Spring源码-AOP是如何实现代理的【8】
实验环境：spring-framework-5.0.2、jdk8、gradle4.3.1- Spring源码-IOC容器简介【1】
- Spring源码-IOC容器初始化过程【2】
- Spring源码-Xml Bean解析注册过程【3】
- Spring源码-自定义IOC容器及Bean解析注册【4】
- Spring源码-Bean实例化过程【5】
- Spring源码-Spring是如何解决Bean循环依赖的【6】
- Spring源码-循环依赖-用实例证明去掉二级缓存会出现什么问题【7】
- Spring源码-AOP是如何实现代理的【8】
AOP代理对象也是在bean的实例化过程中创建的，如果不明白可以看一下bean实例化过程。如果类配置了@EnableAspectJAutoProxy注解 ，Spring则会在BeanPostProcessor.after这一步通过AnnotationAwareAspectJAutoProxyCreator这个BeanPostProcessor创建一个代理对象，替换掉了原有的bean对象返回了出去。
我们再来看一下bean创建流程
创建bean方法如下
doCreateBean方法
/**
* 真正创建bean的方法
*/
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
throws BeanCreationException {
// 封装被创建的bean对象
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
// TODO【重要】 这一步创建了bean实例，只是一个早期的对象，还没有填充属性值
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
final Object bean = instanceWrapper.getWrappedInstance();
// 获取实例化对象的类型
Class<?> beanType = instanceWrapper.getWrappedClass();
if (beanType != NullBean.class) {
mbd.resolvedTargetType = beanType;
}
// 调用post-processors
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
try {
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Post-processing of merged bean definition failed", ex);
}
mbd.postProcessed = true;
}
}
// 向容器中缓存单例模式的bean，防止循环引用
boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
isSingletonCurrentlyInCreation(beanName));
if (earlySingletonExposure) {
if (logger.isDebugEnabled()) {
logger.debug("Eagerly caching bean '" + beanName +
"' to allow for resolving potential circular references");
}
// TODO【重要】 向三级缓存添加bean实例 匿名内部类，防止循环引用，尽早持有对象的引用
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// 初始化bean实例，exposedObject在初始化完成之后返回依赖注入完成之后的bean
Object exposedObject = bean;
try {
// TODO 【重要】 bean属性依赖注入，并且将bean定义中配置的属性值赋值给实例对象
populateBean(beanName, mbd, instanceWrapper);
// TODO 【重要】开始初始化bean对象（含AOP）
// 调用Aware接口方法 -> 调用BeanPostProcessor.before方法 -> 调用init-method方法 -> 调用BeanPostProcessor.after方法
exposedObject = initializeBean(beanName, exposedObject, mbd);
}
catch (Throwable ex) {
if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
throw (BeanCreationException) ex;
}
else {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
}
}
if (earlySingletonExposure) {
// 获取指定名称的已注册的单例bean
Object earlySingletonReference = getSingleton(beanName, false);
if (earlySingletonReference != null) {
// 根据名称获取已注册的bean和正在实例化的bean是同一个
if (exposedObject == bean) {
// 当前实例化的bean初始完成
exposedObject = earlySingletonReference;
}
// 当前bean依赖其他bean，且当发送循环引用时，不允许创建新的实例对象
else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
// 获取当前bean所依赖的其他bean
String[] dependentBeans = getDependentBeans(beanName);
Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
for (String dependentBean : dependentBeans) {
// 对依赖bean进行类型检查
if (!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.");
}
}
}
}
// 注册完成依赖注入的bean
try {
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}
/**
* 初始化容器创建的bean实例对象，为其添加BeanPostProcessor后置处理器
* 调用Aware接口方法 -> 调用BeanPostProcessor.before方法 -> 调用init-method方法 -> 调用BeanPostProcessor.after方法
*/
protected Object initializeBean(final String beanName, final Object bean, @Nullable RootBeanDefinition mbd) {
// 通过JDK的安全机制验证权限
if (System.getSecurityManager() != null) {
// 实现PrivilegedAction接口的匿名内部类
AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
invokeAwareMethods(beanName, bean);
return null;
}, getAccessControlContext());
}
else {
// 为bean实例对象包装相关属性，如名称、类加载器、所属容器等
invokeAwareMethods(beanName, bean);
}
Object wrappedBean = bean;
// 调用BeanPostProcessor后置处理器的回调方法，在bean实例初始化前做一些处理
if (mbd == null || !mbd.isSynthetic()) {
wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
}
try {
// 调用init方法 (@PostConstruct标注的init方法)
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()) {
// 调用BeanPostProcessor后置处理器的回调方法，在bean实例初始化后做一些处理（AOP会在这一步实现代理对象）
wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
}
return wrappedBean;
}
/**
* 后置处理器：在bean初始化之后，init方法之后调用
*/
@Override
public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
throws BeansException {
Object result = existingBean;
for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
// 依次执行BeanPostProcessor的postProcessAfterInitialization实现方法
Object current = beanProcessor.postProcessAfterInitialization(result, beanName);
if (current == null) {
return result;
}
result = current;
}
return result;
}
我们看的有个applyBeanPostProcessorsAfterInitialization方法，会循环调用所有的BeanPostProcessor对当前bean做增强，如果这个bean配置了AOP代理的话，则会通AnnotationAwareAspectJAutoProxyCreator.postProcessAfterInitialization方法创建一个代理对象。我们来看一下其父类AbstractAutoProxyCreator的实现逻辑。
AbstractAutoProxyCreator#postProcessAfterInitialization方法
@Override
public Object postProcessAfterInitialization(@Nullable Object bean, String beanName) throws BeansException {
if (bean != null) {
Object cacheKey = getCacheKey(bean.getClass(), beanName);
if (!this.earlyProxyReferences.contains(cacheKey)) {
// 如果有需要则做一下包装
return wrapIfNecessary(bean, beanName, cacheKey);
}
}
return bean;
}
/**
* 如果有需要则包装一下当前bean
*/
protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
if (StringUtils.hasLength(beanName) && this.targetSourcedBeans.contains(beanName)) {
return bean;
}
// 判断是否应该代理这个bean
if (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) {
return bean;
}
/**
* 判断是否是一些InfrastructureClass或者是否应该跳过这个bean
* InfrastructureClass是指Advice、PointCut、Advisor等接口的实现类
*/
if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) {
this.advisedBeans.put(cacheKey, Boolean.FALSE);
return bean;
}
// Create proxy if we have advice.
// 获取这个bean的通知
Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
if (specificInterceptors != DO_NOT_PROXY) {
this.advisedBeans.put(cacheKey, Boolean.TRUE);
// 创建代理
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;
}
/**
* 创建代理对象
*/
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);
}
}
Advisor[] advisors = buildAdvisors(beanName, specificInterceptors);
proxyFactory.addAdvisors(advisors);
proxyFactory.setTargetSource(targetSource);
customizeProxyFactory(proxyFactory);
proxyFactory.setFrozen(this.freezeProxy);
if (advisorsPreFiltered()) {
proxyFactory.setPreFiltered(true);
}
// 这一步创建了一个代理对象
return proxyFactory.getProxy(getProxyClassLoader());
}
我们看的是通过 proxyFactory.getProxy来创建的代理对象，调用getProxy就会返回CglibAopProxy或JdkDynamicAopProxy代理对象。
ProxyFactory#getProxy方法
public Object getProxy(@Nullable ClassLoader classLoader) {
return createAopProxy().getProxy(classLoader);
}
那么是怎么决定返回的是CglibAopProxy还是JdkDynamicAopProxy代理对象呢？是通过createAopProxy方法来决定的。
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.");
}
// 如果是接口，或本身就是代理类
if (targetClass.isInterface() || Proxy.isProxyClass(targetClass)) {
return new JdkDynamicAopProxy(config);
}
return new ObjenesisCglibAopProxy(config);
}
else {
return new JdkDynamicAopProxy(config);
}
}到此，一个代理对象就创建了出来。
本文来自博客园，作者：wzyy，转载请注明原文链接：https://www.cnblogs.com/wwzyy/p/15863242.html
