这篇文章主要介绍spring aop底层原理及实现方法，文中介绍的非常详细，具有一定的参考价值，感兴趣的小伙伴们一定要看完！
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
<dependency> <groupId>org.springframework</groupId> <artifactId>spring-context</artifactId> <version>5.1.1.RELEASE</version> </dependency>
然后我使用aspectj作为aop的语法实现，和spring整合起来
<dependency> <groupId>org.aspectj</groupId> <artifactId>aspectjweaver</artifactId> <version>1.9.4</version> </dependency>
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
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> 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<T> 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<T> parentProvider = parent.getBeanProvider(requiredType);
if (args != null) {
return parentProvider.getObject(args);
}
else {
return (nonUniqueAsNull ? parentProvider.getIfUnique() : parentProvider.getIfAvailable());
}
}
return null;
}@SuppressWarnings("unchecked")
@Nullable
private <T> NamedBeanHolder<T> 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<String> 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<String, Object> candidates = new LinkedHashMap<>(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<Object>) () -> {
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(getProxyClassLoader());
}
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);
}
}
我以spring aop实现的调用链图来结束这次的总结
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