对Spring平台或者说生态系统来说，AOP是Spring框架的核心功能模块之一。AOP与IOC容器的结合使用, 为应用开发或者Spring自身功能的扩展都提供了许多便利。Spring AOP的实现和其他特性的实现一样，非常丰富，除了可以使用Spring本身提供的AOP实现之外，还封装了业界优秀的AOP解决方案AspectJ来让应用使用。在这里，主要对Spring自身的AOP实现原理做一些解析；在这个AOP实现中，Spring充分利用了IOC容器Proxy代理对象以及AOP拦截器的功能特性，通过这些对AOP基本功能的封装机制，为用户提供了AOP的实现框架。所以，要了解这些AOP的基本实现，需要我们对Java 的Proxy机制有一些基本了解。
AOP实现的基本线索
AOP实现中，可以看到三个主要的步骤，一个是代理对象的生成，然后是拦截器的作用，然后是Aspect编织的实现。AOP框架的丰富，很大程度体现在这三个具体实现中，所具有的丰富的技术选择，以及如何实现与IOC容器的无缝结合。毕竟这也是一个非常核心的模块，需要满足不同的应用需求带来的解决方案需求。
在Spring AOP的实现原理中，我们主要举ProxyFactoryBean(aspectj方案则是AspectJProxyFactory)的实现作为例子和实现的基本线索进行分析；很大一个原因，是因为ProxyFactoryBean是在Spring IoC环境中，创建AOP应用的最底层方法，从中，可以看到一条实现AOP的基本线索。在ProxyFactoryBean中，它的AOP实现需要依赖JDK或者CGLIB提供的Proxy特性。从FactoryBean中获取对象，是从getObject()方法作为入口完成的。然后为proxy代理对象配置advisor链，这个配置是在initializeAdvisorChain方法中完成的，这样就为生成AOP代理对象做好了准备。代码如下;
ProxyFactoryBean
public Object getObject() throws BeansException {
initializeAdvisorChain();
if (isSingleton()) {
return getSingletonInstance();
}
else {
if (this.targetName == null) {
logger.warn("Using non-singleton proxies with singleton targets is often undesirable. " +
"Enable prototype proxies by setting the 'targetName' property.");
}
return newPrototypeInstance();
}
}
ProxyCreatorSupport
protected final synchronized AopProxy createAopProxy() {
if (!this.active) {
activate();
}
return getAopProxyFactory().createAopProxy(this);
}
DefaultAopProxyFactory
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);
}
}
我们回忆一下我们做过的jdk代理的例子：
UserMgr mgr = new UserMgrImpl(); InvocationHandler h = new TransactionHandler(mgr); UserMgr u = (UserMgr) Proxy.newProxyInstance(UserMgr.class, h);
生成的代理是比如$Proxy34，h是$Proxy34的成员变量，
public class Proxy implements java.io.Serializable {
/** prefix for all proxy class names */
private final static String proxyClassNamePrefix = "$Proxy";
/**
* the invocation handler for this proxy instance.
* @serial
*/
protected InvocationHandler h;
}
在spring aop中正是JdkDynamicAopProxy。那么重点来了，我们就从JdkDynamicAopProxy的invoke方法看起：
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);
}
}
}
ReflectiveMethodInvocation implements ProxyMethodInvocation extends MethodInvocation extends Invocation extends Joinpoint
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 { // eg. ExposeInvocationInterceptor
// 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);
}
}
package org.aopalliance.intercept; // Aop联盟
import java.lang.reflect.AccessibleObject;
public interface Joinpoint {
Object proceed() throws Throwable;
Object getThis();
AccessibleObject getStaticPart();
}
interceptor.invoker - eg. MethodBeforeAdviceInterceptor
public Object invoke(MethodInvocation mi) throws Throwable {
this.advice.before(mi.getMethod(), mi.getArguments(), mi.getThis() );
return mi.proceed(); // 又回归到了上面的调用
}
值得注意的是，虽然切面可以只用到一个类的部分方法上，但我们调用其他方法时，仍然会经历上面的逻辑，此时拦截器链里只有一个interceptor - ExposeInvocationInterceptor
public Object invoke(MethodInvocation mi) throws Throwable {
MethodInvocation oldInvocation = invocation.get();
invocation.set(mi);
try {
return mi.proceed();
}
finally {
invocation.set(oldInvocation);
}
}
Aop应用参考：
一、分库分表http://wely.iteye.com/blog/2275725
二、方法性能监控
package com.itlong.bjxizhan.support.web.service.monitor;
import org.aspectj.lang.ProceedingJoinPoint;
import org.aspectj.lang.annotation.Around;
import org.aspectj.lang.annotation.Aspect;
/**
* Created by shenhongxi on 2016/8/10.
*/
@Aspect
public class MonitorAspect {
private String tagPrefix;
@Around(
value = "execution(* *(..)) && @annotation(monitor)",
argNames = "pjp,monitor"
)
public Object doUmpLogging(ProceedingJoinPoint pjp, Monitor monitor) throws Throwable {
// String tag = monitor.tag();
// boolean heart = monitor.heart();
long start = System.currentTimeMillis();
// record invocation (times)
Object obj = null;
try {
obj = pjp.proceed();
} catch (Exception e) {
// record error
throw e;
} finally {
long end = System.currentTimeMillis();
// record time -> end - start
}
return obj;
}
public String getTagPrefix() {
return tagPrefix;
}
public void setTagPrefix(String tagPrefix) {
this.tagPrefix = tagPrefix;
}
}
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD, ElementType.CONSTRUCTOR})
public @interface Monitor {
String DEFAULT_TAG_NAME = "@@USE_METHOD_NAME";
String tag() default "@@USE_METHOD_NAME";
String message() default "";
boolean heart() default false;
}
另外，性能监控拦截器可参考org.springframework.aop.interceptor.PerformanceMonitorInterceptor