springAop的简单介绍

AOP是处理一些横切性问题，AOP的编程思想就是把这些问题和主业务逻辑分开，达到与主业务逻辑解耦的目的。使代码的重用性和开发效率更高。比如在日志记录、权限校验等场景，使用aop实现对代码无入侵效率更高。

• spring aop中的概念

• spring aop与AspectJ

springAop、AspectJ都是Aop的实现，SpringAop有自己的语法，但是语法复杂，所以SpringAop借助了AspectJ的注解，但是底层实现还是自己的。在springAop中启用AspectJ支持需要添加注释@EnableAspectJAutoProxy。

Aop的源码解析

首先声明一个切面：

@Aspect
@Component
public class TestAspect {

	@Pointcut("execution(* test.aop.*.*(..))")
	public void pointCut(){}

	@Before("pointCut()")
	public void advice(){
		System.out.println("aop before---");
	}
}

编写一个测试类

@ComponentScan("test")
@Configuration
@EnableAspectJAutoProxy
public class AppConfig {

}

@Component
public class TestService implements I{
	@Override
	public void query() {
		System.out.println("query-demo");
	}
}

public class TestDemo {
	public static void main(String[] args) {
		AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext(AppConfig.class);
		I i = (I) applicationContext.getBean("testService");
		System.out.println(i instanceof TestService);
		i.query();
	}
}

在spring的初始化过程中会将AppConfig的ComponentScan标注的的包下的bean都加载在上下文容器中，bean初始化bean的时候会调用AbstractBeanFactory的getBean方法，如果bean存在容器中就返回不存在执行创建逻辑。

AbstractBeanFactory.getBean()方法调用了doGetBean()方法

protected <T> T doGetBean(
		String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
		throws BeansException {

	// 如果是别名则转换为规范的bean名称
	String beanName = transformedBeanName(name);
	Object beanInstance;

	// 从容器中获取bean实例，在创建bean的过程中获取到的sharedInstance肯定是null
	Object sharedInstance = getSingleton(beanName);
	if (sharedInstance != null && args == null) {
		
		......
		
	}

	else {
	
		......
		// 这部分的内容获取到的都是null直接跳过

		StartupStep beanCreation = this.applicationStartup.start("spring.beans.instantiate")
				.tag("beanName", name);
		try {
			if (requiredType != null) {
				beanCreation.tag("beanType", requiredType::toString);
			}
			// 如果目标类有父类，会去获取父类的BeanDefinition和目标类的BeanDefinition整合到一起
			RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
			checkMergedBeanDefinition(mbd, beanName, args);

			// Guarantee initialization of beans that the current bean depends on.
			String[] dependsOn = mbd.getDependsOn();
			if (dependsOn != null) {
				for (String dep : dependsOn) {
					if (isDependent(beanName, dep)) {
						throw new BeanCreationException(mbd.getResourceDescription(), beanName,
								"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
					}
					registerDependentBean(dep, beanName);
					try {
						getBean(dep);
					}
					catch (NoSuchBeanDefinitionException ex) {
						throw new BeanCreationException(mbd.getResourceDescription(), beanName,
								"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
					}
				}
			}

			// Create bean instance.
			if (mbd.isSingleton()) {
				// 重点在这里，目标对象为单例，前面的步骤大部分判断条件都不会进去，目标对象的bean在这里创建
				sharedInstance = getSingleton(beanName, () -> {
					try {
						// *创建bean*
						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;
					}
				});
				beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
			}

			......

	return adaptBeanInstance(name, beanInstance, requiredType);
}

createBean方法又调用到AbstractAutowriteCapableBeanFactory中的createBean方法

在createBean的方法中会执行doCreateBean方法

AbstractAutowireCapableBeanFactory

protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
			throws BeanCreationException {

	......
	// 执行一些初始化前操作

	try {
		Object beanInstance = doCreateBean(beanName, mbdToUse, args);
		if (logger.isTraceEnabled()) {
			logger.trace("Finished creating instance of bean '" + beanName + "'");
		}
		return beanInstance;
	}
	......
}

而doCreateBean方法中，首先创建beanWrapper实例化bean，然后在初始化bean的时候执行BeanPostProcessors的postProcessAfterInitialization的回调创建代理类

AbstractAutowireCapableBeanFactory

protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
			throws BeanCreationException {

	// Instantiate the bean.
	BeanWrapper instanceWrapper = null;
	if (mbd.isSingleton()) {
		instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
	}
	if (instanceWrapper == null) {
		// 创建Bean,当前的bean还不是代理类
		instanceWrapper = createBeanInstance(beanName, mbd, args);
	}
	
	Object bean = instanceWrapper.getWrappedInstance();
	......
			
	Object exposedObject = bean;
	try {
		// 实现依赖注入
		populateBean(beanName, mbd, instanceWrapper);
		// 在这里有一个Aop自动生成的后置处理，生成代理类
		exposedObject = initializeBean(beanName, exposedObject, mbd);
	}
	......
	return exposedObject;
}

在方法initializeBean(beanName, exposedObject, mbd)中有一个applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName)的方法，调用初始化后的后置处理

AbstractAutowireCapableBeanFactory

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()) {
		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()) {
		// 在这里有一个AnnotationAwareAspectJAutoProxyCreator自动生成的后置处理，生成代理类
		wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
	}

	return wrappedBean;
}

@Override
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;
}

在testService的后置处理进行时，会有一个特殊的后置处理，通过这个后置处理生成代理类

 AnnotationAwareAspectJAutoProxyCreator的postProcessAfterInitialization()方法又调用了AbstractAutoProxyCreator的postProcessAfterInitialization()方法，在wrapIfNecessary()中执行createProxy()，调用proxyFactory.getProxy创建代理对象

AbstractAutoProxyCreator

@Override
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);
		}
	}
	return bean;
}

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.
	Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
	if (specificInterceptors != DO_NOT_PROXY) {
		this.advisedBeans.put(cacheKey, Boolean.TRUE);
		// 创建代理对象
		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()) {
		// Explicit handling of JDK proxy targets (for introduction advice scenarios)
		if (Proxy.isProxyClass(beanClass)) {
			// Must allow for introductions; can't just set interfaces to the proxy's interfaces only.
			for (Class<?> ifc : beanClass.getInterfaces()) {
				proxyFactory.addInterface(ifc);
			}
		}
	}
	else {
		// No proxyTargetClass flag enforced, let's apply our default checks...
		if (shouldProxyTargetClass(beanClass, beanName)) {
			proxyFactory.setProxyTargetClass(true);
		}
		else {
			evaluateProxyInterfaces(beanClass, proxyFactory);
		}
	}
	// 获取切面中的advice即所有通知
	Advisor[] advisors = buildAdvisors(beanName, specificInterceptors);
	proxyFactory.addAdvisors(advisors);
	proxyFactory.setTargetSource(targetSource);
	customizeProxyFactory(proxyFactory);

	proxyFactory.setFrozen(this.freezeProxy);
	if (advisorsPreFiltered()) {
		proxyFactory.setPreFiltered(true);
	}

	// Use original ClassLoader if bean class not locally loaded in overriding class loader
	ClassLoader classLoader = getProxyClassLoader();
	if (classLoader instanceof SmartClassLoader && classLoader != beanClass.getClassLoader()) {
		classLoader = ((SmartClassLoader) classLoader).getOriginalClassLoader();
	}
	// 创建代理对象
	return proxyFactory.getProxy(classLoader);
}

代理类工厂ProxyFactory

ProxyFactory

public Object getProxy(@Nullable ClassLoader classLoader) {
	return createAopProxy().getProxy(classLoader);
}

其中最重要的是createAopProxy()返回代理类AopProxy执行相应的getProxy方法创建代理类

在DefaultAopProxyFactory中可以看到是使用JDK动态代理还是CGLIB动态代理

public class DefaultAopProxyFactory implements AopProxyFactory, Serializable {

	@Override
	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);
		}
	}
	...
}

可以看出在默认情况下如果目标类实现了接口则使用jdk动态代理，没有实现接口使用cglib动态代理

在JdkDynamicAopProxy和CglibAopProxy中都有相应的getProxy()方法创建代理类

final class JdkDynamicAopProxy implements AopProxy, InvocationHandler, Serializable {

@Override
public Object getProxy(@Nullable ClassLoader classLoader) {
	if (logger.isTraceEnabled()) {
		logger.trace("Creating JDK dynamic proxy: " + this.advised.getTargetSource());
	}
	return Proxy.newProxyInstance(classLoader, this.proxiedInterfaces, this);
}

...


}

使用jdk生成代理的方法Proxy.newProxyInstance()

CglibAopProxy

@Override
public Object getProxy(@Nullable ClassLoader classLoader) {
	......
	// Configure CGLIB Enhancer...
	Enhancer enhancer = createEnhancer();
	if (classLoader != null) {
		enhancer.setClassLoader(classLoader);
		if (classLoader instanceof SmartClassLoader &&
				((SmartClassLoader) classLoader).isClassReloadable(proxySuperClass)) {
			enhancer.setUseCache(false);
		}
	}
	enhancer.setSuperclass(proxySuperClass);
	enhancer.setInterfaces(AopProxyUtils.completeProxiedInterfaces(this.advised));
	enhancer.setNamingPolicy(SpringNamingPolicy.INSTANCE);
	enhancer.setStrategy(new ClassLoaderAwareGeneratorStrategy(classLoader));

	Callback[] callbacks = getCallbacks(rootClass);
	Class<?>[] types = new Class<?>[callbacks.length];
	for (int x = 0; x < types.length; x++) {
		types[x] = callbacks[x].getClass();
	}
	// fixedInterceptorMap only populated at this point, after getCallbacks call above
	enhancer.setCallbackFilter(new ProxyCallbackFilter(
			this.advised.getConfigurationOnlyCopy(), this.fixedInterceptorMap, this.fixedInterceptorOffset));
	enhancer.setCallbackTypes(types);

	// Generate the proxy class and create a proxy instance.
	return createProxyClassAndInstance(enhancer, callbacks);
}

使用字节码增强工具Enhancer创建一个代理类

代理类是如何执行的

JdkDynamicAopProxy实现了JDK动态代理执行所需要的接口InvocationHandler，生成代理类时

Proxy.newProxyInstance(classLoader, this.proxiedInterfaces, this)

InvocationHandler的实现类也指向了JdkDynamicAopProxy，因此，代理类调用的时候会先执行JdkDynamicAopProxy中的invoke()方法。在invoke方法中，会获取满足条件的所有切面的advice，然后组成一个调用链chain，然后invocation.proceed()按照责任链的模式依次执行一个切点的前置、后置等方法

final class JdkDynamicAopProxy implements AopProxy, InvocationHandler, Serializable {
....
@Override
@Nullable
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
	Object oldProxy = null;
	boolean setProxyContext = false;

	TargetSource targetSource = this.advised.targetSource;
	Object target = null;

	try {
		......
		target = targetSource.getTarget();
		Class<?> targetClass = (target != null ? target.getClass() : null);

		// Get the interception chain for this method.
		// 获取满足条件的所有切面的advice，然后组成一个调用链chain，然后依次执行一个切点的前置、后置等方法
		List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);

		// Check whether we have any advice. If we don't, we can fallback on direct
		// reflective invocation of the target, and avoid creating a MethodInvocation.
		if (chain.isEmpty()) {
			// 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 = AopUtils.invokeJoinpointUsingReflection(target, method, argsToUse);
		}
		else {
			// We need to create a method invocation...
			MethodInvocation invocation =
					new ReflectiveMethodInvocation(proxy, target, method, args, targetClass, chain);
			// Proceed to the joinpoint through the interceptor chain.
			retVal = invocation.proceed();
		}

		....
		return retVal;
	}
	finally {
		if (target != null && !targetSource.isStatic()) {
			// Must have come from TargetSource.
			targetSource.releaseTarget(target);
		}
		if (setProxyContext) {
			// Restore old proxy.
			AopContext.setCurrentProxy(oldProxy);
		}
	}
}

....
}

同理使用CGLIB动态代理，是在创建代理类的时候enhancer设置了callback，最后执行callback的MethodInterceptor的intercept方法