在使用Ibatis的时候，如果某个sql的定义出现在引用sql的定义之后的话，笨笨的ibatis是会报错的。。这让用惯了spring的人会感到烦躁，为什么ibatis不能和spring一样，做到xml定义的时候与顺序无关。。。但是 spring 真的能够做到完全与bean定义的顺序无关么？下面的代码，会让我们警醒下：
Xml代码
1. <?xml version="1.0" encoding="UTF-8"?>
2. <beans xmlns="http://www.springframework.org/schema/beans"
3. xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
4. xmlns:aop="http://www.springframework.org/schema/aop"
5. xmlns:tx="http://www.springframework.org/schema/tx"
6. xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.0.xsd
7. http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.0.xsd
8. http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-2.0.xsd"
9. default-autowire="byName">
10. <bean id="a" class="org.springframework.aop.framework.ProxyFactoryBean">
11. <property name="target" ref="targetA" />
12. <property name="interceptorNames">
13. <list>
14. <value>beforeAdvisor</value>
15. </list>
16. </property>
17. </bean>
18. <bean id="targetA" class="reference.A"></bean>
19. <bean id="targetB" class="reference.B"></bean>
20. <bean id="beforeAdvice" class="reference.BeforeAdvice" />
21. <bean id="beforeAdvisor" class="reference.BeforeAdvisor">
22. <property name="advice" ref="beforeAdvice"/>
23. </bean>
24. <bean id="b" class="org.springframework.aop.framework.ProxyFactoryBean">
25. <property name="target" ref="targetB" />
26. <property name="interceptorNames">
27. <list>
28. <value>beforeAdvisor</value>
29. </list>
30. </property>
31. </bean>
32. </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>
一个简单的循环引用 + ProxyFactoryBean 定义。
启动spring的代码：
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();
}
}接着就是悲剧的error:
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 more好，那么我们换下bean定义的顺序吧:
Xml代码
1. <bean id="targetA" class="reference.A"></bean>
2. <bean id="a" class="org.springframework.aop.framework.ProxyFactoryBean">
3. <property name="target" ref="targetA" />
4. <property name="interceptorNames">
5. <list>
6. <value>beforeAdvisor</value>
7. </list>
8. </property>
9. </bean>
10. <bean id="targetB" class="reference.B"></bean>
11. <bean id="beforeAdvice" class="reference.BeforeAdvice" />
12. <bean id="beforeAdvisor" class="reference.BeforeAdvisor">
13. <property name="advice" ref="beforeAdvice"/>
14. </bean>
15. <bean id="b" class="org.springframework.aop.framework.ProxyFactoryBean">
16. <property name="target" ref="targetB" />
17. <property name="interceptorNames">
18. <list>
19. <value>beforeAdvisor</value>
20. </list>
21. </property>
22. </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：
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 ? 这是为什么呢？
这里的问题确实是出在bean定义的顺序上面。applicationContext 在refresh的时候，是按照bean定义的顺序来加载singleton bean 的(除开BeanPostProcessor,BeanPostProcessor 在singleton加载之前被bean化)。如果仅仅只是加载普通的Bean或者普通的FactoryBean的话，Spring已经通过某种方式很好的解决了singleton循环依赖的问题。
导致这个问题的原因，是由2个原因叠加产生的：
1 FactoryBean的getObject(),需要开发者自己去做处理。如果是new 个对象，那么这个对象就享受不到spring 的IOC 的好处，因为它脱离了spring 容器的管辖。 而ProxyFactoryBean 的 getObject() 在获取代理对象的时候，会对target的属性有依赖，如果target的某些值为空，会抛错。(这里的target 通指 targetName,target)
2 spring 再 bean化 bean的时候，是分为2步的，第一步可以简单的认为new个对象出来，第二步为这个new出来的对象设置属性。就是去容器里面把这个bean需要的其他bean给注入进来。在第2步的时候，注入给其他的bean的bean 可能没有被完全bean化,很有可能只是完成了第一步的bean，还是个“半成品”。但是在整个容器初始化结束的时候，这些“半成品”bean会被变成“合格品”。
1 + 2 ，恩，就是ProxyFactoryBean在getObject()的时候，依赖了一个“半成品”，结果就悲剧了。
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;
}这里的targetSource 是ProxyFactoryBean 在设置target属性的时候被设置进去的，很可惜的是，这个时候的 target 指向的Bean 还在创建中，无法走到调用setTarget() 这一步。所以，这个时候的targetSource 就是个 EMPTY_TARGET_SOURCE,它返回的targetClass 就是null. 所以就失败了。。。。
原因找到了，那么怎么解决呢？
解决也有2个方法，就是针对导致问题的2个原因作出处理么？
A方案：
针对 ProxyFactoryBean 在getObject的时候，一定要求 targetSource的targetClass不为空，那么我就让他fresh下吧。
Xml代码
1. <?xml version="1.0" encoding="UTF-8"?>
2. <beans xmlns="http://www.springframework.org/schema/beans"
3. xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
4. xmlns:aop="http://www.springframework.org/schema/aop"
5. xmlns:tx="http://www.springframework.org/schema/tx"
6. xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.0.xsd
7. http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.0.xsd
8. http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-2.0.xsd"
9. default-autowire="byName">
10.
11.
12. <bean id="a" class="org.springframework.aop.framework.ProxyFactoryBean">
13. <property name="targetName" value="targetA" />
14. <property name="interceptorNames">
15. <list>
16. <value>beforeAdvisor</value>
17. </list>
18. </property>
19. </bean>
20.
21. <bean id="targetA" class="reference.A"></bean>
22. <bean id="targetB" class="reference.B"></bean>
23. <bean id="beforeAdvice" class="reference.BeforeAdvice" />
24. <bean id="beforeAdvisor" class="reference.BeforeAdvisor">
25. <property name="advice" ref="beforeAdvice"/>
26. </bean>
27. <bean id="b" class="org.springframework.aop.framework.ProxyFactoryBean">
28. <property name="targetName" value="targetB" />
29. <property name="interceptorNames">
30. <list>
31. <value>beforeAdvisor</value>
32. </list>
33. </property>
34. </bean>
35. </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,换成targetName 就可以了
B方案：
因为spring 在解决循环依赖的时候，化了个好大好大的圈，以至于我们需要的某个属性迟迟还没有set进来。
那么我们可以把这个大大的圈分成N个小小的圈吗。。
既然spring初始化singleton bean是从xml文件第一个bean开始的(除开BeanPostProcessor)，那么我们就好好利用下这个第一个Bean么。。就把bean定义的位置给换下吧。。
spring 还是和bean定义的顺序有关的。希望大家在使用spring的时候，有所警醒。这个几率的问题不是很大，在使用ProxyFactoryBean 的时候最好使用 targetName 属性。另外，在使用FactoryBean的时候，需要借鉴下ProxyFactoryBean所导致的问题，对于某些属性依赖的话，不要太相信spring会在那个时候给你设置进去。
