spring源码分析之spring注解@Aspect是如何工作的？

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。