Spring核心——AOP

Concepts

• Aspect: A modularization of a concern that cuts across multiple classes. Transaction management is a good example of a crosscutting concern in enterprise Java applications. In Spring AOP, aspects are implemented by using regular classes (the schema-based approach (opens new window)) or regular classes annotated with the @Aspect annotation (the @AspectJ style (opens new window)).
• Join point: A point during the execution of a program, such as the execution of a method or the handling of an exception. In Spring AOP, a join point always represents a method execution.
• Advice: Action taken by an aspect at a particular join point. Different types of advice include “around”, “before” and “after” advice. (Advice types are discussed later.) Many AOP frameworks, including Spring, model an advice as an interceptor and maintain a chain of interceptors around the join point.
• Pointcut: A predicate that matches join points. Advice is associated with a pointcut expression and runs at any join point matched by the pointcut (for example, the execution of a method with a certain name). The concept of join points as matched by pointcut expressions is central to AOP, and Spring uses the AspectJ pointcut expression language by default.
• Introduction: Declaring additional methods or fields on behalf of a type. Spring AOP lets you introduce new interfaces (and a corresponding implementation) to any advised object. For example, you could use an introduction to make a bean implement an IsModified interface, to simplify caching. (An introduction is known as an inter-type declaration in the AspectJ community.)
• Target object: An object being advised by one or more aspects. Also referred to as the “advised object”. Since Spring AOP is implemented by using runtime proxies, this object is always a proxied object.
• AOP proxy: An object created by the AOP framework in order to implement the aspect contracts (advise method executions and so on). In the Spring Framework, an AOP proxy is a JDK dynamic proxy or a CGLIB proxy.
• Weaving: linking aspects with other application types or objects to create an advised object. This can be done at compile time (using the AspectJ compiler, for example), load time, or at runtime. Spring AOP, like other pure Java AOP frameworks, performs weaving at runtime.

其中切面的概念较为抽象，理解为对多个切入点按类别进行抽取，模块化。按业务或系统需要，比如日志，事务，安全校验等。本质上就是动态代理。

直接进入源码

源码

测试代码

@Component
public class Student2 {

    public void study() {
        System.out.println("好好学习，");
    }

}
@Aspect
@Component
@EnableAspectJAutoProxy(proxyTargetClass = true)
public class Aspect1 {
    @After("execution(* com.hireek.webdemo.aspect.*.study(..))")
    public void ttxs() {
        System.out.println("天天向上。");
    }
}
/**
 * Test
 *
 * @author Hireek
 * @date 2022/1/19 19:53
 */
public class Test {
    public static void main(String[] args) {
        AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext("com.hireek.webdemo.aspect");
        Student2 student = (Student2) context.getBean("student2");
        student.study();
        System.out.println(student);
    }
}
/*
好好学习，
天天向上。
com.hireek.webdemo.aspect.Student2@29e495ff
*/

internalAutoProxyCreator

preInstantiateSingletons方法好像比之前多了个类，org.springframework.aop.config.internalAutoProxyCreator。

我们知道前五个后置处理器是通过AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry)注册的;

看下internalAutoProxyCreator链路我们可知是扫描EnableAspectJAutoProxy注解可得。

绑定 AnnotationAwareAspectJAutoProxyCreator类。后续的后置处理器的后置增强就靠这个类。

return registerOrEscalateApcAsRequired(AnnotationAwareAspectJAutoProxyCreator.class, registry, source);

@Nullable
private static BeanDefinition registerOrEscalateApcAsRequired(
  Class<?> cls, BeanDefinitionRegistry registry, @Nullable Object source) {

  Assert.notNull(registry, "BeanDefinitionRegistry must not be null");

  if (registry.containsBeanDefinition(AUTO_PROXY_CREATOR_BEAN_NAME)) {
    BeanDefinition apcDefinition = registry.getBeanDefinition(AUTO_PROXY_CREATOR_BEAN_NAME);
    if (!cls.getName().equals(apcDefinition.getBeanClassName())) {
      int currentPriority = findPriorityForClass(apcDefinition.getBeanClassName());
      int requiredPriority = findPriorityForClass(cls);
      if (currentPriority < requiredPriority) {
        apcDefinition.setBeanClassName(cls.getName());
      }
    }
    return null;
  }

  RootBeanDefinition beanDefinition = new RootBeanDefinition(cls);
  beanDefinition.setSource(source);
  beanDefinition.getPropertyValues().add("order", Ordered.HIGHEST_PRECEDENCE);
  beanDefinition.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
  registry.registerBeanDefinition(AUTO_PROXY_CREATOR_BEAN_NAME, beanDefinition);
  return beanDefinition;
}

并通过PostProcessorRegistrationDelegate.registerBeanPostProcessors(beanFactory, this)注册。

List<BeanPostProcessor> orderedPostProcessors = new ArrayList<>(orderedPostProcessorNames.size());
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);

DI的过程之前已详细解析，aop如何实现？

initializeBean

protected Object initializeBean(String beanName, Object bean, @Nullable RootBeanDefinition mbd) {
   if (System.getSecurityManager() != null) {
      AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
         invokeAwareMethods(beanName, bean);
         return null;
      }, getAccessControlContext());
   }
   else {
      invokeAwareMethods(beanName, bean);
   }

   Object wrappedBean = bean;
   if (mbd == null || !mbd.isSynthetic()) {
      // 调用所有后置处理器的postProcessBeforeInitialization方法
      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()) {
      // 初始化bean之后增强，动态代理就在该方法实现
      wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
   }

   return wrappedBean;
}

AbstractAutowireCapableBeanFactory.applyBeanPostProcessorsAfterInitialization

public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
      throws BeansException {

   Object result = existingBean;
   for (BeanPostProcessor processor : getBeanPostProcessors()) {
      Object current = processor.postProcessAfterInitialization(result, beanName);
      if (current == null) {
         return result;
      }
      result = current;
   }
   return result;
}

调用AnnotationAwareAspectJAutoProxyCreator的父类AbstractAutoProxyCreator。

public Object postProcessAfterInitialization(@Nullable Object bean, String beanName) {
   if (bean != null) {
      Object cacheKey = getCacheKey(bean.getClass(), beanName);
      if (this.earlyProxyReferences.remove(cacheKey) != bean) {
         return wrapIfNecessary(bean, beanName, cacheKey); // 创建代理类
      }
   }
   return bean;
}

使用什么代理？

public AopProxy createAopProxy(AdvisedSupport config) throws AopConfigException {
   if (!NativeDetector.inNativeImage() &&
         (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);
   }
}

cglib创建代理类。

protected Object createProxyClassAndInstance(Enhancer enhancer, Callback[] callbacks) {
   Class<?> proxyClass = enhancer.createClass();
   Object proxyInstance = null;

   if (objenesis.isWorthTrying()) {
      try {
         proxyInstance = objenesis.newInstance(proxyClass, enhancer.getUseCache()); // 生成代理类
      }
      catch (Throwable ex) {
         logger.debug("Unable to instantiate proxy using Objenesis, " +
               "falling back to regular proxy construction", ex);
      }
   }

   if (proxyInstance == null) {
      // Regular instantiation via default constructor...
      try {
         Constructor<?> ctor = (this.constructorArgs != null ?
               proxyClass.getDeclaredConstructor(this.constructorArgTypes) :
               proxyClass.getDeclaredConstructor());
         ReflectionUtils.makeAccessible(ctor);
         proxyInstance = (this.constructorArgs != null ?
               ctor.newInstance(this.constructorArgs) : ctor.newInstance());
      }
      catch (Throwable ex) {
         throw new AopConfigException("Unable to instantiate proxy using Objenesis, " +
               "and regular proxy instantiation via default constructor fails as well", ex);
      }
   }

   ((Factory) proxyInstance).setCallbacks(callbacks);
   return proxyInstance;
}

查看cglib字节码反编译可知，最后自定义的aop调用的是DynamicAdvisedInterceptor（动态）的方法。

DynamicAdvisedInterceptor.intercept

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);
      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);
      }
   }
}

cglib的原理在设计模式 (opens new window)精讲过，这里就不赘述了。

至于spring创建代理类的？应该是cpp代码，下次探究。

加油，愿你能跨越山海，披星戴月！