注:这里不阐述Spring和AOP的一些基本概念和用法,直接进入正题。
流程
Spring所管理的对象大体会经过确定实例化对象类型、推断构造方法创建对象(实例化)、设置属性、初始化等等步骤。在对象初始化阶段,Spring为开发者提供了一个BeanPostProcessor接口,它会在对象初始化之前和初始化之后被调用(初始化,不是实例化,对应实例化的是InstantiationAwareBeanPostProcessor接口)。
public interface BeanPostProcessor {
//初始化之前
Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException;
//初始化之后
Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException;
}
在对象初始化之后会调用postProcessAfterInitialization方法,该方法返回一个Object。如果成功返回了一个对象,那么容器中相应beanName对应的实例就将会是这个对象。
本文主要分析动态代理,我们着重看AnnotationAwareAspectJAutoProxyCreator。先来看一下它的继承关系:
AnnotationAwareAspectJAutoProxyCreator最终实现了BeanPostProcessor接口(也实现了InstantiationAwareBeanPostProcessor接口),可以看到继承关系比较复杂。当前我们关注的postProcessAfterInitialization方法实现在它的父类AbstractAutoProxyCreator中(只保留了部分代码):
public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
if (bean != null) {
Object cacheKey = getCacheKey(bean.getClass(), beanName);
if (!this.earlyProxyReferences.contains(cacheKey)) {
return wrapIfNecessary(bean, beanName, cacheKey);
}
}
return bean;
}
这里主要看看wrapIfNecessary方法(只保留了部分代码):
Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
......
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;
}
......
}
其中核心的是两个方法调用,分别是getAdvicesAndAdvisorsForBean和createProxy。getAdvicesAndAdvisorsForBean会返回一个对象数组,包含aop相关的一些对象,如果是一个普通的不需要代理的对象会返回一个空Object数组,也就是DO_NOT_PROXY;createProxy方法则是创建代理类。
先看看getAdvicesAndAdvisorsForBean方法:
protected abstract Object[] getAdvicesAndAdvisorsForBean(Class<?> beanClass, String beanName, TargetSource customTargetSource) throws BeansException;
getAdvicesAndAdvisorsForBean方法在当前类(AbstractAutoProxyCreator)中是一个抽象方法,由子类AbstractAdvisorAutoProxyCreator实现:
public abstract class AbstractAdvisorAutoProxyCreator extends AbstractAutoProxyCreator {
@Override
protected Object[] getAdvicesAndAdvisorsForBean(Class<?> beanClass, String beanName, TargetSource targetSource) {
List<Advisor> advisors = findEligibleAdvisors(beanClass, beanName);
if (advisors.isEmpty()) {
return DO_NOT_PROXY;
}
return advisors.toArray();
}
}
代码很清晰,我们进入findEligibleAdvisors方法,看方法名也知道它会完成寻找Advisor的工作:
protected List<Advisor> findEligibleAdvisors(Class<?> beanClass, String beanName) {
//寻找Advisor
List<Advisor> candidateAdvisors = findCandidateAdvisors();
//针对指定的bean,过滤可用的Advisor,比如根据注解匹配
List<Advisor> eligibleAdvisors = findAdvisorsThatCanApply(candidateAdvisors, beanClass, beanName);
extendAdvisors(eligibleAdvisors);
if (!eligibleAdvisors.isEmpty()) {
eligibleAdvisors = sortAdvisors(eligibleAdvisors);
}
return eligibleAdvisors;
}
首先进入findCandidateAdvisors方法:
protected List<Advisor> findCandidateAdvisors() {
// Add all the Spring advisors found according to superclass rules.
List<Advisor> advisors = super.findCandidateAdvisors();
// Build Advisors for all AspectJ aspects in the bean factory.
advisors.addAll(this.aspectJAdvisorsBuilder.buildAspectJAdvisors());
return advisors;
}
我们这里主要看看aspectj的逻辑,所以看看aspectJAdvisorsBuilder.buildAspectJAdvisors方法(只保留了主要代码):
public List<Advisor> buildAspectJAdvisors() {
List<String> aspectNames = null;
......
synchronized (this) {
aspectNames = this.aspectBeanNames;
if (aspectNames == null) {
//获取所有管理的beanName
String[] beanNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(this.beanFactory, Object.class, true, false);
//遍历每个beanName
for (String beanName : beanNames) {
//从beanFactory获取Class
Class<?> beanType = this.beanFactory.getType(beanName);
//检查对应的Class是否实现Aspect注解
if (this.advisorFactory.isAspect(beanType)) {
//说明这个beanName对应的类是一个切面
aspectNames.add(beanName);
AspectMetadata amd = new AspectMetadata(beanType, beanName);
if (amd.getAjType().getPerClause().getKind() == PerClauseKind.SINGLETON) {
MetadataAwareAspectInstanceFactory factory =
new BeanFactoryAspectInstanceFactory(this.beanFactory, beanName);
//获取Advisor,主要是解析对象中关于AOP的注解,比如Pointcut
List<Advisor> classAdvisors = this.advisorFactory.getAdvisors(factory);
if (this.beanFactory.isSingleton(beanName)) {
//就放入缓存,后面就不用重新解析了
this.advisorsCache.put(beanName, classAdvisors);
}
advisors.addAll(classAdvisors);
}
}
}
this.aspectBeanNames = aspectNames;
return advisors;
}
}
......
}
会从beanFactory中寻找所有管理的beanName,返回一个String数组,然后遍历数组,从beanFactory中根据beanName获取对应的Class,然后再看对应的Class是否有Aspect注解,如果有对应的注解,那么就表示这个对象是一个切面。接下来就需要进行解析,生成真正的Advisor对象,最后放入缓存。
可以看看isAspect方法是如何判断的:
@Override
public boolean isAspect(Class<?> clazz) {
return (hasAspectAnnotation(clazz) && !compiledByAjc(clazz));
}
private boolean hasAspectAnnotation(Class<?> clazz) {
return (AnnotationUtils.findAnnotation(clazz, Aspect.class) != null);
}
逻辑很清晰,主要就是看有没有Aspect注解。 但是这里要注意,这个buildAspectJAdvisors方法通常不是在这里调用的(”这里“的意思是postProcessAfterInitialization的流程)。回到AnnotationAwareAspectJAutoProxyCreator继承关系图中,它也实现了InstantiationAwareBeanPostProcessor接口,同样在其父类AbstractAutoProxyCreator中实现了postProcessBeforeInstantiation方法,这个方法会在对象实例化(不是初始化)之前调用,在该方法的逻辑里通常会首先触发buildAspectJAdvisors方法的执行,执行之后会把结果缓存起来。
好了,再回到findEligibleAdvisors方法,上面代码已经贴了,这里就不贴了。获取到Advisor列表之后,要从中找到能用于指定类的Advisor列表,然后返回。接下来就要为指定的对象创建代理对象了,也就是AbstractAutoProxyCreator类的createProxy方法:
protected Object createProxy(
Class<?> beanClass, String beanName, Object[] specificInterceptors, TargetSource targetSource) {
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);
for (Advisor advisor : advisors) {
proxyFactory.addAdvisor(advisor);
}
proxyFactory.setTargetSource(targetSource);
customizeProxyFactory(proxyFactory);
proxyFactory.setFrozen(this.freezeProxy);
if (advisorsPreFiltered()) {
proxyFactory.setPreFiltered(true);
}
return proxyFactory.getProxy(getProxyClassLoader());
}
代理对象是由ProxyFactory代理工厂创建的,我们先看看这个工厂是如何创建代理对象的,也就是proxyFactory.getProxy方法:
public Object getProxy(ClassLoader classLoader) {
return createAopProxy().getProxy(classLoader);
}
createAopProxy方法会返回一个AopProxy,该方法定义在ProxyFactory的父类ProxyCreatorSupport中:
public class ProxyCreatorSupport extends AdvisedSupport {
private AopProxyFactory aopProxyFactory;
public ProxyCreatorSupport() {
//设置默认的代理工厂DefaultAopProxyFactory
this.aopProxyFactory = new DefaultAopProxyFactory();
}
public AopProxyFactory getAopProxyFactory() {
//获取代理工厂,默认就是DefaultAopProxyFactory
return this.aopProxyFactory;
}
protected final synchronized AopProxy createAopProxy() {
//先获取代理工厂,然后调用工厂的createAopProxy方法创建AopProxy
return getAopProxyFactory().createAopProxy(this);
}
}
上面贴出了关键代码,getAopProxyFactory默认返回的是一个DefaultAopProxyFactory工厂类,来看看DefaultAopProxyFactory的createAopProxy方法:
public AopProxy createAopProxy(AdvisedSupport config) throws AopConfigException {
if (config.isOptimize() || config.isProxyTargetClass() || hasNoUserSuppliedProxyInterfaces(config)) {
Class<?> targetClass = config.getTargetClass();
if (targetClass.isInterface()) {
return new JdkDynamicAopProxy(config);
}
return new ObjenesisCglibAopProxy(config);
}
else {
return new JdkDynamicAopProxy(config);
}
}
代码中有一些代理配置的判断,这里不用关心。可以看到它提供了两个AopProxy,分别是基于JDK的JdkDynamicAopProxy和基于cglib的ObjenesisCglibAopProxy。由于JDK提供的动态代理实现最终生成的代理类默认会继承Proxy类,实现被代理类实现的接口,因为Java是单继承,所以只能通过接口实现,也就限制了要使用JDK提供的动态代理,必须要基于接口。而使用cglib基于字节码的改造则没有这个限制,所以Spring提供了这两种方式,根据被代理类的实际情况来选择。
关于每个AopProxy是如何创建代理类的,这里就先不跟了~
总结
总的来说,动态代理是实现AOP的重要手段,Spring提供的动态代理主要依靠其提供的BeanPostProcessor,也称之为后置处理器。除了BeanPostProcessor之外,还有InstantiationAwareBeanPostProcessor(也继承了BeanPostProcessor),它们会在bean的生命周期的特定阶段被调用,以开放给开发者处理和调整对象的入口或者手段。动态代理依托后置处理器,在后置处理器的逻辑中使用AopProxy创建了被代理对象的代理类,然后代替原有类存入Spring的bean工厂中,之后根据beanName获取的实例对象就不再是原对象实例,而是代理类。而AopProxy是由AopProxyFactory接口生成,目前该接口只有DefaultAopProxyFactory实现类,其提供了两种AopProxy,分别基于原生JDK提供的动态代理和cgib,根据实际情况选择。
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