac = new ClassPathXmlApplicationContext("/files/reference.xml");InterfaceA a = (InterfaceA) ac.getBean("a");a.ok();}
}
?接著就是悲劇的error:
Java代碼 ?
Caused?by:?org.springframework.beans.factory.BeanCurrentlyInCreationException:?Error?creating?bean?with?name?'a' :?org.springframework.beans.factory.FactoryBeanNotInitializedException:?Cannot?determine?target?class ?for ?proxy ?? ????at?org.springframework.beans.factory.support.FactoryBeanRegistrySupport.doGetObjectFromFactoryBean(FactoryBeanRegistrySupport.java:147 ????at?org.springframework.beans.factory.support.FactoryBeanRegistrySupport.getObjectFromFactoryBean(FactoryBeanRegistrySupport.java:109 ????at?org.springframework.beans.factory.support.AbstractBeanFactory.getObjectForBeanInstance(AbstractBeanFactory.java:1387 ????at?org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:244 ????at?org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:189 ????at?org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireByName(AbstractAutowireCapableBeanFactory.java:1085 ????at?org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1035 ????at?org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:511 ????...?39
Caused by: org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'a': org.springframework.beans.factory.FactoryBeanNotInitializedException: Cannot determine target class for proxyat org.springframework.beans.factory.support.FactoryBeanRegistrySupport.doGetObjectFromFactoryBean(FactoryBeanRegistrySupport.java:147)at org.springframework.beans.factory.support.FactoryBeanRegistrySupport.getObjectFromFactoryBean(FactoryBeanRegistrySupport.java:109)at org.springframework.beans.factory.support.AbstractBeanFactory.getObjectForBeanInstance(AbstractBeanFactory.java:1387)at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:244)at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:189)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireByName(AbstractAutowireCapableBeanFactory.java:1085)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1035)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:511)... 39 more
??好，那么我們換下bean定義的順序吧:
Xml代碼 ?
< bean id "targetA" class "reference.A" > </ bean > < bean id "a" class "org.springframework.aop.framework.ProxyFactoryBean" > ????< property name "target" ref "targetA" /> ????< property name "interceptorNames" > ????????< list > ????????????< value > </ value > ????????</ list > ????</ property > </ bean > < bean id "targetB" class "reference.B" > </ bean > < bean id "beforeAdvice" class "reference.BeforeAdvice" /> < bean id "beforeAdvisor" class "reference.BeforeAdvisor" > ????< property name "advice" ref "beforeAdvice" /> </ bean > < bean id "b" class "org.springframework.aop.framework.ProxyFactoryBean" > ????< property name "target" ref "targetB" /> ????< property name "interceptorNames" > ????????< list > ????????????< value > </ value > ????????</ list > ????</ property > </ bean >
<bean id="targetA" class="reference.A"></bean> <bean id="a" class="org.springframework.aop.framework.ProxyFactoryBean"><property name="target" ref="targetA" /> <property name="interceptorNames"><list><value>beforeAdvisor</value></list></property></bean> <bean id="targetB" class="reference.B"></bean> <bean id="beforeAdvice" class="reference.BeforeAdvice" /><bean id="beforeAdvisor" class="reference.BeforeAdvisor"><property name="advice" ref="beforeAdvice"/></bean><bean id="b" class="org.springframework.aop.framework.ProxyFactoryBean"><property name="target" ref="targetB" /> <property name="interceptorNames"><list><value>beforeAdvisor</value></list></property></bean>
?再run一次：
Java代碼 ?
2010 7 4 23 09 09 信息:?Refreshing?org.springframework.context.support.ClassPathXmlApplicationContext@32fb4f 04 23 09 09 2010 2010 7 4 23 09 09 信息:?Loading?XML?bean?definitions?from?class ?path?resource?[files/reference.xml] ?? 2010 7 4 23 09 09 信息:?Pre-instantiating?singletons?in?org.springframework.beans.factory.support.DefaultListableBeanFactory@1d7ad1c xxx ?? $Proxy1??
2010-7-4 23:09:09 org.springframework.context.support.AbstractApplicationContext prepareRefresh
信息: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@32fb4f: startup date [Sun Jul 04 23:09:09 CST 2010]; root of context hierarchy
2010-7-4 23:09:09 org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
信息: Loading XML bean definitions from class path resource [files/reference.xml]
2010-7-4 23:09:09 org.springframework.beans.factory.support.DefaultListableBeanFactory preInstantiateSingletons
信息: Pre-instantiating singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory@1d7ad1c: defining beans [targetA,a,targetB,beforeAdvice,beforeAdvisor,b]; root of factory hierarchy
xxx
$Proxy1
?
成功了。。。。。
?
duo xi die ? 這是為什么呢？
?
這里的問題確實是出在bean定義的順序上面。applicationContext 在refresh的時候，是按照bean定義的順序來加載singleton bean 的(除開BeanPostProcessor,BeanPostProcessor 在singleton加載之前被bean化)。如果僅僅只是加載普通的Bean或者普通的FactoryBean的話，Spring已經通過某種方式很好的解決了singleton循環依賴的問題。
?
導致這個問題的原因，是由2個原因疊加產生的：
1??FactoryBean的getObject(),需要開發者自己去做處理。如果是new 個對象，那么這個對象就享受不到spring 的IOC 的好處，因為它脫離了spring 容器的管轄。 而ProxyFactoryBean 的 getObject() 在獲取代理對象的時候，會對target的屬性有依賴，如果target的某些值為空，會拋錯。(這里的target 通指 targetName,target)
?
2 spring 再 bean化 bean的時候，是分為2步的，第一步可以簡單的認為new個對象出來，第二步為這個new出來的對象設置屬性。就是去容器里面把這個bean需要的其他bean給注入進來。在第2步的時候，注入給其他的bean的bean 可能沒有被完全bean化,很有可能只是完成了第一步的bean，還是個“半成品”。但是在整個容器初始化結束的時候，這些“半成品”bean會被變成“合格品”。
?
1 + 2 ，恩，就是ProxyFactoryBean在getObject()的時候，依賴了一個“半成品”，結果就悲劇了。
Java代碼 ?
private ?synchronized ?Object?getSingletonInstance()?{ ??????if ?(this .singletonInstance?==?null )?{ ?? ????????this .targetSource?=?freshTargetSource(); ?? ????????if ?(this .autodetectInterfaces?&&?getProxiedInterfaces().length?==?0 ?????????????? ????????????Class?targetClass?=?getTargetClass(); ?? ????????????if ?(targetClass?==?null )?{ ?? ????????????????throw ?new ?FactoryBeanNotInitializedException("Cannot?determine?target?class?for?proxy" ); ?? ????????????} ?? ????????????setInterfaces(ClassUtils.getAllInterfacesForClass(targetClass,?this .proxyClassLoader)); ?? ????????} ?? ?????????? ????????super .setFrozen(this .freezeProxy); ?? ????????this .singletonInstance?=?getProxy(createAopProxy()); ?? ????} ?? ????return ?this .singletonInstance; ?? }??
private synchronized Object getSingletonInstance() {if (this.singletonInstance == null) {this.targetSource = freshTargetSource();if (this.autodetectInterfaces && getProxiedInterfaces().length == 0 && !isProxyTargetClass()) {// Rely on AOP infrastructure to tell us what interfaces to proxy.Class targetClass = getTargetClass();if (targetClass == null) {throw new FactoryBeanNotInitializedException("Cannot determine target class for proxy");}setInterfaces(ClassUtils.getAllInterfacesForClass(targetClass, this.proxyClassLoader));}// Initialize the shared singleton instance.super.setFrozen(this.freezeProxy);this.singletonInstance = getProxy(createAopProxy());}return this.singletonInstance;}
?這里的targetSource 是ProxyFactoryBean 在設置target屬性的時候被設置進去的，很可惜的是，這個時候的 target 指向的Bean?還在創建中，無法走到調用setTarget() 這一步。所以，這個時候的targetSource 就是個 EMPTY_TARGET_SOURCE,它返回的targetClass? 就是null. 所以就失敗了。。。。
?
原因找到了，那么怎么解決呢？
解決也有2個方法，就是針對導致問題的2個原因作出處理么？
A方案：
針對 ProxyFactoryBean 在getObject的時候，一定要求 targetSource的targetClass不為空，那么我就讓他fresh下吧。
Xml代碼 ?
<? xml version "1.0" encoding "UTF-8" ?> < beans xmlns "http://www.springframework.org/schema/beans" ?????????xmlns:xsi "http://www.w3.org/2001/XMLSchema-instance" ?????????xmlns:aop "http://www.springframework.org/schema/aop" ?????????xmlns:tx "http://www.springframework.org/schema/tx" ?????????xsi:schemaLocation ???????????http://www.springframework.org/schema/aop?http://www.springframework.org/schema/aop/spring-aop-2.0.xsd??? ?? ???????????http://www.springframework.org/schema/tx?http://www.springframework.org/schema/tx/spring-tx-2.0.xsd" ?? ???????????default-autowire "byName" > ??????? ?? ???? ?? ????< bean id "a" class "org.springframework.aop.framework.ProxyFactoryBean" > ????????< property name "targetName" value "targetA" /> ????????< property name "interceptorNames" > ????????????< list > ????????????????< value > </ value > ????????????</ list > ????????</ property > ????</ bean > ???? ?? ????< bean id "targetA" class "reference.A" > </ bean > ????< bean id "targetB" class "reference.B" > </ bean > ????< bean id "beforeAdvice" class "reference.BeforeAdvice" /> ????< bean id "beforeAdvisor" class "reference.BeforeAdvisor" > ????????< property name "advice" ref "beforeAdvice" /> ????</ bean > ????< bean id "b" class "org.springframework.aop.framework.ProxyFactoryBean" > ????????< property name "targetName" value "targetB" /> ????????< property name "interceptorNames" > ????????????< list > ????????????????< value > </ value > ????????????</ list > ????????</ property > ????</ bean > </ beans >
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop" xmlns:tx="http://www.springframework.org/schema/tx" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.0.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.0.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-2.0.xsd"default-autowire="byName"> <bean id="a" class="org.springframework.aop.framework.ProxyFactoryBean"><property name="targetName" value="targetA" /> <property name="interceptorNames"><list><value>beforeAdvisor</value></list></property></bean> <bean id="targetA" class="reference.A"></bean> <bean id="targetB" class="reference.B"></bean> <bean id="beforeAdvice" class="reference.BeforeAdvice" /><bean id="beforeAdvisor" class="reference.BeforeAdvisor"><property name="advice" ref="beforeAdvice"/></bean><bean id="b" class="org.springframework.aop.framework.ProxyFactoryBean"><property name="targetName" value="targetB" /> <property name="interceptorNames"><list><value>beforeAdvisor</value></list></property></bean>
</beans>
?把 target,換成targetName 就可以了
?
B方案：
因為spring 在解決循環依賴的時候，化了個好大好大的圈，以至于我們需要的某個屬性遲遲還沒有set進來。
那么我們可以把這個大大的圈分成N個小小的圈嗎。。
既然spring初始化singleton bean是從xml文件第一個bean開始的(除開BeanPostProcessor)，那么我們就好好利用下這個第一個Bean么。。就把bean定義的位置給換下吧。。
?
?
spring 還是和bean定義的順序有關的。希望大家在使用spring的時候，有所警醒。這個幾率的問題不是很大，在使用ProxyFactoryBean 的時候最好使用 targetName 屬性。另外，在使用FactoryBean的時候，需要借鑒下ProxyFactoryBean所導致的問題，對于某些屬性依賴的話，不要太相信spring會在那個時候給你設置進去。
?
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