摘要：版本從注解開始由于在本人實(shí)際應(yīng)用中使用的是注解配置，也更傾向于了解的整個(gè)實(shí)現(xiàn)，而不僅僅是關(guān)鍵實(shí)現(xiàn)。于是本篇源碼解析，將會(huì)從注解開始。那么，便從的源碼引用開始吧。的引用先從源碼中找有引用到，用來判斷是否有該注解的代碼。

spring 5.0.8.BUILD-SNAPSHOT

aspectjweaver 1.8.13

由于在本人實(shí)際應(yīng)用中使用的是注解配置AOP，也更傾向于了解Spring AOP的整個(gè)實(shí)現(xiàn)，而不僅僅是關(guān)鍵實(shí)現(xiàn)。于是本篇源碼解析，將會(huì)從注解開始。了解Spring AOP是怎么掃描Aspect配置，匹配，并生成AOP代理的。

注解@Aspect定了一個(gè)類為AOP的配置。那么，便從@Aspect的源碼引用開始吧。

先從源碼中找有引用到@Aspect，用來判斷Class是否有該注解的代碼。找到方法。

/org/springframework/aop/aspectj/annotation/AbstractAspectJAdvisorFactory.java
...
@Override
public boolean isAspect(Class clazz) {
    return (hasAspectAnnotation(clazz) && !compiledByAjc(clazz));
}

private boolean hasAspectAnnotation(Class clazz) {
    return (AnnotationUtils.findAnnotation(clazz, Aspect.class) != null);
}

/**
 * We need to detect this as "code-style" AspectJ aspects should not be
 * interpreted by Spring AOP.
 */
private boolean compiledByAjc(Class clazz) {
    // The AJTypeSystem goes to great lengths to provide a uniform appearance between code-style and
    // annotation-style aspects. Therefore there is no "clean" way to tell them apart. Here we rely on
    // an implementation detail of the AspectJ compiler.
    for (Field field : clazz.getDeclaredFields()) {
        if (field.getName().startsWith(AJC_MAGIC)) {
            return true;
        }
    }
    return false;
}
...

isAspect(Class clazz)用來判斷clazz對(duì)象是否包含Aspect.class注解并且未被AspectJ編譯過，其中hasAspectAnnotation(Class clazz)內(nèi)容很明顯就不多提。倒是compiledByAjc(Class clazz)的實(shí)現(xiàn)比較特別。它的實(shí)現(xiàn)是用字段前綴來判斷是否為"code-style" aspects，看起來是一種比較Hack的方法。

這里我有一個(gè)疑惑點(diǎn)，就是"code-style" aspects和"annotation-style" aspects的具體所指，查了一圈也沒有看到明確的解釋。只在IDEA的幫助文檔Overview of AspectJ support這段上有看到相關(guān)的解釋。我的理解是"code-style"是由AspectJ Language所定義的aspect，會(huì)由AspectJ來編譯，而"annotation-style"則是由使用了@Aspect注解的Java語言所定義的aspect，如有錯(cuò)誤煩請(qǐng)指出。

通過一步步閱讀和調(diào)試，可以一層一層向上找到org/springframework/aop/framework/autoproxy/AbstractAutoProxyCreator.java中有兩個(gè)入口。

postProcessBeforeInstantiation和postProcessAfterInitialization，是Bean生命周期中的兩個(gè)步驟：

postProcessBeforeInstantiation: 在Bean實(shí)例化之前執(zhí)行

postProcessAfterInitialization: 在Bean初始化之后執(zhí)行

/org/springframework/aop/framework/autoproxy/AbstractAutoProxyCreator.java
...
@Override
public Object postProcessBeforeInstantiation(Class beanClass, String beanName) throws BeansException {
    Object cacheKey = getCacheKey(beanClass, beanName);

    if (!StringUtils.hasLength(beanName) || !this.targetSourcedBeans.contains(beanName)) {
        if (this.advisedBeans.containsKey(cacheKey)) {
            return null;
        }
        if (isInfrastructureClass(beanClass) || shouldSkip(beanClass, beanName)) {
            this.advisedBeans.put(cacheKey, Boolean.FALSE);
            return null;
        }
    }

    // Create proxy here if we have a custom TargetSource.
    // Suppresses unnecessary default instantiation of the target bean:
    // The TargetSource will handle target instances in a custom fashion.
    TargetSource targetSource = getCustomTargetSource(beanClass, beanName);
    if (targetSource != null) {
        if (StringUtils.hasLength(beanName)) {
            this.targetSourcedBeans.add(beanName);
        }
        Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(beanClass, beanName, targetSource);
        Object proxy = createProxy(beanClass, beanName, specificInterceptors, targetSource);
        this.proxyTypes.put(cacheKey, proxy.getClass());
        return proxy;
    }

    return null;
}
...

這個(gè)方法的目的是將含有custom TargetSource的bean進(jìn)行增強(qiáng)處理?？煞譃閮刹糠?，前半部分利用緩存和幾個(gè)方法判斷是否需要增強(qiáng)。后半部分則進(jìn)入主題判斷是否含有custom TargetSource。不過這里我對(duì)custom TargetSource不是特別理解，也沒有細(xì)看，因?yàn)橥ㄟ^@Aspect注解配置不會(huì)執(zhí)行這里面的代碼，留著以后有時(shí)間再看。
這里還有另外兩個(gè)方法：

isInfrastructureClass(是否是基礎(chǔ)類，如Advice、Pointcut、Advisor、AopInfrastructureBean 及其超類)

shouldSkip(主要目的是判斷是否是已注冊(cè)的@Aspect配置Bean，其實(shí)掃描@Aspect注解配置的方法就在這里面被調(diào)用到了，這個(gè)后面再說)

/org/springframework/aop/framework/autoproxy/AbstractAutoProxyCreator.java
....
/**
 * Create a proxy with the configured interceptors if the bean is
 * identified as one to proxy by the subclass.
 * @see #getAdvicesAndAdvisorsForBean
 */
@Override
public Object postProcessAfterInitialization(@Nullable Object bean, String beanName) {
    if (bean != null) {
        Object cacheKey = getCacheKey(bean.getClass(), beanName);
        if (!this.earlyProxyReferences.contains(cacheKey)) {
            return wrapIfNecessary(bean, beanName, cacheKey);
        }
    }
    return bean;
}

/**
 * Wrap the given bean if necessary, i.e. if it is eligible for being proxied.
 * @param bean the raw bean instance
 * @param beanName the name of the bean
 * @param cacheKey the cache key for metadata access
 * @return a proxy wrapping the bean, or the raw bean instance as-is
 */
protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
    if (StringUtils.hasLength(beanName) && this.targetSourcedBeans.contains(beanName)) {
        return bean;
    }
    if (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) {
        return bean;
    }
    if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) {
        this.advisedBeans.put(cacheKey, Boolean.FALSE);
        return bean;
    }

    // Create proxy if we have advice.
    Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
    if (specificInterceptors != DO_NOT_PROXY) {
        this.advisedBeans.put(cacheKey, Boolean.TRUE);
        Object proxy = createProxy(
                bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));
        this.proxyTypes.put(cacheKey, proxy.getClass());
        return proxy;
    }

    this.advisedBeans.put(cacheKey, Boolean.FALSE);
    return bean;
}

可以看到關(guān)鍵內(nèi)容就在wrapIfNecessary里面。顧名思義：必要時(shí)轉(zhuǎn)成AOP代理。前半部分判斷是否是不需要增強(qiáng)的，跟postProcessBeforeInstantiation的前半部分有點(diǎn)類似。后半部分根據(jù)是否有合適的Advice方法，有則將Bean轉(zhuǎn)成代理。
好了，這里其實(shí)就是整個(gè)流程最關(guān)鍵的兩個(gè)地方了：

getAdvicesAndAdvisorsForBean(獲取適合該Bean的Advice方法，里面包含了掃描@Aspect注解配置Bean的方法)

createProxy(創(chuàng)建AOP代理，里面包含了AOP代理的實(shí)現(xiàn))

這兩個(gè)方法的具體內(nèi)容，將在接下來的文章介紹