Spring技术内幕：Spring AOP的实现原理（三）

生成SingleTon代理对象在getSingleTonInstance方法中完毕，这种方法时ProxyFactoryBean生成AopProxy对象的入口。代理对象会封装对target目标对象的调用。也就是说针对target对象的方法调用行为会被这里生成的代理对象所拦截。

详细的生成过程是首先读取ProxyFactoryBean配置，为生成代理对象做好准备。Spring通过AopProxy类来详细生成代理对象。对于getSingleTonInstance方法中生成代理对象的步骤例如以下：

/**
* Return the singleton instance of this class's proxy object,
* lazily creating it if it hasn't been created already.
* @return the shared singleton proxy
*/
private synchronized Object getSingletonInstance() {
if (this.singletonInstance == null) {
this.targetSource = freshTargetSource();
if (this.autodetectInterfaces && getProxiedInterfaces().length == 0 && !isProxyTargetClass()) {
// Rely on AOP infrastructure to tell us what interfaces to proxy.
// 依据AOP框架来推断须要代理的接口
Class targetClass = getTargetClass();
if (targetClass == null) {
throw new FactoryBeanNotInitializedException("Cannot determine target class for proxy");
}
// 这里设置代理对象的接口
setInterfaces(ClassUtils.getAllInterfacesForClass(targetClass, this.proxyClassLoader));
}
// Initialize the shared singleton instance.
super.setFrozen(this.freezeProxy);
// 这里会使用ProxyFactoryBean来生成须要的proxy对象
this.singletonInstance = getProxy(createAopProxy());
}
return this.singletonInstance;
}

/**
* Return the proxy object to expose.
* <p>The default implementation uses a {@code getProxy} call with
* the factory's bean class loader. Can be overridden to specify a
* custom class loader.
* @param aopProxy the prepared AopProxy instance to get the proxy from
* @return the proxy object to expose
* @see AopProxy#getProxy(ClassLoader)
*/
protected Object getProxy(AopProxy aopProxy) {
return aopProxy.getProxy(this.proxyClassLoader);
}

这里出现了AopProxy对象类型，Spring利用AOPProxy接口类把AOP代理对象的实现与框架其它部分有效隔离开来。

AopProxy接口有两个子类实现，一个Cglib2AopProxy，还有一个是JdkDynamicProxy。

详细代理对象的生成是在ProxyFactoryBean的基类AdvisedSupport中实现，借助AopProxyFactory完毕，这个对象要么从JDK中生成，要么借助CGLIB获得。

以下看看ProxyCreatorSupport中是怎样生成代理对象的。

/**
* 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();
}
// 通过AopProxyFactory取得AopProxy，这个AopProxyFactory是在初始化函数中定义的，使用的是DefaultAopProxyFactory
return getAopProxyFactory().createAopProxy(this);
}

AopProxy代理对象的生成有两种方式。假设目标对象是接口类使用JDK来生成，否则Spring会使用CGLIB来生成目标的代理对象。

以下看看在DefaultAopProxyFactory是怎样生成AopProxy目标代理对象的：

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 CglibProxyFactory.createCglibProxy(config);
}
else {
return new JdkDynamicAopProxy(config);
}
}

在AopProxy代理对象的生成过程中。首先要从AdviseSupport对象中取得配置的目标对象。AOP完毕的是切面应用对目标应用对象的增强。假设这里没有配置目标对象会直接抛出异常。一般而言，默认方式是使用JDK来产生AopProxy代理对象，但假设配置的目标对象不是接口类的实现，会使用CGLIB来产生AopProxy代理对象；在使用CGLIB来产生AopProxy代理对象时，由于CGLIB是第三方类库，本身不在JDK基类库中，全部须要在classPath中正确配置，以便可以载入和利用。在Spring中。使用JDK和CGLIB来生成AopProxy代理对象的工作，是由JdkDynamicAopProxy和CglibProxyFactory来完毕。

4、JDK生成AopProxy对象（接口实现类）

通过上面我们已经知道生成AopProxy对象有两种方式，以下看下类图：



我们先看下JdkDynamicAopProxy是怎样生成AopProxy对象的：

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);
// 调用JDK生成Proxy
return Proxy.newProxyInstance(classLoader, proxiedInterfaces, this);
}

5、CGLIB生成AopProxy对象（非接口实现类）

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...
// 来自advised的IOC配置。比方使用AOP的DynamicAdvisedInterceptor
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 MemorySafeUndeclaredThrowableStrategy(UndeclaredThrowableException.class));
enhancer.setInterceptDuringConstruction(false);
// 通过设置DynamicAdvisedInterceptor拦截器来完毕AOP功能，getCallBacks方法例如以下：
// Callback aopInterceptor = new DynamicAdvisedInterceptor(this.advised)
Callback[] callbacks = getCallbacks(rootClass);
Class<?>[] types = new Class<?>[callbacks.length];
for (int x = 0; x < types.length; x++) {
types[x] = callbacks[x].getClass();
}
enhancer.setCallbackFilter(new ProxyCallbackFilter(
this.advised.getConfigurationOnlyCopy(), this.fixedInterceptorMap, this.fixedInterceptorOffset));
enhancer.setCallbackTypes(types);
enhancer.setCallbacks(callbacks);
// Generate the proxy class and create a proxy instance.
Object proxy;
if (this.constructorArgs != null) {
proxy = enhancer.create(this.constructorArgTypes, this.constructorArgs);
}
else {
proxy = enhancer.create();
}
return proxy;
}
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);
}
}

四、Spring AOP拦截器调用的实现

1、设计原理

在Spring AOP通过JDK的Proxy方式或CGLIB方式生成代理对象的时候，相关的拦截器已经配置到代理对象中去了，拦截器在代理对象中起作用是通过对这些方法的回调来完毕的。

假设使用JDK的Proxy来生成代理对象，那么须要InvocationHandler来设置拦截器回调，而假设使用CGLIB来生成代理对象。通过DynamicAdvisedInterceptor来完毕回调。

2、JdkDynamicAopProxy的invoke拦截

在JDKDynamicAopProxy生成代理对象时，他的AopProxy代理对象生成调用：

Proxy.newProxyInstance(classLoader, proxiedInterfaces, this);

this指的是InvocationHandler对象，InvocationHandler是JDK定义反射类的一个接口。这个接口定义了invoke方法，此方法为回调方法。

通过invoke的详细实现。来完毕对目标对象方法调用的拦截器或者功能增强工作。

在这种方法中。包括一个完整的拦截器链对目标对象的拦截过程。比方获取拦截器链中的拦截器进行配置。逐个执行拦截器链里的拦截器增强，知道最后的目标对象方法的执行。以下看下invoke的源代码

/**
* Implementation of {@code InvocationHandler.invoke}.
* <p>Callers will see exactly the exception thrown by the target,
* unless a hook method throws an exception.
*/
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.
// 获得定义好的拦截器
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.
// 假设没有拦截器直接调用target的相应方法
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.
// 假设有拦截器设定，那么须要调用拦截器之后才调用目标对象相应的方法
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);
}
}
}

3、CglibAopProxy的intercept拦截器

使用CglibAopProxy生成AopProxy对象时候。对于AOP拦截器调用。回调的是DynamicAdvisedInterceptor对象生成的。

回调的方法时intercept，以下看看回调方法的源代码：

public Object intercept(Object proxy, Method method, Object[] args, MethodProxy methodProxy) throws Throwable {
Object oldProxy = null;
boolean setProxyContext = false;
Class<?> targetClass = null;
Object target = null;
try {
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 = getTarget();
if (target != null) {
targetClass = target.getClass();
}
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.
retVal = methodProxy.invoke(target, args);
}
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) {
releaseTarget(target);
}
if (setProxyContext) {
// Restore old proxy.
AopContext.setCurrentProxy(oldProxy);
}
}
}