spring aop底层原理及如何实现

相信每天工作都要用spring框架的大家一定使用过spring aop，aop的概念是面向切面编程，相对与传统的面向对象编程oop，aop更关注的是横向的逻辑，比如说一个大型系统中的日志记录，异常处理，性能监控等等，都是各个模块都需要的操作，那样代表着这些操作会散落在系统的各个地方，不易管理且杂乱无章，而aop就是关注的这些，aop将这些操作与业务代码分离，统一成一个个的切面，针对这些个切面进行编程处理。spring aop使得我们的aop开发工作变得简单，这次我就给大家讲讲spring aop的底层原理和实现

要分析spring aop的底层原理，首先要会使用，先创建一个普通maven webapp项目，引入spring-context依赖，版本为5.1.1RELEASE

<dependency> <groupId>org.springframework</groupId> <artifactId>spring-context</artifactId> <version>5.1.1.RELEASE</version> </dependency>

然后我使用aspectj作为aop的语法实现，和spring整合起来

<dependency> <groupId>org.aspectj</groupId> <artifactId>aspectjweaver</artifactId> <version>1.9.4</version> </dependency>

接下来我全称用注解的形式来模拟spring aop的使用，先创建一个配置类去扫描包，开启aspectJ的自动代理支持

@Configuration@EnableAspectJAutoProxy@ComponentScan('com.ww')public class Wconfig {}

然后新建一个接口和接口的实现类

public interface Dao { void query();}@Componentpublic class IndexDao implements Dao{ @Override public void query() {System.out.println('query......'); }}

创建切面

//代表是一个切面@Aspect@Componentpublic class WAspect { /** * execution表达式，可以百度写法 */ @Pointcut('execution(* com.ww.dao.*.*(..))') public void point(){ } /** * 在切点上进行前置通知 */ @Before('point()') public void beforeAd(){System.out.println('before-------------'); }}

创建测试方法

public class TestAspect { public static void main(String[] args) {AnnotationConfigApplicationContext configApplicationContext = new AnnotationConfigApplicationContext(Wconfig.class);Dao dao = configApplicationContext.getBean(Dao.class);dao.query(); }}

执行方法，可以看到在打印query...之前打印了before----------

这个时候我们很想知道为什么这句before会打印在query之前呢，稍微对spring aop有所了解的人应该知道，spring是通过动态代理和字节码技术来实现aop操作的，也就是经常说的jdk动态代理和cglib动态代理两种模式，那么，spring究竟是怎么创建的代理对象，又是什么时候产生的代理对象呢，下面我们来一起探究一下源码，来揭开这个谜底

首先我们透过现象看本质，我先把断点打在测试方法的最后一行，我们来看这个时候的dao对象

那么接下来我们就要去找到什么时候这个dao对象变成了动态代理对象的，既然在最后一行的时候对象已经变成了代理对象，那么我门自然就猜想是在上一句代码的位置spring执行了创建代理对象的操作，我们把断点移到上一行，debug进去

再往下走

这行代码我看方法名觉得应该是有用的代码，方法意思应该是spring处理好的bean，跟进去看看

@Nullableprivate <T> T resolveBean(ResolvableType requiredType, @Nullable Object[] args, boolean nonUniqueAsNull) {//这行代码最有用，处理有beanName的bean，debug进入看一下NamedBeanHolder<T> namedBean = resolveNamedBean(requiredType, args, nonUniqueAsNull);if (namedBean != null) {return namedBean.getBeanInstance();}BeanFactory parent = getParentBeanFactory();if (parent instanceof DefaultListableBeanFactory) {return ((DefaultListableBeanFactory) parent).resolveBean(requiredType, args, nonUniqueAsNull);}else if (parent != null) {ObjectProvider<T> parentProvider = parent.getBeanProvider(requiredType);if (args != null) {return parentProvider.getObject(args);}else {return (nonUniqueAsNull ? parentProvider.getIfUnique() : parentProvider.getIfAvailable());}}return null;}

@SuppressWarnings('unchecked')@Nullableprivate <T> NamedBeanHolder<T> resolveNamedBean(ResolvableType requiredType, @Nullable Object[] args, boolean nonUniqueAsNull) throws BeansException {Assert.notNull(requiredType, 'Required type must not be null');Class<?> clazz = requiredType.getRawClass();Assert.notNull(clazz, 'Required type must have a raw Class');//候选name列表，直觉告诉我这行代码比较重要String[] candidateNames = getBeanNamesForType(requiredType);if (candidateNames.length > 1) {List<String> autowireCandidates = new ArrayList<>(candidateNames.length);for (String beanName : candidateNames) {if (!containsBeanDefinition(beanName) || getBeanDefinition(beanName).isAutowireCandidate()) {autowireCandidates.add(beanName);}}if (!autowireCandidates.isEmpty()) {candidateNames = StringUtils.toStringArray(autowireCandidates);}}//因为我的代码里只有一个bean，所以我觉得应该会执行这里的代码if (candidateNames.length == 1) {String beanName = candidateNames[0];return new NamedBeanHolder<>(beanName, (T) getBean(beanName, clazz, args));}else if (candidateNames.length > 1) {Map<String, Object> candidates = new LinkedHashMap<>(candidateNames.length);for (String beanName : candidateNames) {if (containsSingleton(beanName) && args == null) {Object beanInstance = getBean(beanName);candidates.put(beanName, (beanInstance instanceof NullBean ? null : beanInstance));}else {candidates.put(beanName, getType(beanName));}}String candidateName = determinePrimaryCandidate(candidates, clazz);if (candidateName == null) {candidateName = determineHighestPriorityCandidate(candidates, clazz);}if (candidateName != null) {Object beanInstance = candidates.get(candidateName);if (beanInstance == null || beanInstance instanceof Class) {beanInstance = getBean(candidateName, clazz, args);}return new NamedBeanHolder<>(candidateName, (T) beanInstance);}if (!nonUniqueAsNull) {throw new NoUniqueBeanDefinitionException(requiredType, candidates.keySet());}}return null;}

执行完getBeanNamesForType(requiredType)后，我们看idea的变量显示，果然有一个bean，name是IndexDao

那么接下来自然会进到length==1的那个代码块，这时候我再debug进入，这里还是一个getBean方法

在spring容器中还有一些没有name的其他的bean需要被创建，所以这里我用上了条件断点，当beanName等于indexDao的时候，才会进入断点，但是当我F8跑完这行代码的时候，出乎意料的事情发生了

惊不惊喜，意不意外，getSingleton这行代码执行结束之后，代理对象就已经被创建了，所以需要debug进入这行代码去看

protected Object getSingleton(String beanName, boolean allowEarlyReference) {//spring所有的bean被放在ioc容器中的地方，就是这个singletonObjects，这是一个concorrentHashMap。Object singletonObject = this.singletonObjects.get(beanName);if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {synchronized (this.singletonObjects) {singletonObject = this.earlySingletonObjects.get(beanName);if (singletonObject == null && allowEarlyReference) {ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);if (singletonFactory != null) {singletonObject = singletonFactory.getObject();this.earlySingletonObjects.put(beanName, singletonObject);this.singletonFactories.remove(beanName);}}}}return singletonObject;}

但是我在这里只看到了get方法，那么这些bean是什么时候放到singletonObjects里的呢，我来找找

protected void addSingleton(String beanName, Object singletonObject) {synchronized (this.singletonObjects) {this.singletonObjects.put(beanName, singletonObject);this.singletonFactories.remove(beanName);this.earlySingletonObjects.remove(beanName);this.registeredSingletons.add(beanName);}}

在DefaultSingletonBeanRegistry注册器中，我找到了singletonObjects.put方法，代表bean是这个时候被放到这个map里去的，接下来我在这行代码上进行条件断点，然后我们来看它的调用链，找出是什么时候执行的addSingleton方法，其实从这个时候我已经知道，断点打在测试方法的倒数第二行是不对的，在getBean之前其实代理对象已经产生了

// Create bean instance.if (mbd.isSingleton()) {sharedInstance = getSingleton(beanName, () -> {try { //创建bean，核心代码return createBean(beanName, mbd, args);}catch (BeansException ex) {// Explicitly remove instance from singleton cache: It might have been put there// eagerly by the creation process, to allow for circular reference resolution.// Also remove any beans that received a temporary reference to the bean.destroySingleton(beanName);throw ex;}});bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);}

在createBean方法上，我也加上条件断点，然后debug进入

@Overrideprotected 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.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 { //核心代码，创建bean实例，需要断点进入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);}}

接下来我debug进入doCreateBean方法

debug跟进initializeBean方法，条件断点在两个初始化处理器上，我隐约觉得代理对象就是从这两个方法中产生的，我们拭目以待

protected Object initializeBean(final String beanName, final Object bean, @Nullable RootBeanDefinition mbd) {if (System.getSecurityManager() != null) {AccessController.doPrivileged((PrivilegedAction<Object>) () -> {invokeAwareMethods(beanName, bean);return null;}, getAccessControlContext());}else {invokeAwareMethods(beanName, bean);}//包装的beanObject wrappedBean = bean;if (mbd == null || !mbd.isSynthetic()) { //执行前置初始化beanPostProcessor处理器wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);}try {invokeInitMethods(beanName, wrappedBean, mbd);}catch (Throwable ex) {throw new BeanCreationException((mbd != null ? mbd.getResourceDescription() : null),beanName, 'Invocation of init method failed', ex);}if (mbd == null || !mbd.isSynthetic()) { //执行初始化后的beanPostProcessor处理器wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);}return wrappedBean;}

执行完applyBeanPostProcessorsBeforeInitialization方法，这个时候我们看到warppedBean还是indexDao，并没有产生代理对象

我猜想在下一个后置处理器中，代理对象将被创建，我debug进去

public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)throws BeansException {Object result = existingBean;for (BeanPostProcessor processor : getBeanPostProcessors()) { //经过处理器处理后的bean，我先看看有多少个处理器Object current = processor.postProcessAfterInitialization(result, beanName);if (current == null) {return result;}result = current;}return result;}

可以看到我的猜想被证明是正确的，运行完这个后置处理器，代理对象就被创建出来了。 到了这里我们知道了代理对象是从哪里来的了，但是还是没搞清楚代理对象是怎么创建出来的，这时候我们就需要debug进入到这个处理器内部去瞧瞧了。

@Overridepublic Object postProcessAfterInitialization(@Nullable Object bean, String beanName) {if (bean != null) { //获取缓存key,不重要Object cacheKey = getCacheKey(bean.getClass(), beanName);if (!this.earlyProxyReferences.contains(cacheKey)) {//重要方法，需要debug进去看return wrapIfNecessary(bean, beanName, cacheKey);}}return bean;}

于是乎我又进到了wrapIfNecessary这个方法内部

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.//看到上面这行注释，可以确定代理类就是从这里产生的，下面这个方法得到的都是一些包括切点信息，通知类型等等的信息Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);if (specificInterceptors != DO_NOT_PROXY) {this.advisedBeans.put(cacheKey, Boolean.TRUE); //顾名思义，创建代理，bebug进入看看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;}

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);}}Advisor[] advisors = buildAdvisors(beanName, specificInterceptors);proxyFactory.addAdvisors(advisors);proxyFactory.setTargetSource(targetSource);customizeProxyFactory(proxyFactory);proxyFactory.setFrozen(this.freezeProxy);if (advisorsPreFiltered()) {proxyFactory.setPreFiltered(true);}//重要的地方在这里，代理对象是通过这个方法生成的return proxyFactory.getProxy(getProxyClassLoader());} public Object getProxy(@Nullable ClassLoader classLoader) {//debug进去看看return createAopProxy().getProxy(classLoader);}

protected final synchronized AopProxy createAopProxy() {if (!this.active) {activate();}//同样需要debug进入看看return getAopProxyFactory().createAopProxy(this);}

我们看到这里有一个if语句，当config中的isOptimize和isProxyTargetClass还有hasNoUserSuppliedProxyInterfaces三个判断条件只要有一个满足的话，spring就会选择cglib的方式进行动态代理，而config中的两个boolean变量的默认值都是false，而我们的被代理对象又是实现接口的，所以spring会选择jdk动态代理的实现形式来完成动态代理，当然，我们也可以在这种情况下手动的配置config值来让spring选择cglib作为动态代理的实现方式，稍后我会演示

@Overridepublic 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 { //现在知道为什么我们的代理类是动态代理了吗return new JdkDynamicAopProxy(config);}}

我以spring aop实现的调用链图来结束这次的总结

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