這里只是列出來一些必要high level的點(diǎn)简十，可以幫助理解AOP底層是怎么工作的县遣。
1. Spring AOP的原理-動態(tài)代理
Spring AOP 的原理很簡單会涎，就是動態(tài)代理否彩，它和 AspectJ 不一樣，AspectJ 是直接修改掉你的字節(jié)碼寺枉。
代理模式很簡單抑淫，接口 + 真實(shí)實(shí)現(xiàn)類 + 代理類绷落，其中 真實(shí)實(shí)現(xiàn)類 和 代理類 都要實(shí)現(xiàn)接口姥闪，實(shí)例化的時候要使用代理類。所以砌烁，Spring AOP 需要做的是生成這么一個代理類筐喳，然后替換掉真實(shí)實(shí)現(xiàn)類來對外提供服務(wù)催式。
替換的過程怎么理解呢？在 Spring IOC 容器中非常容易實(shí)現(xiàn)避归，就是在 getBean(…) 的時候返回的實(shí)際上是代理類的實(shí)例荣月，而這個代理類我們自己沒寫代碼，它是 Spring 采用 JDK Proxy 或 CGLIB 動態(tài)生成的梳毙。
getBean(…) 方法用于查找或?qū)嵗萜髦械?bean哺窄，這也是為什么 Spring AOP 只能作用于 Spring 容器中的 bean 的原因，對于不是使用 IOC 容器管理的對象账锹，Spring AOP 是無能為力的萌业。
XML配置中，總是會聲明如下bean：
<bean class='org.springframework.aop.framework.autoproxy.BeanNameAutoProxyCreator' />
或者是其他的*AutoProxyCreator奸柬，如AbstractAutoProxyCreator等生年，其實(shí)他們都實(shí)現(xiàn)了SmartInstantiationAwareBeanPostProcessor，最終繼承自BeanPostProcessor廓奕。postProcessAfterInitialization方法正式生成自動代理類來替換具體實(shí)現(xiàn)Bean的地方抱婉。
/**
* Create a proxy with the configured interceptors if the bean is
* identified as one to proxy by the subclass.
* @see #getAdvicesAndAdvisorsForBean
*/
@Override
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;
}
/**
* Wrap the given bean if necessary, i.e. if it is eligible for being proxied.
* @param bean the raw bean instance
* @param beanName the name of the bean
* @param cacheKey the cache key for metadata access
* @return a proxy wrapping the bean, or the raw bean instance as-is
*/
protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
if (beanName != null && 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);
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;
}
IoC容器實(shí)例化Bean的時候，getBean -> initializeBean -> beanPostProcessor.postProcessAfterInitialization桌粉。
2. JDK/CGLib 動態(tài)代理
相關(guān)源碼：接上面createProxy的調(diào)用:
protected Object createProxy(
Class<?> beanClass, String beanName, Object[] specificInterceptors, TargetSource targetSource) {
ProxyFactory proxyFactory = new ProxyFactory();
// Copy our properties (proxyTargetClass etc) inherited from ProxyConfig.
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(this.proxyClassLoader);
}
到ProxyFactory中g(shù)etProxy方法：
/**
* Create a new proxy according to the settings in this factory.
* <p>Can be called repeatedly. Effect will vary if we've added
* or removed interfaces. Can add and remove interceptors.
* <p>Uses the given class loader (if necessary for proxy creation).
* @param classLoader the class loader to create the proxy with
* (or {@code null} for the low-level proxy facility's default)
* @return the proxy object
*/
public Object getProxy(ClassLoader classLoader) {
return createAopProxy().getProxy(classLoader);
}
//ProxyCreatorSupport.java
/**
* Subclasses should call this to get a new AOP proxy. They should <b>not</b>
* create an AOP proxy with {@code this} as an argument.
*/
protected final synchronized AopProxy createAopProxy() {
if (!this.active) {
activate();
}
return getAopProxyFactory().createAopProxy(this);
}
//DefaultAopProxyFactory.java
@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()) {
return new JdkDynamicAopProxy(config);
}
return new ObjenesisCglibAopProxy(config);
}
else {
return new JdkDynamicAopProxy(config);
}
}
得結(jié)論：
createAopProxy 方法有可能返回 JdkDynamicAopProxy 實(shí)例，也有可能返回 ObjenesisCglibAopProxy 實(shí)例铃肯。
- 如果被代理的目標(biāo)類實(shí)現(xiàn)了一個或多個自定義的接口，那么就會使用 JDK 動態(tài)代理窍育；
- 如果沒有實(shí)現(xiàn)任何接口，會使用 CGLIB 實(shí)現(xiàn)代理宴胧；
- 如果設(shè)置了 proxy-target-class="true"漱抓，那么都會使用 CGLIB恕齐。
JDK 動態(tài)代理基于接口，所以只有接口中的方法會被增強(qiáng)显歧；
CGLIB 基于類繼承仪或，可以增強(qiáng)類中非final和非private的方法。
3. 動態(tài)代理-方法的調(diào)用
AopProxy接口的兩個實(shí)現(xiàn)類：JdkDynamicAopProxy和CglibAopProxy士骤，分別對應(yīng)JDK和CGLIB動態(tài)代理的實(shí)現(xiàn)。
public interface AopProxy {
/**
* Create a new proxy object.
* <p>Uses the AopProxy's default class loader (if necessary for proxy creation):
* usually, the thread context class loader.
* @return the new proxy object (never {@code null})
* @see Thread#getContextClassLoader()
*/
Object getProxy();
/**
* Create a new proxy object.
* <p>Uses the given class loader (if necessary for proxy creation).
* {@code null} will simply be passed down and thus lead to the low-level
* proxy facility's default, which is usually different from the default chosen
* by the AopProxy implementation's {@link #getProxy()} method.
* @param classLoader the class loader to create the proxy with
* (or {@code null} for the low-level proxy facility's default)
* @return the new proxy object (never {@code null})
*/
Object getProxy(ClassLoader classLoader);
}
//JdkDynamicAopProxy
@Override
public Object getProxy(ClassLoader classLoader) {
if (logger.isDebugEnabled()) {
logger.debug("Creating JDK dynamic proxy: target source is " + this.advised.getTargetSource());
}
Class<?>[] proxiedInterfaces = AopProxyUtils.completeProxiedInterfaces(this.advised);
findDefinedEqualsAndHashCodeMethods(proxiedInterfaces);
return Proxy.newProxyInstance(classLoader, proxiedInterfaces, this);
}
//CglibAopProxy
@Override
public Object getProxy(ClassLoader classLoader) {
if (logger.isDebugEnabled()) {
logger.debug("Creating CGLIB proxy: target source is " + this.advised.getTargetSource());
}
try {
Class<?> rootClass = this.advised.getTargetClass();
Assert.state(rootClass != null, "Target class must be available for creating a CGLIB proxy");
Class<?> proxySuperClass = rootClass;
if (ClassUtils.isCglibProxyClass(rootClass)) {
proxySuperClass = rootClass.getSuperclass();
Class<?>[] additionalInterfaces = rootClass.getInterfaces();
for (Class<?> additionalInterface : additionalInterfaces) {
this.advised.addInterface(additionalInterface);
}
}
// Validate the class, writing log messages as necessary.
validateClassIfNecessary(proxySuperClass);
// 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 UndeclaredThrowableStrategy(UndeclaredThrowableException.class));
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);
}
catch (CodeGenerationException ex) {
throw new AopConfigException("Could not generate CGLIB subclass of class [" +
this.advised.getTargetClass() + "]: " +
"Common causes of this problem include using a final class or a non-visible class",
ex);
}
catch (IllegalArgumentException ex) {
throw new AopConfigException("Could not generate CGLIB subclass of class [" +
this.advised.getTargetClass() + "]: " +
"Common causes of this problem include using a final class or a non-visible class",
ex);
}
catch (Exception ex) {
// TargetSource.getTarget() failed
throw new AopConfigException("Unexpected AOP exception", ex);
}
}
結(jié)合Spring AOP之原理解析（一）看它們的實(shí)現(xiàn)本質(zhì)。簡單來說采呐，JDK的實(shí)現(xiàn)基于InvocationHandler，JdkDynamicAopProxy本身就是先了InvocationHandler斧吐；CGLIB則基于Enhancer仲器。
這里重點(diǎn)看下織入的切面方法是怎么被調(diào)用的。
/**
* Implementation of {@code InvocationHandler.invoke}.
* <p>Callers will see exactly the exception thrown by the target,
* unless a hook method throws an exception.
*/
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
MethodInvocation invocation;
Object oldProxy = null;
boolean setProxyContext = false;
TargetSource targetSource = this.advised.targetSource;
Class<?> targetClass = null;
Object target = null;
try {
if (!this.equalsDefined && AopUtils.isEqualsMethod(method)) {
// The target does not implement the equals(Object) method itself.
return equals(args[0]);
}
if (!this.hashCodeDefined && AopUtils.isHashCodeMethod(method)) {
// The target does not implement the hashCode() method itself.
return hashCode();
}
if (!this.advised.opaque && method.getDeclaringClass().isInterface() &&
method.getDeclaringClass().isAssignableFrom(Advised.class)) {
// Service invocations on ProxyConfig with the proxy config...
return AopUtils.invokeJoinpointUsingReflection(this.advised, method, args);
}
Object retVal;
if (this.advised.exposeProxy) {
// Make invocation available if necessary.
oldProxy = AopContext.setCurrentProxy(proxy);
setProxyContext = true;
}
// May be null. Get as late as possible to minimize the time we "own" the target,
// in case it comes from a pool.
target = targetSource.getTarget();
if (target != null) {
targetClass = target.getClass();
}
// Get the interception chain for this method.
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.
retVal = AopUtils.invokeJoinpointUsingReflection(target, method, args);
}
else {
// We need to create a method invocation...
invocation = new ReflectiveMethodInvocation(proxy, target, method, args, targetClass, chain);
// Proceed to the joinpoint through the interceptor chain.
retVal = invocation.proceed();
}
// Massage return value if necessary.
Class<?> returnType = method.getReturnType();
if (retVal != null && retVal == target && returnType.isInstance(proxy) &&
!RawTargetAccess.class.isAssignableFrom(method.getDeclaringClass())) {
// Special case: it returned "this" and the return type of the method
// is type-compatible. Note that we can't help if the target sets
// a reference to itself in another returned object.
retVal = proxy;
} else if (retVal == null && returnType != Void.TYPE && returnType.isPrimitive()) {
throw new AopInvocationException("Null return value from advice does not match primitive return type for: " + method);
}
return retVal;
}
finally {
if (target != null && !targetSource.isStatic()) {
// Must have come from TargetSource.
targetSource.releaseTarget(target);
}
if (setProxyContext) {
// Restore old proxy.
AopContext.setCurrentProxy(oldProxy);
}
}
}
重點(diǎn)在這里：
// Get the interception chain for this method.
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.
retVal = AopUtils.invokeJoinpointUsingReflection(target, method, args);
}
else {
// We need to create a method invocation...
invocation = new ReflectiveMethodInvocation(proxy, target, method, args, targetClass, chain);
// Proceed to the joinpoint through the interceptor chain.
retVal = invocation.proceed();
}
//ReflectiveMethodInvocation
protected ReflectiveMethodInvocation(
Object proxy, Object target, Method method, Object[] arguments,
Class<?> targetClass, List<Object> interceptorsAndDynamicMethodMatchers) {
this.proxy = proxy;
this.target = target;
this.targetClass = targetClass;
this.method = BridgeMethodResolver.findBridgedMethod(method);
this.arguments = arguments;
this.interceptorsAndDynamicMethodMatchers = interceptorsAndDynamicMethodMatchers;
}
@Override
public Object proceed() throws Throwable {
// We start with an index of -1 and increment early.
if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1) {
return invokeJoinpoint();
}
Object interceptorOrInterceptionAdvice =
this.interceptorsAndDynamicMethodMatchers.get(++this.currentInterceptorIndex);
if (interceptorOrInterceptionAdvice instanceof InterceptorAndDynamicMethodMatcher) {
// Evaluate dynamic method matcher here: static part will already have
// been evaluated and found to match.
InterceptorAndDynamicMethodMatcher dm =
(InterceptorAndDynamicMethodMatcher) interceptorOrInterceptionAdvice;
if (dm.methodMatcher.matches(this.method, this.targetClass, this.arguments)) {
return dm.interceptor.invoke(this);
}
else {
// Dynamic matching failed.
// Skip this interceptor and invoke the next in the chain.
return proceed();
}
}
else {
// It's an interceptor, so we just invoke it: The pointcut will have
// been evaluated statically before this object was constructed.
return ((MethodInterceptor) interceptorOrInterceptionAdvice).invoke(this);
}
}
這里就是各個Advice或MethodInterceptor執(zhí)行的地方裳涛。仔細(xì)閱讀众辨，其實(shí)這里是個遞歸調(diào)用，如果有多個增強(qiáng)的話郊闯，會分別執(zhí)行。
CGLIB代理的調(diào)用是通過callback實(shí)現(xiàn)的蛛株，最終也是落到上面ReflectiveMethodInvocation的proceed方法。重點(diǎn)的源碼片段如下：
protected Object createProxyClassAndInstance(Enhancer enhancer, Callback[] callbacks) {
enhancer.setInterceptDuringConstruction(false);
enhancer.setCallbacks(callbacks);
return (this.constructorArgs != null ?
enhancer.create(this.constructorArgTypes, this.constructorArgs) :
enhancer.create());
}
每個Callback是MethodInterceptor的實(shí)現(xiàn)欢摄，以DynamicUnadvisedExposedInterceptor為例：
/**
* Interceptor for unadvised dynamic targets when the proxy needs exposing.
*/
private static class DynamicUnadvisedExposedInterceptor implements MethodInterceptor, Serializable {
private final TargetSource targetSource;
public DynamicUnadvisedExposedInterceptor(TargetSource targetSource) {
this.targetSource = targetSource;
}
@Override
public Object intercept(Object proxy, Method method, Object[] args, MethodProxy methodProxy) throws Throwable {
Object oldProxy = null;
Object target = this.targetSource.getTarget();
try {
oldProxy = AopContext.setCurrentProxy(proxy);
Object retVal = methodProxy.invoke(target, args);
return processReturnType(proxy, target, method, retVal);
}
finally {
AopContext.setCurrentProxy(oldProxy);
this.targetSource.releaseTarget(target);
}
}
}
