SpringAOP源码解析

1.环境搭建

pom文件加入aspects的依赖

<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-context</artifactId>
<version>5.1.9.RELEASE</version>
</dependency>
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-aspects</artifactId>
<version>5.1.9.RELEASE</version>
</dependency>

定义一个需要被切入的bean

public class MathCaculator {

public int div(int i,int j){
System.out.println("MathCaculator.div.......");
return i/j;
}
}

定义切面类

@Aspect
public class LogAspects {

//抽取公共的切点表达式
//1.本类引用
@Pointcut("execution(public int com.shiki.aop.MathCaculator.*(..))")
public void pointCut(){};

//@Before在目标方法之前切入，切入点表达式（指定在哪个方法切入）
@Before("pointCut()")
public void logStart(JoinPoint joinPoint){
Object[] args = joinPoint.getArgs();
System.out.println(""+joinPoint.getSignature().getName()+"除法运行...参数列表式:{"+ Arrays.asList(args)+"}");
}

@After("pointCut()")
public void logEnd(JoinPoint joinPoint){
System.out.println(""+joinPoint.getSignature().getName()+"除法结束...");
}

//JoinPoint一定要出现在参数表的第一位
@AfterReturning(value = "pointCut()",returning = "result")
public void logReturn(JoinPoint joinPoint,Object result){
System.out.println("除法运行...返回值:{"+result+"}");
}

@AfterThrowing(value = "pointCut()",throwing = "exception")
public void logException(Exception exception){
System.out.println("除法运行...异常信息:{"+exception+"}");
}
}

将业务逻辑类和切面类注册到容器中

@EnableAspectJAutoProxy
@Configuration
public class MainConfigOfAOP {

//业务逻辑类加入容器中
@Bean
public MathCaculator caculator(){
return new MathCaculator();
}

//切面类加入容器中
@Bean
public LogAspects logAspects(){
return new LogAspects();
}
}

测试代码

public class IOCTest_AOP {

@Test
public void test01(){
AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(MainConfigOfAOP.class);
MathCaculator caculator = context.getBean(MathCaculator.class);
caculator.div(1,1);
context.close();
}

}

测试执行结果：

Spring注解版AOP分为三步：

1）、将业务逻辑组件和切面类都加入到容器中，告诉Sprinf哪个是切面类(@Aspect)
2）、在切面类上的每一个通知方法上标注通知注解，告诉Spring何时何地运行（切入点表达式）
3）、开启基于注解的AOP模式

*如何切入AOP源码进行分析？

核心思想：看给容器中注册类什么组件，这个组件什么时候工作，提供了什么功能

从@EnableAspectJAutoProxy入手，查看@EnableAspectJAutoProxy源码

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import({AspectJAutoProxyRegistrar.class})
public @interface EnableAspectJAutoProxy {
boolean proxyTargetClass() default false;

boolean exposeProxy() default false;
}

注意到EnableAspectJAutoProxy向容器中导入了AspectJAutoProxyRegistrar这个组件，点进源码，在方法入口处打上断点

debug方式启动容器，跟进到这个方法

发现返回了这个bean，即internalAutoProxyCreator=AnnotationAwareAspectJAutoProxyCreator。

可见利用AspectJAutoProxyRegistrar自定义给容器中注册Bean，这个bean就是AnnotationAwareAspectJAutoProxyCreator，顾名思义是“注解通知切面自动代理创建”。

下面研究AnnotationAwareAspectJAutoProxyCreator

2.AnnotationAwareAspectJAutoProxyCreator分析

首先查看AnnotationAwareAspectJAutoProxyCreator的继承关系



AnnotationAwareAspectJAutoProxyCreator的父类AbstractAutoProxyCreator实现了SmartInstantiationAwareBeanPostProcessor和BeanFactoryAware接口。因此AnnotationAwareAspectJAutoProxyCreator具备了bean后置处理器和beanFactory的功能，我们重点来研究AnnotationAwareAspectJAutoProxyCreator作为后置处理器和BeanFactoryAware做了哪些工作，只要知道了这个，那么SpringAOP的流程就差不多清楚了。

在AnnotationAwareAspectJAutoProxyCreator类中没有发现从SmartInstantiationAwareBeanPostProcessor和BeanFactoryAware接口重写的方法，所以到AnnotationAwareAspectJAutoProxyCreator的父类里寻找相关方法。

首先从直接实现SmartInstantiationAwareBeanPostProcessor和BeanFactoryAware接口的AbstractAutoProxyCreator开始找。AbstractAutoProxyCreator里有重写的setBeanFactory方法和后置处理器相关的方法，在这些方法上打上断点。

继续查看AbstractAutoProxyCreator的直接子类AbstractAdvisorAutoProxyCreator，虽然它的父类重写了setBeanFactory方法，但是它又重写了这个方法，先打上断点。

注意到AbstractAdvisorAutoProxyCreator的setBeanFactory方法里调用了一个initBeanFactory方法。

继续查看AbstractAdvisorAutoProxyCreator的子类AspectJAwareAdvisorAutoProxyCreator，没有发现可疑的方法。最后查看AnnotationAwareAspectJAutoProxyCreator，发现了从父类重写的initBeanFactory方法。

它的父类里的initBeanFactory方法里会调用到这里，在这个方法上打上断点。
现在我们已经找出了AnnotationAwareAspectJAutoProxyCreator及其父类中所有与后置处理器和BeanFactoryAware有关的方法并在有意义的方法上（直接返回的不算）打上断点了。整理一下思路。

AbstractAutoProxyCreator.setBeanFactory()
AbstractAutoProxyCreator.postProcessBeforeInstantiation()
AbstractAutoProxyCreator.postProcessAfterInitialization
AbstractAdvisorAutoProxyCreator.setBeanFactory()->initBeanFactory
AspectJAwareAdvisorAutoProxyCreator 无
AnnotationAwareAspectJAutoProxyCreator.initBeanFactory()

3.创建和注册AnnotationAwareAspectJAutoProxyCreator

debug方式启动容器，启动之后先来到了AbstractAutoProxyCreator的setBeanFactory方法，这个方法是从我自己写的测试方法过来的，向后跟踪栈轨迹。

注册bean processor用于拦截bean的创建

public abstract class AbstractApplicationContext extends DefaultResourceLoader
implements ConfigurableApplicationContext {

@Override
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// Prepare this context for refreshing.
prepareRefresh();

// Tell the subclass to refresh the internal bean factory.
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

// Prepare the bean factory for use in this context.
prepareBeanFactory(beanFactory);

try {
// Allows post-processing of the bean factory in context subclasses.
postProcessBeanFactory(beanFactory);

// Invoke factory processors registered as beans in the context.
invokeBeanFactoryPostProcessors(beanFactory);

// Register bean processors that intercept bean creation.
// 注册BeanPostProcessor来拦截bean的创建
registerBeanPostProcessors(beanFactory);

// Initialize message source for this context.
initMessageSource();

// Initialize event multicaster for this context.
initApplicationEventMulticaster();

// Initialize other special beans in specific context subclasses.
onRefresh();

// Check for listener beans and register them.
registerListeners();

// Instantiate all remaining (non-lazy-init) singletons.
finishBeanFactoryInitialization(beanFactory);

// Last step: publish corresponding event.
finishRefresh();
}

catch (BeansException ex) {
if (logger.isWarnEnabled()) {
logger.warn("Exception encountered during context initialization - " +
"cancelling refresh attempt: " + ex);
}

// Destroy already created singletons to avoid dangling resources.
destroyBeans();

// Reset 'active' flag.
cancelRefresh(ex);

// Propagate exception to caller.
throw ex;
}

finally {
// Reset common introspection caches in Spring's core, since we
// might not ever need metadata for singleton beans anymore...
resetCommonCaches();
}
}
}

}

3.1BeanPostProcessor注册的过程

这一步就是在创建internalAutoProxyCreator的BeanPostProcessor即AnnotationAwareAspectJAutoProxyCreator

final class PostProcessorRegistrationDelegate {
public static void registerBeanPostProcessors(
ConfigurableListableBeanFactory beanFactory, AbstractApplicationContext applicationContext) {

//1.先获取ioc容器已经定义了的需要创建对象的BeanPostProcessor
String[] postProcessorNames = beanFactory.getBeanNamesForType(BeanPostProcessor.class, true, false);

int beanProcessorTargetCount = beanFactory.getBeanPostProcessorCount() + 1 + postProcessorNames.length;
//2.给容器中加别的BeanPostProcessor
beanFactory.addBeanPostProcessor(new BeanPostProcessorChecker(beanFactory, beanProcessorTargetCount));

List<BeanPostProcessor> priorityOrderedPostProcessors = new ArrayList<>();
List<BeanPostProcessor> internalPostProcessors = new ArrayList<>();
List<String> orderedPostProcessorNames = new ArrayList<>();
List<String> nonOrderedPostProcessorNames = new ArrayList<>();
for (String ppName : postProcessorNames) {
//3.优先注册实现了PriorityOrdered接口的BeanPostProcessor
if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
priorityOrderedPostProcessors.add(pp);
if (pp instanceof MergedBeanDefinitionPostProcessor) {
internalPostProcessors.add(pp);
}
}
//4.然后给容器中注册实现了Ordered接口的BeanPostProcessor
else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
orderedPostProcessorNames.add(ppName);
}
//5.最后注册没实现优先级接口的BeanPostProcessor
else {
nonOrderedPostProcessorNames.add(ppName);
}
}

// First, register the BeanPostProcessors that implement PriorityOrdered.
sortPostProcessors(priorityOrderedPostProcessors, beanFactory);
registerBeanPostProcessors(beanFactory, priorityOrderedPostProcessors);

// Next, register the BeanPostProcessors that implement Ordered.
List<BeanPostProcessor> orderedPostProcessors = new ArrayList<>();
//6.注册BeanPostProcessor，实际上就是创建BeanPostProcessor对象，保存在容器中
for (String ppName : orderedPostProcessorNames) {
BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
orderedPostProcessors.add(pp);
if (pp instanceof MergedBeanDefinitionPostProcessor) {
internalPostProcessors.add(pp);
}
}
sortPostProcessors(orderedPostProcessors, beanFactory);
registerBeanPostProcessors(beanFactory, orderedPostProcessors);

// Now, register all regular BeanPostProcessors.
List<BeanPostProcessor> nonOrderedPostProcessors = new ArrayList<>();
for (String ppName : nonOrderedPostProcessorNames) {
BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
nonOrderedPostProcessors.add(pp);
if (pp instanceof MergedBeanDefinitionPostProcessor) {
internalPostProcessors.add(pp);
}
}
registerBeanPostProcessors(beanFactory, nonOrderedPostProcessors);

// Finally, re-register all internal BeanPostProcessors.
sortPostProcessors(internalPostProcessors, beanFactory);
registerBeanPostProcessors(beanFactory, internalPostProcessors);

// Re-register post-processor for detecting inner beans as ApplicationListeners,
// moving it to the end of the processor chain (for picking up proxies etc).
//7.把注册BeanPostProcessor注册到BeanFactory中
beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(applicationContext));
}
}

3.2AnnotationAwareAspectJAutoProxyCreator的创建过程

public abstract class AbstractAutowireCapableBeanFactory extends AbstractBeanFactory implements AutowireCapableBeanFactory {
protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) throws BeanCreationException {
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = (BeanWrapper)this.factoryBeanInstanceCache.remove(beanName);
}

if (instanceWrapper == null) {
instanceWrapper = this.createBeanInstance(beanName, mbd, args);
}
//1.创建bean的实例
Object bean = instanceWrapper.getWrappedInstance();
Class<?> beanType = instanceWrapper.getWrappedClass();
if (beanType != NullBean.class) {
mbd.resolvedTargetType = beanType;
}

Object var7 = mbd.postProcessingLock;
synchronized(mbd.postProcessingLock) {
if (!mbd.postProcessed) {
try {
this.applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
} catch (Throwable var17) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Post-processing of merged bean definition failed", var17);
}

mbd.postProcessed = true;
}
}

boolean earlySingletonExposure = mbd.isSingleton() && this.allowCircularReferences && this.isSingletonCurrentlyInCreation(beanName);
if (earlySingletonExposure) {
if (this.logger.isTraceEnabled()) {
this.logger.trace("Eagerly caching bean '" + beanName + "' to allow for resolving potential circular references");
}

this.addSingletonFactory(beanName, () -> {
return this.getEarlyBeanReference(beanName, mbd, bean);
});
}

Object exposedObject = bean;

try {
//2.给bean的各种属性赋值
this.populateBean(beanName, mbd, instanceWrapper);
//3.初始化bean
exposedObject = this.initializeBean(beanName, exposedObject, mbd);
} catch (Throwable var18) {
if (var18 instanceof BeanCreationException && beanName.equals(((BeanCreationException)var18).getBeanName())) {
throw (BeanCreationException)var18;
}

throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Initialization of bean failed", var18);
}

if (earlySingletonExposure) {
Object earlySingletonReference = this.getSingleton(beanName, false);
if (earlySingletonReference != null) {
if (exposedObject == bean) {
exposedObject = earlySingletonReference;
} else if (!this.allowRawInjectionDespiteWrapping && this.hasDependentBean(beanName)) {
String[] dependentBeans = this.getDependentBeans(beanName);
Set<String> actualDependentBeans = new LinkedHashSet(dependentBeans.length);
String[] var12 = dependentBeans;
int var13 = dependentBeans.length;

for(int var14 = 0; var14 < var13; ++var14) {
String dependentBean = var12[var14];
if (!this.removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
actualDependentBeans.add(dependentBean);
}
}

if (!actualDependentBeans.isEmpty()) {
throw new BeanCurrentlyInCreationException(beanName, "Bean with name '" + beanName + "' has been injected into other beans [" + StringUtils.collectionToCommaDelimitedString(actualDependentBeans) + "] in its raw version as part of a circular reference, but has eventually been wrapped. This means that said other beans do not use the final version of the bean. This is often the result of over-eager type matching - consider using 'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
}
}
}
}

try {
this.registerDisposableBeanIfNecessary(beanName, bean, mbd);
return exposedObject;
} catch (BeanDefinitionValidationException var16) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", var16);
}
}

}

3.3AnnotationAwareAspectJAutoProxyCreato的初始化过程

public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements SingletonBeanRegistry {
protected Object initializeBean(String beanName, Object bean, @Nullable RootBeanDefinition mbd) {
//1.处理Aware接口的方法回调
if (System.getSecurityManager() != null) {
AccessController.doPrivileged(() -> {
this.invokeAwareMethods(beanName, bean);
return null;
}, this.getAccessControlContext());
} else {
this.invokeAwareMethods(beanName, bean);
}

Object wrappedBean = bean;
//2.执行后置处理器的postProcessorsBeforeInitialization
if (mbd == null || !mbd.isSynthetic()) {
wrappedBean = this.applyBeanPostProcessorsBeforeInitialization(bean, beanName);
}

try {
//3.执行自定义的初始化方法
this.invokeInitMethods(beanName, wrappedBean, mbd);
} catch (Throwable var6) {
throw new BeanCreationException(mbd != null ? mbd.getResourceDescription() : null, beanName, "Invocation of init method failed", var6);
}
//4.执行后置处理器的postProcessorsAfterInitialization
if (mbd == null || !mbd.isSynthetic()) {
wrappedBean = this.applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
}

return wrappedBean;
}

}

至此，AnnotationAwareAspectJAutoProxyCreator创建成功并注册到了容器中。下一节研究AnnotationAwareAspectJAutoProxyCreator的执行时机。

4.AnnotationAwareAspectJAutoProxyCreator执行时机

AnnotationAwareAspectJAutoProxyCreator已经创建并注册到容器中，它会在bean创建时进行拦截。因此我们只要重点关注AnnotationAwareAspectJAutoProxyCreator是如何对自定义并注册进容器的业务逻辑类进行拦截和处理的。

将断点打到业务逻辑类注册的地方，向前追溯栈轨迹。箭头所指的finishBeanFactoryInitialization方法的作用正是完成BeanFactory的初始化，创建剩下的单实例bean，即当前的业务逻辑类MathCaculator。

4.1自定义的业务逻辑类的创建过程

1）首先遍历获取容器中所有bean，依次创建对象

getBean()–>doGetBean()–>getSingleton–>…

public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFactory
implements ConfigurableListableBeanFactory, BeanDefinitionRegistry, Serializable {
@Override
public void preInstantiateSingletons() throws BeansException {
if (logger.isTraceEnabled()) {
logger.trace("Pre-instantiating singletons in " + this);
}

// Iterate over a copy to allow for init methods which in turn register new bean definitions.
// While this may not be part of the regular factory bootstrap, it does otherwise work fine.
List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);

// Trigger initialization of all non-lazy singleton beans...
//遍历获取容器中所有bean，依次创建对象
for (String beanName : beanNames) {
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
if (isFactoryBean(beanName)) {
Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
if (bean instanceof FactoryBean) {
final FactoryBean<?> factory = (FactoryBean<?>) bean;
boolean isEagerInit;
if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
isEagerInit = AccessController.doPrivileged((PrivilegedAction<Boolean>)
((SmartFactoryBean<?>) factory)::isEagerInit,
getAccessControlContext());
}
else {
isEagerInit = (factory instanceof SmartFactoryBean &&
((SmartFactoryBean<?>) factory).isEagerInit());
}
if (isEagerInit) {
getBean(beanName);
}
}
}
else {
getBean(beanName);
}
}
}

// Trigger post-initialization callback for all applicable beans...
for (String beanName : beanNames) {
Object singletonInstance = getSingleton(beanName);
if (singletonInstance instanceof SmartInitializingSingleton) {
final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
if (System.getSecurityManager() != null) {
AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
smartSingleton.afterSingletonsInstantiated();
return null;
}, getAccessControlContext());
}
else {
smartSingleton.afterSingletonsInstantiated();
}
}
}
}

}

2）创建bean

public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements SingletonBeanRegistry {
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(beanName, "Bean name must not be null");
synchronized (this.singletonObjects) {
//1.先从缓存中获取当前bean，如果能获取到，说明bean是之前被创建过的，直接使用，否则再创建
//只要创建好的bean都会被缓存起来(singletonObjects->ConcurrentHashMap)
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
if (this.singletonsCurrentlyInDestruction) {
throw new BeanCreationNotAllowedException(beanName,
"Singleton bean creation not allowed while singletons of this factory are in destruction " +
"(Do not request a bean from a BeanFactory in a destroy method implementation!)");
}
if (logger.isDebugEnabled()) {
logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
}
beforeSingletonCreation(beanName);
boolean newSingleton = false;
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet<>();
}
try {
singletonObject = singletonFactory.getObject();
newSingleton = true;
}
catch (IllegalStateException ex) {
// Has the singleton object implicitly appeared in the meantime ->
// if yes, proceed with it since the exception indicates that state.
singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
throw ex;
}
}
catch (BeanCreationException ex) {
if (recordSuppressedExceptions) {
for (Exception suppressedException : this.suppressedExceptions) {
ex.addRelatedCause(suppressedException);
}
}
throw ex;
}
finally {
if (recordSuppressedExceptions) {
this.suppressedExceptions = null;
}
afterSingletonCreation(beanName);
}
if (newSingleton) {
addSingleton(beanName, singletonObject);
}
}
return singletonObject;
}
}

}

public abstract class AbstractAutowireCapableBeanFactory extends AbstractBeanFactory
implements AutowireCapableBeanFactory {
@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {

if (logger.isTraceEnabled()) {
logger.trace("Creating instance of bean '" + beanName + "'");
}
RootBeanDefinition mbdToUse = mbd;

// Make sure bean class is actually resolved at this point, and
// clone the bean definition in case of a dynamically resolved Class
// which cannot be stored in the shared merged bean definition.
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}

// Prepare method overrides.
try {
mbdToUse.prepareMethodOverrides();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
beanName, "Validation of method overrides failed", ex);
}

try {
// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
//希望后置处理器能返回一个代理对象，如果能返回代理对象就使用，如果不能就继续
Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
if (bean != null) {
return bean;
}
}
catch (Throwable ex) {
throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,
"BeanPostProcessor before instantiation of bean failed", ex);
}

try {
//如果不能返回代理对象，才真正的去创建一个对象
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
if (logger.isTraceEnabled()) {
logger.trace("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
// A previously detected exception with proper bean creation context already,
// or illegal singleton state to be communicated up to DefaultSingletonBeanRegistry.
throw ex;
}
catch (Throwable ex) {
throw new BeanCreationException(
mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
}
}

}

后置处理器是如何返回代理对象的？

public abstract class AbstractAutowireCapableBeanFactory extends AbstractBeanFactory
implements AutowireCapableBeanFactory {
*/
@Nullable
protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {
Object bean = null;
if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {
// Make sure bean class is actually resolved at this point.
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
Class<?> targetType = determineTargetType(beanName, mbd);
if (targetType != null) {
//
bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);
if (bean != null) {
bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);
}
}
}
mbd.beforeInstantiationResolved = (bean != null);
}
return bean;
}

@Nullable
protected Object applyBeanPostProcessorsBeforeInstantiation(Class<?> beanClass, String beanName) {
//拿到所有BeanPostProccessor，如果是InstantiationAwareBeanPostProcessor，就执行postProcessBeforeInstantiation方法
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
Object result = ibp.postProcessBeforeInstantiation(beanClass, beanName);
if (result != null) {
return result;
}
}
}
return null;
}

}

而AnnotationAwareAspectJAutoProxyCreator正是一个InstantiationAwareBeanPostProcessor。

*InstantiationAwareBeanPostProcessor和BeanPostProcessor的区别：

BeanPostProcessor是在bean创建初始化完成后调用的；

InstantiationAwareBeanPostProcessor是在bean实例化之前尝试用后置处理器返回对象的。

所以AnnotationAwareAspectJAutoProxyCreator会在任何bean创建之前先尝试返回bean的实例。

5.创建AOP动态代理

由上一节可知，AnnotationAwareAspectJAutoProxyCreator在所有bean创建之前会有一个拦截，会调用postProcessBeforeInstantiation。因此这节重点研究postProcessBeforeInstantiation方法做了什么，重点关注自定义的业务逻辑类MathCaculator和切面类LogAspects。

public abstract class AbstractAutoProxyCreator extends ProxyProcessorSupport
implements SmartInstantiationAwareBeanPostProcessor, BeanFactoryAware {
@Override
public Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) {
Object cacheKey = getCacheKey(beanClass, beanName);

if (!StringUtils.hasLength(beanName) || !this.targetSourcedBeans.contains(beanName)) {
//1.判断当前bean是否在advisedBeans中（保存了所有需要增强的bean）
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;
}

//2.判断当前bean是否是基础类型的(Advice,Pointcut,Advisor,AopInfrastructureBean类型)
protected boolean isInfrastructureClass(Class<?> beanClass) {
boolean retVal = Advice.class.isAssignableFrom(beanClass) ||
Pointcut.class.isAssignableFrom(beanClass) ||
Advisor.class.isAssignableFrom(beanClass) ||
AopInfrastructureBean.class.isAssignableFrom(beanClass);
if (retVal && logger.isTraceEnabled()) {
logger.trace("Did not attempt to auto-proxy infrastructure class [" + beanClass.getName() + "]");
}
return retVal;
}

//3.是否需要跳过
protected boolean shouldSkip(Class<?> beanClass, String beanName) {
return AutoProxyUtils.isOriginalInstance(beanName, beanClass);
}

}

//创建代理对象
@Override
public Object postProcessAfterInitialization(@Nullable Object bean, String beanName) {
if (bean != null) {
Object cacheKey = getCacheKey(bean.getClass(), beanName);
if (this.earlyProxyReferences.remove(cacheKey) != bean) {
//如果需要就包装对象
return wrapIfNecessary(bean, beanName, cacheKey);
}
}
return bean;
}

//包装对象
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.
//1.获取当前bean的所有增强器（通知方法）
Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
if (specificInterceptors != DO_NOT_PROXY) {
//2.保存当前bean在advisedBeans中
this.advisedBeans.put(cacheKey, Boolean.TRUE);
//3.如果当前bean需要增强，创建当前对象代理对象
Object proxy = createProxy(
bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));
this.proxyTypes.put(cacheKey, proxy.getClass());
return proxy;
}
//4.给容器中返回了当前对象使用cglib增强了的代理对象
//以后容器中获取到的就是这个组件增强后的代理对象，代理对象就会执行通知方法的流程
this.advisedBeans.put(cacheKey, Boolean.FALSE);
return bean;
}

public abstract class AbstractAutoProxyCreator extends ProxyProcessorSupport
implements SmartInstantiationAwareBeanPostProcessor, BeanFactoryAware {

//如果当前bean需要增强，创建当前对象代理对象
protected Object createProxy(Class<?> beanClass, @Nullable String beanName,
@Nullable Object[] specificInterceptors, TargetSource targetSource) {

if (this.beanFactory instanceof ConfigurableListableBeanFactory) {
AutoProxyUtils.exposeTargetClass((ConfigurableListableBeanFactory) this.beanFactory, beanName, beanClass);
}

ProxyFactory proxyFactory = new ProxyFactory();
proxyFactory.copyFrom(this);

if (!proxyFactory.isProxyTargetClass()) {
if (shouldProxyTargetClass(beanClass, beanName)) {
proxyFactory.setProxyTargetClass(true);
}
else {
evaluateProxyInterfaces(beanClass, proxyFactory);
}
}
//1.获取所有增强器（通知方法）
Advisor[] advisors = buildAdvisors(beanName, specificInterceptors);
//2.保存到proxyFactory
proxyFactory.addAdvisors(advisors);
proxyFactory.setTargetSource(targetSource);
customizeProxyFactory(proxyFactory);

proxyFactory.setFrozen(this.freezeProxy);
if (advisorsPreFiltered()) {
proxyFactory.setPreFiltered(true);
}
//3.创建代理对象
return proxyFactory.getProxy(getProxyClassLoader());
}
}

public class DefaultAopProxyFactory implements AopProxyFactory, Serializable {

//由spring决定是使用jdk动态代理还是cglib来创建代理对象
@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() || Proxy.isProxyClass(targetClass)) {
return new JdkDynamicAopProxy(config);
}
return new ObjenesisCglibAopProxy(config);
}
else {
return new JdkDynamicAopProxy(config);
}
}

/**
* Determine whether the supplied {@link AdvisedSupport} has only the
* {@link org.springframework.aop.SpringProxy} interface specified
* (or no proxy interfaces specified at all).
*/
private boolean hasNoUserSuppliedProxyInterfaces(AdvisedSupport config) {
Class<?>[] ifcs = config.getProxiedInterfaces();
return (ifcs.length == 0 || (ifcs.length == 1 && SpringProxy.class.isAssignableFrom(ifcs[0])));
}

}

至此，藉由SpringAOP创建出了目标类的代理对象，以后容器中获取到的就是这个组件增强后的代理对象，代理对象就会执行通知方法的流程。

6.各种增强器的执行顺序是如何和方法调用进行结合的？

由上一节可知，容器中保存了组件的代理对象（cglib增强后的对象），这个对象里面保存了详细信息（比如增强器，目标对象，xxx）。

目标方法的执行进入了这里。

class CglibAopProxy implements AopProxy, Serializable {

//拦截目标方法的执行
@Override
@Nullable
public Object intercept(Object proxy, Method method, Object[] args, MethodProxy methodProxy) throws Throwable {
Object oldProxy = null;
boolean setProxyContext = false;
Object target = null;
TargetSource targetSource = this.advised.getTargetSource();
try {
if (this.advised.exposeProxy) {
// Make invocation available if necessary.
oldProxy = AopContext.setCurrentProxy(proxy);
setProxyContext = true;
}
// Get as late as possible to minimize the time we "own" the target, in case it comes from a pool...
target = targetSource.getTarget();
Class<?> targetClass = (target != null ? target.getClass() : null);
//根据proxyFactory获取目标方法的拦截器链
//拦截器链（每一个通知方法又被包装为方法拦截器，利用MethodInterceptor机制）
List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);
Object retVal;
// Check whether we only have one InvokerInterceptor: that is,
// no real advice, but just reflective invocation of the target.
if (chain.isEmpty() && Modifier.isPublic(method.getModifiers())) {
// We can skip creating a MethodInvocation: just invoke the target directly.
// Note that the final invoker must be an InvokerInterceptor, so we know
// it does nothing but a reflective operation on the target, and no hot
// swapping or fancy proxying.
Object[] argsToUse = AopProxyUtils.adaptArgumentsIfNecessary(method, args);
retVal = methodProxy.invoke(target, argsToUse);
}
else {
// We need to create a method invocation...
retVal = new CglibMethodInvocation(proxy, target, method, args, targetClass, chain, methodProxy).proceed();
}
retVal = processReturnType(proxy, target, method, retVal);
return retVal;
}
finally {
if (target != null && !targetSource.isStatic()) {
targetSource.releaseTarget(target);
}
if (setProxyContext) {
// Restore old proxy.
AopContext.setCurrentProxy(oldProxy);
}
}
}

}

当前的代理对象包含一个默认的ExposeInvocationInterceptor拦截器和4个自定义的拦截器。

public class DefaultAdvisorAdapterRegistry implements AdvisorAdapterRegistry, Serializable {
@Override
public MethodInterceptor[] getInterceptors(Advisor advisor) throws UnknownAdviceTypeException {
List<MethodInterceptor> interceptors = new ArrayList<>(3);
Advice advice = advisor.getAdvice();
//如果是MethodInterceptor直接加入数组
if (advice instanceof MethodInterceptor) {
interceptors.add((MethodInterceptor) advice);
}
//如果不是用AdvisorAdapter转为MethodInterceptor
for (AdvisorAdapter adapter : this.adapters) {
if (adapter.supportsAdvice(advice)) {
interceptors.add(adapter.getInterceptor(advisor));
}
}
if (interceptors.isEmpty()) {
throw new UnknownAdviceTypeException(advisor.getAdvice());
}
//返回MethodInterceptor数组
return interceptors.toArray(new MethodInterceptor[0]);
}
}

/**
* 包裹MethodBeforeAdvice的方法拦截器
*/
@SuppressWarnings("serial")
public class MethodBeforeAdviceInterceptor implements MethodInterceptor, BeforeAdvice, Serializable {

private final MethodBeforeAdvice advice;

/**
* Create a new MethodBeforeAdviceInterceptor for the given advice.
* @param advice the MethodBeforeAdvice to wrap
*/
public MethodBeforeAdviceInterceptor(MethodBeforeAdvice advice) {
Assert.notNull(advice, "Advice must not be null");
this.advice = advice;
}

@Override
public Object invoke(MethodInvocation mi) throws Throwable {
this.advice.before(mi.getMethod(), mi.getArguments(), mi.getThis());
return mi.proceed();
}

}

/**
*包裹AfterReturningAdvice的方法拦截器
*/
@SuppressWarnings("serial")
public class AfterReturningAdviceInterceptor implements MethodInterceptor, AfterAdvice, Serializable {

private final AfterReturningAdvice advice;

/**
* Create a new AfterReturningAdviceInterceptor for the given advice.
* @param advice the AfterReturningAdvice to wrap
*/
public AfterReturningAdviceInterceptor(AfterReturningAdvice advice) {
Assert.notNull(advice, "Advice must not be null");
this.advice = advice;
}

@Override
public Object invoke(MethodInvocation mi) throws Throwable {
Object retVal = mi.proceed();
this.advice.afterReturning(retVal, mi.getMethod(), mi.getArguments(), mi.getThis());
return retVal;
}

}

/**
*包裹AspectJAfterAdvice的方法拦截器
*/
public class AspectJAfterAdvice extends AbstractAspectJAdvice
implements MethodInterceptor, AfterAdvice, Serializable {

public AspectJAfterAdvice(
Method aspectJBeforeAdviceMethod, AspectJExpressionPointcut pointcut, AspectInstanceFactory aif) {

super(aspectJBeforeAdviceMethod, pointcut, aif);
}

@Override
public Object invoke(MethodInvocation mi) throws Throwable {
try {
return mi.proceed();
}
finally {
invokeAdviceMethod(getJoinPointMatch(), null, null);
}
}

@Override
public boolean isBeforeAdvice() {
return false;
}

@Override
public boolean isAfterAdvice() {
return true;
}

}

在拦截器链中，每一个拦截器最后都会调用MethodInvocation的proceed方法，如果简单的按拦截器链进行链式调用的话，MethodInvocation的proceed方法就会执行N次（N等于拦截器个数），更直观的讲，我们调用了caculator.div(1,1)方法，按照这个逻辑，会打印出4条记录（4个自定义拦截器）

MathCaculator.div.......
MathCaculator.div.......
MathCaculator.div.......
MathCaculator.div.......
MathCaculator.div.......

这显然不是我们想要的结果。那么Spring是如何控制执行链逻辑的？

public class ReflectiveMethodInvocation implements ProxyMethodInvocation, Cloneable {

protected final Object proxy;

@Nullable
protected final Object target;

protected final Method method;

protected Object[] arguments = new Object[0];

@Nullable
private final Class<?> targetClass;

/**
* Lazily initialized map of user-specific attributes for this invocation.
*/
@Nullable
private Map<String, Object> userAttributes;

/**
* List of MethodInterceptor and InterceptorAndDynamicMethodMatcher
* that need dynamic checks.
*/
protected final List<?> interceptorsAndDynamicMethodMatchers;

/**
* Index from 0 of the current interceptor we're invoking.
* -1 until we invoke: then the current interceptor.
*/
private int currentInterceptorIndex = -1;

protected ReflectiveMethodInvocation(
Object proxy, @Nullable Object target, Method method, @Nullable Object[] arguments,
@Nullable Class<?> targetClass, List<Object> interceptorsAndDynamicMethodMatchers) {

this.proxy = proxy;
this.target = target;
this.targetClass = targetClass;
this.method = BridgeMethodResolver.findBridgedMethod(method);
this.arguments = AopProxyUtils.adaptArgumentsIfNecessary(method, arguments);
this.interceptorsAndDynamicMethodMatchers = interceptorsAndDynamicMethodMatchers;
}

@Override
public final Object getProxy() {
return this.proxy;
}

@Override
@Nullable
public final Object getThis() {
return this.target;
}

@Override
public final AccessibleObject getStaticPart() {
return this.method;
}

@Override
public final Method getMethod() {
return this.method;
}

@Override
public final Object[] getArguments() {
return this.arguments;
}

@Override
public void setArguments(Object... arguments) {
this.arguments = arguments;
}

@Override
@Nullable
public Object proceed() throws Throwable {
//	We start with an index of -1 and increment early.
//1.如果没有拦截器直接执行目标方法，或者拦截器索引和拦截器数组-1大小一样（执行到了最后一个拦截器）执行目标方法
if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1) {
return invokeJoinpoint();
}

Object interceptorOrInterceptionAdvice =
this.interceptorsAndDynamicMethodMatchers.get(++this.currentInterceptorIndex);
if (interceptorOrInterceptionAdvice instanceof InterceptorAndDynamicMethodMatcher) {
// Evaluate dynamic method matcher here: static part will already have
// been evaluated and found to match.
InterceptorAndDynamicMethodMatcher dm =
(InterceptorAndDynamicMethodMatcher) interceptorOrInterceptionAdvice;
Class<?> targetClass = (this.targetClass != null ? this.targetClass : this.method.getDeclaringClass());
//2.链式获取每一个拦截器，拦截器执行invoke方法，每一个拦截器等待上一个拦截器执行完成返回后再执行
if (dm.methodMatcher.matches(this.method, targetClass, this.arguments)) {
return dm.interceptor.invoke(this);
}
else {
// Dynamic matching failed.
// Skip this interceptor and invoke the next in the chain.
return proceed();
}
}
else {
// It's an interceptor, so we just invoke it: The pointcut will have
// been evaluated statically before this object was constructed.
return ((MethodInterceptor) interceptorOrInterceptionAdvice).invoke(this);
}
}

@Nullable
protected Object invokeJoinpoint() throws Throwable {
return AopUtils.invokeJoinpointUsingReflection(this.target, this.method, this.arguments);
}

@Override
public MethodInvocation invocableClone() {
Object[] cloneArguments = this.arguments;
if (this.arguments.length > 0) {
// Build an independent copy of the arguments array.
cloneArguments = new Object[this.arguments.length];
System.arraycopy(this.arguments, 0, cloneArguments, 0, this.arguments.length);
}
return invocableClone(cloneArguments);
}

@Override
public MethodInvocation invocableClone(Object... arguments) {
// Force initialization of the user attributes Map,
// for having a shared Map reference in the clone.
if (this.userAttributes == null) {
this.userAttributes = new HashMap<>();
}

// Create the MethodInvocation clone.
try {
ReflectiveMethodInvocation clone = (ReflectiveMethodInvocation) clone();
clone.arguments = arguments;
return clone;
}
catch (CloneNotSupportedException ex) {
throw new IllegalStateException(
"Should be able to clone object of type [" + getClass() + "]: " + ex);
}
}

@Override
public void setUserAttribute(String key, @Nullable Object value) {
if (value != null) {
if (this.userAttributes == null) {
this.userAttributes = new HashMap<>();
}
this.userAttributes.put(key, value);
}
else {
if (this.userAttributes != null) {
this.userAttributes.remove(key);
}
}
}

@Override
@Nullable
public Object getUserAttribute(String key) {
return (this.userAttributes != null ? this.userAttributes.get(key) : null);
}

public Map<String, Object> getUserAttributes() {
if (this.userAttributes == null) {
this.userAttributes = new HashMap<>();
}
return this.userAttributes;
}

@Override
public String toString() {
// Don't do toString on target, it may be proxied.
StringBuilder sb = new StringBuilder("ReflectiveMethodInvocation: ");
sb.append(this.method).append("; ");
if (this.target == null) {
sb.append("target is null");
}
else {
sb.append("target is of class [").append(this.target.getClass().getName()).append(']');
}
return sb.toString();
}

}

上述代码proceed方法简单理解就是，在拦截器链中，每一个拦截器都是将后续所有拦截器作为一个整体来调用proceed方法。画图比较直观：

7.总结

至此，SpringAOP的原理差不多介绍完毕。我认为需要掌握以下几点：

1.AnnotationAwareAspectJAutoProxyCreator的创建和注册过程；

2.AnnotationAwareAspectJAutoProxyCreator是如何拦截bean的创建的；

3.AnnotationAwareAspectJAutoProxyCreator是如何生成代理对象的；

4.代理对象的拦截器链的构造过程和执行机制。

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