34--SpringAop获取增强（二）

引

在上一篇结尾，我们得到了增强的提取工作交给了List classAdvisors = this.advisorFactory.getAdvisors(factory);。接着分析。

1. getAdvisors获取增强简析

@Override
public List getAdvisors(MetadataAwareAspectInstanceFactory aspectInstanceFactory) {
    // 1、预处理工作，包括获取切面类，名称，验证等
    Class aspectClass = aspectInstanceFactory.getAspectMetadata().getAspectClass();
    String aspectName = aspectInstanceFactory.getAspectMetadata().getAspectName();
    validate(aspectClass);

    // 我们需要用一个装饰器包装MetadataAwareAspectInstanceFactory，这样它只会实例化一次。
    MetadataAwareAspectInstanceFactory lazySingletonAspectInstanceFactory =
            new LazySingletonAspectInstanceFactoryDecorator(aspectInstanceFactory);

    // 2、提取增强
    // 2.1、获取切面类的所有方法，循环判断提取合适的切入点，并创建增强
    List advisors = new ArrayList<>();
    for (Method method : getAdvisorMethods(aspectClass)) {
        // 获取增强
        Advisor advisor = getAdvisor(method, lazySingletonAspectInstanceFactory, advisors.size(), aspectName);
        if (advisor != null) {
            advisors.add(advisor);
        }
    }

    // If it's a per target aspect, emit the dummy instantiating aspect.
    // 2.2、处理perthis和pertarget切面类
    if (!advisors.isEmpty() && lazySingletonAspectInstanceFactory.getAspectMetadata().isLazilyInstantiated()) {
        // 创建SyntheticInstantiationAdvisor实例
        Advisor instantiationAdvisor = new SyntheticInstantiationAdvisor(lazySingletonAspectInstanceFactory);
        // 将SyntheticInstantiationAdvisor实例加入到advisors集合首位，注意：不是替换
        advisors.add(0, instantiationAdvisor);
    }

    // Find introduction fields.
    // 2.3、处理引入，获取所有的引入并循环创建DeclareParentsAdvisor
    for (Field field : aspectClass.getDeclaredFields()) {
        Advisor advisor = getDeclareParentsAdvisor(field);
        if (advisor != null) {
            advisors.add(advisor);
        }
    }

    return advisors;
}

在该方法中提取工作一共分为了三步：提取普通增强、处理处理perthis和pertarget、引介增强。篇幅有限，只分析普通增强的处理过程,该过程也是大家最关心的过程。来看代码：

@Override
@Nullable
public Advisor getAdvisor(Method candidateAdviceMethod, MetadataAwareAspectInstanceFactory aspectInstanceFactory,
        int declarationOrderInAspect, String aspectName) {

    // 1、验证
    validate(aspectInstanceFactory.getAspectMetadata().getAspectClass());

    // 2、提取切入点
    AspectJExpressionPointcut expressionPointcut = getPointcut(
            candidateAdviceMethod,
            aspectInstanceFactory.getAspectMetadata().getAspectClass());

    if (expressionPointcut == null) {
        return null;
    }

    // 3、创建增强
    return new InstantiationModelAwarePointcutAdvisorImpl(expressionPointcut, candidateAdviceMethod,
            this, aspectInstanceFactory, declarationOrderInAspect, aspectName);
}

在该方法中，终于看到了核心的切点提取和创建增强。下面分别来看这两步是如何实现的。

2.提取切入点

@Nullable
private AspectJExpressionPointcut getPointcut(Method candidateAdviceMethod, Class candidateAspectClass) {
    // 1、从候选切入点上找出增强表达式
    AspectJAnnotation aspectJAnnotation = AbstractAspectJAdvisorFactory.findAspectJAnnotationOnMethod(candidateAdviceMethod);
    if (aspectJAnnotation == null) {
        return null;
    }

    // 2、创建AspectJExpressionPointcut对象
    AspectJExpressionPointcut ajexp = new AspectJExpressionPointcut(candidateAspectClass, new String[0], new Class[0]);
    ajexp.setExpression(aspectJAnnotation.getPointcutExpression());
    if (this.beanFactory != null) {
        ajexp.setBeanFactory(this.beanFactory);
    }
    return ajexp;
}

2.1 从候选切入点上找出增强表达式

/**
 * Find and return the first AspectJ annotation on the given method
 * (there should only be one anyway...).
 */
@SuppressWarnings("unchecked")
@Nullable
protected static AspectJAnnotation findAspectJAnnotationOnMethod(Method method) {
    Class[] classesToLookFor = new Class[] {
            Before.class, Around.class, After.class, AfterReturning.class, AfterThrowing.class, Pointcut.class};
    for (Class c : classesToLookFor) {
        AspectJAnnotation foundAnnotation = findAnnotation(method, (Class) c);
        if (foundAnnotation != null) {
            return foundAnnotation;
        }
    }
    return null;
}

该过程主要通过findAnnotation方法完成，但是该方法调用较深，也不属于我们分析的范畴，感兴趣的同学可以自己跟踪调试。

2.2 创建AspectJExpressionPointcut对象

/**
 * Create a new AspectJExpressionPointcut with the given settings.
 * @param declarationScope the declaration scope for the pointcut
 * @param paramNames the parameter names for the pointcut
 * @param paramTypes the parameter types for the pointcut
 */
public AspectJExpressionPointcut(Class declarationScope, String[] paramNames, Class[] paramTypes) {
    this.pointcutDeclarationScope = declarationScope;
    if (paramNames.length != paramTypes.length) {
        throw new IllegalStateException(
                "Number of pointcut parameter names must match number of pointcut parameter types");
    }
    this.pointcutParameterNames = paramNames;
    this.pointcutParameterTypes = paramTypes;
}

该创建过程比较简单，将提取的切点表达式的信息实例化为AspectJExpressionPointcut对象即可。

3.创建增强

public InstantiationModelAwarePointcutAdvisorImpl(
            AspectJExpressionPointcut declaredPointcut,
            Method aspectJAdviceMethod,
            AspectJAdvisorFactory aspectJAdvisorFactory,
            MetadataAwareAspectInstanceFactory aspectInstanceFactory,
            int declarationOrder,
            String aspectName) {

    this.declaredPointcut = declaredPointcut;
    this.declaringClass = aspectJAdviceMethod.getDeclaringClass();
    this.methodName = aspectJAdviceMethod.getName();
    this.parameterTypes = aspectJAdviceMethod.getParameterTypes();
    this.aspectJAdviceMethod = aspectJAdviceMethod;
    this.aspectJAdvisorFactory = aspectJAdvisorFactory;
    this.aspectInstanceFactory = aspectInstanceFactory;
    this.declarationOrder = declarationOrder;
    this.aspectName = aspectName;

    // 1、延迟初始化
    // 在Spring AOP中，切面类的实例只有一个，比如前面我们一直使用的MyAspect类，
    // 假设我们使用的切面类需要具有某种状态，以适用某些特殊情况的使用，比如多线程环境，此时单例的切面类就不符合我们的要求了。
    // 在Spring AOP中，切面类默认都是单例的，但其还支持另外两种多例的切面实例的切面，即perthis和pertarget，
    // 需要注意的是perthis和pertarget都是使用在切面类的@Aspect注解中的。
    // 这里perthis和pertarget表达式中都是指定一个切面表达式，其语义与前面讲解的this和target非常的相似，
    // perthis表示如果某个类的代理类符合其指定的切面表达式，那么就会为每个符合条件的目标类都声明一个切面实例；
    // pertarget表示如果某个目标类符合其指定的切面表达式，那么就会为每个符合条件的类声明一个切面实例。
    // 从上面的语义可以看出，perthis和pertarget的含义是非常相似的。如下是perthis和pertarget的使用语法：
    // perthis(pointcut-expression)
    // pertarget(pointcut-expression)
    if (aspectInstanceFactory.getAspectMetadata().isLazilyInstantiated()) {
        // Static part of the pointcut is a lazy type.
        Pointcut preInstantiationPointcut = Pointcuts.union(
                aspectInstanceFactory.getAspectMetadata().getPerClausePointcut(),
                this.declaredPointcut);

        // Make it dynamic: must mutate from pre-instantiation to post-instantiation state.
        // If it's not a dynamic pointcut, it may be optimized out
        // by the Spring AOP infrastructure after the first evaluation.
        this.pointcut = new PerTargetInstantiationModelPointcut(
                this.declaredPointcut,
                preInstantiationPointcut,
                aspectInstanceFactory);
        this.lazy = true;
    }
    // 2、立刻初始化
    else {
        // A singleton aspect.
        this.pointcut = this.declaredPointcut;
        this.lazy = false;
        // 初始化增强
        this.instantiatedAdvice = instantiateAdvice(this.declaredPointcut);
    }
}

创建过程中又涉及到perthis和pertarget，无需理会，还是先看单例模式下的创建过程：

/**
 * 根据pointcut初始化增强
 * @param pointcut
 * @return
 */
private Advice instantiateAdvice(AspectJExpressionPointcut pointcut) {
    Advice advice = this.aspectJAdvisorFactory.getAdvice(
            this.aspectJAdviceMethod, pointcut,
            this.aspectInstanceFactory,
            this.declarationOrder,
            this.aspectName);
    return (advice != null ? advice : EMPTY_ADVICE);
}

@Override
@Nullable
public Advice getAdvice(Method candidateAdviceMethod, AspectJExpressionPointcut expressionPointcut,
        MetadataAwareAspectInstanceFactory aspectInstanceFactory, int declarationOrder, String aspectName) {

    // 1、获取增强之前的处理
    Class candidateAspectClass = aspectInstanceFactory.getAspectMetadata().getAspectClass();
    validate(candidateAspectClass);

    AspectJAnnotation aspectJAnnotation = AbstractAspectJAdvisorFactory.findAspectJAnnotationOnMethod(candidateAdviceMethod);
    if (aspectJAnnotation == null) {
        return null;
    }

    // If we get here, we know we have an AspectJ method.
    // Check that it's an AspectJ-annotated class
    if (!isAspect(candidateAspectClass)) {
        throw new AopConfigException("Advice must be declared inside an aspect type: " +
                "Offending method '" + candidateAdviceMethod + "' in class [" +
                candidateAspectClass.getName() + "]");
    }


    // 2、针对各种不同的增强，做不同的处理
    AbstractAspectJAdvice springAdvice;
    switch (aspectJAnnotation.getAnnotationType()) {
            // 1、前置增强
        case AtBefore:
            springAdvice = new AspectJMethodBeforeAdvice(candidateAdviceMethod, expressionPointcut, aspectInstanceFactory);
            break;
            // 2、后置增强
        case AtAfter:
            springAdvice = new AspectJAfterAdvice(candidateAdviceMethod, expressionPointcut, aspectInstanceFactory);
            break;
            // 3、后置返回增强
        case AtAfterReturning:
            springAdvice = new AspectJAfterReturningAdvice(candidateAdviceMethod, expressionPointcut, aspectInstanceFactory);
            AfterReturning afterReturningAnnotation = (AfterReturning) aspectJAnnotation.getAnnotation();
            if (StringUtils.hasText(afterReturningAnnotation.returning())) {
                springAdvice.setReturningName(afterReturningAnnotation.returning());
            }
            break;
            // 4、后置异常增强
        case AtAfterThrowing:
            springAdvice = new AspectJAfterThrowingAdvice(candidateAdviceMethod, expressionPointcut, aspectInstanceFactory);
            AfterThrowing afterThrowingAnnotation = (AfterThrowing) aspectJAnnotation.getAnnotation();
            if (StringUtils.hasText(afterThrowingAnnotation.throwing())) {
                springAdvice.setThrowingName(afterThrowingAnnotation.throwing());
            }
            break;
            // 5、环绕增强
        case AtAround:
            springAdvice = new AspectJAroundAdvice(
                    candidateAdviceMethod, expressionPointcut, aspectInstanceFactory);
            break;
            // 6、如果是Pointcut，则不做处理
        case AtPointcut:
            return null;
            // 7、未能满足case条件，抛出异常
        default:
            throw new UnsupportedOperationException("Unsupported advice type on method: " + candidateAdviceMethod);
    }

    // 3、获取增强方法之后，对增强方法进行配置
    springAdvice.setAspectName(aspectName);
    springAdvice.setDeclarationOrder(declarationOrder);
    String[] argNames = this.parameterNameDiscoverer.getParameterNames(candidateAdviceMethod);
    if (argNames != null) {
        springAdvice.setArgumentNamesFromStringArray(argNames);
    }
    springAdvice.calculateArgumentBindings();

    return springAdvice;
}

看到这里，是不是有种豁然开朗的感觉，Spring针对各种不同的增强创建过程，做了不同的处理。到这里Spring创建增强的过程就完成了，对于不同的增强类型创建，大家可以自己debug跟踪，这里不一一赘述了。

4.关于Advisor的疑问

这两篇在介绍获取增强，但是最受获取到的并不是Advice而是Advisor，可能大家会有所疑问。举例说明一下：
如果我们定义了一个DogAspect类，并用@AspectJ对其进行注解,那么该类仅仅代表一个切面类，会被Spring扫描并解析，仅此而已，该类不代表SpringAop概念中的切面。那么Spring如果通过解析该类得到具体的切面呢?

首先,关于SpringAop中的切面概念，可以理解为切面=连接点+增强

其次，而标记了@AspectJ注解的类在被Spring解析的时候,

提取该类的方法上的切点表达式注解：例如-->@Pointcut("execution(* com.lyc.cn.v2.day07..(..))")，解析之后,就可以的到具体的切点.
提取该类的方法上的增强注解:例如：@Before("test()")解析之后,就可以得到具体的增强代码

最后,通过第一步和第二步的操作,就可以得到切点+增强,那么自然就构成了一个切面

但是Advisor接口里只包含了一个Advice,并且Advisor一般不直接提供给用户使用,所以这里也可以理解为获取增强，当然如果理解为切面也是没有问题的。