spring aop底层原理及如何实现

spring aop底层原理及如何实现

前言

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

使用

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

org.springframework

spring-context

5.1.1.RELEASE

然后我使用aspectj作为aop的语法实现，和spring整合起来

org.aspectj

aspectjweaver

1.9.4

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();

}

@Component

public class IndexDao implements Dao{

@Override

public void query() {

System.out.println("query......");

}

}

创建切面

//代表是一个切面

@Aspect

@Component

public 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

源码分析

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

@Nullable

private 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 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 parentProvider = parent.getBeanProvider(requiredType);

if (args != null) {

return parentProvider.getObject(args);

}

else {

return (nonUniqueAsNull ? parentProvider.getIfUnique() : parentProvider.getIfAvailable());

}

}

return null;

}

@SuppressWarnings("unchecked")

@Nullable

private NamedBeanHolder 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 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(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

@Override

protected 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

invokeAwareMethods(beanName, bean);

return null;

}, getAccessControlContext());

}

else {

invokeAwareMethods(beanName, bean);

}

//包装的bean

Object 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。

@Override

public 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(getProxyClassLoadergjUQsvfzHq());

}

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

@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.");

}

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);

}

}

总结

我以spring aop实现的调用链图来结束这次的总结

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