Spring AOP的原理是 JDK 动态代理和CGLIB字节码增强技术，前者需要被代理类实现相应接口，也只有接口中的方法可以被JDK动态代理技术所处理；后者实际上是生成一个子类，来覆盖被代理类，那么父类的final方法就不能代理，因为父类的final方法不能被子类所覆盖。一般而言Spring默认优先使用JDK动态代理技术，只有在被代理类没有实现接口时，才会选择使用CGLIB技术来实现AOP。
但是也提供了配置参数来强制选择使用 CGLIB 技术，如下：
<aop:config proxy-target-class="true" />
proxy-target-class="true" 表示强制使用 CGLIB 技术来实现AOP，因为CGLIB是生成子类也就是代理类来实现的，所以proxy-target-class，表示是否代理目标类。<aop:config /> 就会由spring来选择，spring优先使用JDK动态代理来实现AOP。
<aop:config /> 那么这句配置，会起到什么作用呢？首先它是 aop 命名空间中的配置，所以：
/** * NamespaceHandler for the aop namespace. * @author Rob Harrop * @author Adrian Colyer * @author Juergen Hoeller * @since 2.0 */public class AopNamespaceHandler extends NamespaceHandlerSupport { /** * Register the {@link BeanDefinitionParser BeanDefinitionParsers} for the * '{@code config}', '{@code spring-configured}', '{@code aspectj-autoproxy}' * and '{@code scoped-proxy}' tags. */ @Override public void init() { // In 2.0 XSD as well as in 2.1 XSD. registerBeanDefinitionParser("config", new ConfigBeanDefinitionParser()); registerBeanDefinitionParser("aspectj-autoproxy", new AspectJAutoProxyBeanDefinitionParser()); registerBeanDefinitionDecorator("scoped-proxy", new ScopedProxyBeanDefinitionDecorator()); // Only in 2.0 XSD: moved to context namespace as of 2.1 registerBeanDefinitionParser("spring-configured", new SpringConfiguredBeanDefinitionParser()); }}
上面的代码表名，aop命名空间有三个元素：<aop:config />, <aop:aspectj-autoproxy />, <aop:scoped-proxy />,而spring-configured被移到了context命名空间了，也就是变成了： <context:spring-configured />
<aop:config /> 所有的配置，由 ConfigBeanDefinitionParser 来解析：
class ConfigBeanDefinitionParser implements BeanDefinitionParser { @Override public BeanDefinition parse(Element element, ParserContext parserContext) { CompositeComponentDefinition compositeDef = new CompositeComponentDefinition(element.getTagName(), parserContext.extractSource(element)); parserContext.pushContainingComponent(compositeDef); configureAutoProxyCreator(parserContext, element);
/**
* Configures the auto proxy creator needed to support the BeanDefinitions
* created by the <aop:config/> tag. Will force class proxying if the 'proxy-target-class' attribute is set to 'true'.
* @see AopNamespaceUtils
*/
private void configureAutoProxyCreator(ParserContext parserContext, Element element) {
AopNamespaceUtils.registerAspectJAutoProxyCreatorIfNecessary(parserContext, element);
}
继续追踪 AopNamespaceUtils.registerAspectJAutoProxyCreatorIfNecessary(parserContext, element)：
/** * Utility class for handling registration of auto-proxy creators used internally by the 'aop' namespace tags. * Only a single auto-proxy creator can be registered and multiple tags may wish * to register different concrete implementations. As such this class delegates to * AopConfigUtils which wraps a simple escalation protocol. Therefore classes * may request a particular auto-proxy creator and know that class, or a subclass * thereof, will eventually be resident in the application context. * * @author Rob Harrop * @author Juergen Hoeller * @author Mark Fisher * @since 2.0 * @see AopConfigUtils */public abstract class AopNamespaceUtils { /** * The proxy-target-class attribute as found on AOP-related XML tags. */ public static final String PROXY_TARGET_CLASS_ATTRIBUTE = "proxy-target-class"; /** * The expose-proxy attribute as found on AOP-related XML tags. */ private static final String EXPOSE_PROXY_ATTRIBUTE = "expose-proxy"; public static void registerAutoProxyCreatorIfNecessary(ParserContext parserContext, Element sourceElement) { BeanDefinition beanDefinition = AopConfigUtils.registerAutoProxyCreatorIfNecessary( parserContext.getRegistry(), parserContext.extractSource(sourceElement)); useClassProxyingIfNecessary(parserContext.getRegistry(), sourceElement); registerComponentIfNecessary(beanDefinition, parserContext); } public static void registerAspectJAutoProxyCreatorIfNecessary(ParserContext parserContext, Element sourceElement) { BeanDefinition beanDefinition = AopConfigUtils.registerAspectJAutoProxyCreatorIfNecessary( parserContext.getRegistry(), parserContext.extractSource(sourceElement)); useClassProxyingIfNecessary(parserContext.getRegistry(), sourceElement); registerComponentIfNecessary(beanDefinition, parserContext); } public static void registerAspectJAnnotationAutoProxyCreatorIfNecessary(ParserContext parserContext, Element sourceElement) { BeanDefinition beanDefinition = AopConfigUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary( parserContext.getRegistry(), parserContext.extractSource(sourceElement)); useClassProxyingIfNecessary(parserContext.getRegistry(), sourceElement); registerComponentIfNecessary(beanDefinition, parserContext); }
很显然的这里针对Spring AOP的三种配置方法分别提供了相应的注册 AutoProxyCreator 的方法：
<aop:config /> 方式对应的注册AutoProxyCreator 的方法是：registerAspectJAutoProxyCreatorIfNecessary；
<aop:aspectj-autoproxy/> 方式对应的注册AutoProxyCreator 的方法是：registerAspectJAnnotationAutoProxyCreatorIfNecessary；
DefaultAdvisorAutoProxyCreator 方式对应的注册AutoProxyCreator 的方法是：registerAutoProxyCreatorIfNecessary；
注：DefaultAdvisorAutoProxyCreator的配置方式一般如下所示：
<bean id="lifecycleBeanPostProcessor" class="org.apache.shiro.spring.LifecycleBeanPostProcessor"/> <bean class="org.springframework.aop.framework.autoproxy.DefaultAdvisorAutoProxyCreator" depends-on="lifecycleBeanPostProcessor" />
这三个方法最终调用的都是 AopConfigUtils 类同一个方法：
private static BeanDefinition registerOrEscalateApcAsRequired(Class<?> cls, BeanDefinitionRegistry registry, Object source) { Assert.notNull(registry, "BeanDefinitionRegistry must not be null"); if (registry.containsBeanDefinition(AUTO_PROXY_CREATOR_BEAN_NAME)) { BeanDefinition apcDefinition = registry.getBeanDefinition(AUTO_PROXY_CREATOR_BEAN_NAME); if (!cls.getName().equals(apcDefinition.getBeanClassName())) { int currentPriority = findPriorityForClass(apcDefinition.getBeanClassName()); int requiredPriority = findPriorityForClass(cls); if (currentPriority < requiredPriority) { apcDefinition.setBeanClassName(cls.getName()); } } return null; } RootBeanDefinition beanDefinition = new RootBeanDefinition(cls); beanDefinition.setSource(source); beanDefinition.getPropertyValues().add("order", Ordered.HIGHEST_PRECEDENCE); beanDefinition.setRole(BeanDefinition.ROLE_INFRASTRUCTURE); registry.registerBeanDefinition(AUTO_PROXY_CREATOR_BEAN_NAME, beanDefinition); return beanDefinition; }
很显然，AOP的三种方式配置，无论如果是最后在bean factory中是只能存在一个AUTO_PROXY_CREATOR_BEAN的，它的name或者说id就是：
/** * The bean name of the internally managed auto-proxy creator. */ public static final String AUTO_PROXY_CREATOR_BEAN_NAME = "org.springframework.aop.config.internalAutoProxyCreator";
但是，如果在多个配置文件中，混用了上面所说的AOP的三种配置方法，那么就有可能产生混乱，产生错误，比如下面的几个配置就会报错：
1>
<aop:config /> <bean id="lifecycleBeanPostProcessor" class="org.apache.shiro.spring.LifecycleBeanPostProcessor"/> <bean class="org.springframework.aop.framework.autoproxy.DefaultAdvisorAutoProxyCreator" depends-on="lifecycleBeanPostProcessor" /> <bean class="org.apache.shiro.spring.security.interceptor.AuthorizationAttributeSourceAdvisor"> <property name="securityManager" ref="securityManager"/> </bean>
上面采用了 <aop:config /> 来配置 AUTO_PROXY_CREATOR_BEAN，而下面有采用了DefaultAdvisorAutoProxyCreator来配置AUTO_PROXY_CREATOR_BEAN，发布时没有报错，但是运行时最终报错：
java.lang.IllegalStateException: The mapped controller method class 'com.xx.controller.xxController' is not an instance of the
actual controller bean instance 'com.sun.proxy.$Proxy45'. If the controller requires proxying (e.g. due to @Transactional),
please use class-based proxying.
HandlerMethod details:
Controller [com.sun.proxy.$Proxy45]
Method [public void com.xx.controller.xxController.xxo(xxx)]
2>
如果将上面的 <aop:config /> 改成 <aop:config proxy-target-class="true" /> 也是一样报相同的错误。
3>
但是如果将上面的DefaultAdvisorAutoProxyCreator修改成下面这样，不管是 <aop:config /> 还是 <aop:config proxy-target-class="true" /> 却都是可以的。
<bean class="org.springframework.aop.framework.autoproxy.DefaultAdvisorAutoProxyCreator" depends-on="lifecycleBeanPostProcessor"> <property name="proxyTargetClass" value="true"/> </bean>
4>
将 <aop:aspectj-autoproxy /> 与 DefaultAdvisorAutoProxyCreator 配置在一起，哪怕是通过<import resource="" /> 放在一起，都会报错。
<aop:aspectj-autoproxy expose-proxy="true"/> <import resource="../shiro/spring-shiro.xml"/> <bean class="org.apache.shiro.spring.security.interceptor.AuthorizationAttributeSourceAdvisor"> <property name="securityManager" ref="securityManager"/> </bean> <bean id="lifecycleBeanPostProcessor" class="org.apache.shiro.spring.LifecycleBeanPostProcessor"/> <bean class="org.springframework.aop.framework.autoproxy.DefaultAdvisorAutoProxyCreator" depends-on="lifecycleBeanPostProcessor"> <property name="proxyTargetClass" value="true"/> </bean>
报错信息：
2015-07-06 17:50:02,270 WARN [org.springframework.beans.factory.support.DefaultListableBeanFactory] -
Bean creation exception on FactoryBean type check: org.springframework.beans.factory.BeanCurrentlyInCreationException:
Error creating bean with name 'shiroFilter': Requested bean is currently in creation: Is there an unresolvable circular reference?而将<aop:aspectj-autoproxy /> 与 DefaultAdvisorAutoProxyCreator 分开配置在不同xml文件中，则不存在该问题。
总结：
1. Spring AOP有三种配置方式，<aop:config />，<aop:aspectj-autoproxy />, DefaultAdvisorAutoProxyCreator. 如果在一个文件中进行混用，那么就可能会产生错误。最好在同一个文件中只采用一种配置方式，推荐使用 <aop:config /> 和 <aop:aspectj-autoproxy />，淘汰DefaultAdvisorAutoProxyCreator的配置方式。
2. Spring的有很多配置都是只在文件级起作用的。
ps:
在iteye找到一篇类似文章 http://jinnianshilongnian.iteye.com/blog/1894465