AOP（Aspect Orient Programming），面向切面编程，是面向对象编程OOP的一种补充。面向对象编程是从静态角度考虑程序的结构，面向切面编程是从动态的角度考虑程序运行过程。
AOP底层，就是采用动态代理模式实现的。采用两种代理：JDK的动态代理，与CGLIB的动态代理。JDK的动态代理是面向接口的，CGLIB既可以实现有接口的，又可以实现没有接口的。（对动态代理不了解的可以看看我的其关于动态代理的介绍）
面向切面编程，就是将交叉业务逻辑封装成切面，利用AOP容器的功能将切面植入到主业务逻辑中。所谓交叉业务逻辑是指：通用的，与主业务逻辑无关的代码，如安全检查，事务日志等。
@Aspect可以使用切点函数定义切点，我们还可以使用逻辑运算符对切点进行复核运算得到复合的切点，为了在切面中重用切点，我们还可以对切点进行命名，以便在其他的地方引用定义过的切点。当一个连接点匹配多个切点时，需要考虑织入顺序的问题，此外一个重要的问题是如何再增强中访问连接点上下文的信息。
1、@Aspect
在xml定义：<aop:aspectj-autoproxy />，
其定义在http://www.springframework.org/schema/aop/spring-aop-3.0.xsd
- <xsd:element name="aspectj-autoproxy"> - <xsd:annotation> - <xsd:documentation source="java:org.springframework.aop.aspectj.annotation.AnnotationAwareAspectJAutoProxyCreator"> - <![CDATA[ Enables the use of the @AspectJ style of Spring AOP. ]]> </xsd:documentation> </xsd:annotation> - <xsd:complexType> - <xsd:sequence> - <xsd:element name="include" type="includeType" minOccurs="0" maxOccurs="unbounded"> - <xsd:annotation> - <xsd:documentation> - <![CDATA[ Indicates that only @AspectJ beans with names matched by the (regex) pattern will be considered as defining aspects to use for Spring autoproxying. ]]> </xsd:documentation> </xsd:annotation> </xsd:element> </xsd:sequence> - <xsd:attribute name="proxy-target-class" type="xsd:boolean" default="false"> - <xsd:annotation> - <xsd:documentation> - <![CDATA[ Are class-based (CGLIB) proxies to be created? By default, standard Java interface-based proxies are created. ]]> </xsd:documentation> </xsd:annotation> </xsd:attribute> - <xsd:attribute name="expose-proxy" type="xsd:boolean" default="false"> - <xsd:annotation> - <xsd:documentation> - <![CDATA[ Indicate that the proxy should be exposed by the AOP framework as a ThreadLocal for retrieval via the AopContext class. Off by default, i.e. no guarantees that AopContext access will work. ]]> </xsd:documentation> </xsd:annotation> </xsd:attribute> </xsd:complexType> </xsd:element>1.1 注册
org.springframework.aop.aspectj.annotation.AnnotationAwareAspectJAutoProxyCreator
的继承关系如下：
1.2 解析过程
AspectJAutoProxyBeanDefinitionParser.java#parse()方法
public BeanDefinition parse(Element element, ParserContext parserContext) { AopNamespaceUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary(parserContext, element); extendBeanDefinition(element, parserContext); return null; }注册过程：
public static void registerAspectJAnnotationAutoProxyCreatorIfNecessary( ParserContext parserContext, Element sourceElement) { BeanDefinition beanDefinition = AopConfigUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary( parserContext.getRegistry(), parserContext.extractSource(sourceElement)); useClassProxyingIfNecessary(parserContext.getRegistry(), sourceElement); registerComponentIfNecessary(beanDefinition, parserContext); }调用实现类：
public static BeanDefinition registerAspectJAnnotationAutoProxyCreatorIfNecessary(BeanDefinitionRegistry registry, Object source) { return registerOrEscalateApcAsRequired(AnnotationAwareAspectJAutoProxyCreator.class, registry, source); }1.3 具体实现类为：AbstractAutoProxyCreator的postProcessAfterInitialization()方法
DefaultAopProxyFactory#createAopProxy()方法
@Override public AopProxy createAopProxy(AdvisedSupport config) throws AopConfigException { if (config.isOptimize() || config.isProxyTargetClass() || hasNoUserSuppliedProxyInterfaces(config)) { Class<?> targetClass = config.getTargetClass(); if (targetClass == null) { throw new AopConfigException("TargetSource cannot determine target class: " + "Either an interface or a target is required for proxy creation."); } if (targetClass.isInterface()) { return new JdkDynamicAopProxy(config); } return new ObjenesisCglibAopProxy(config); } else { return new JdkDynamicAopProxy(config); } }默认使用jdk自带的代理，还有一种cglib方式。