Spring 事务与自定义AOP 是如何执行的？

 我们知道 事务是基于 AOP 实现的，如果我们项目中用到了事务，同时又需要做一些自定义的 AOP 操作，这个时候会不会冲突呢？

先说结论： 不会冲突，并且事务会把自定义的AOP 操作也包含在内。

为什么？这事就得从 CglibAopProxy 聊起，spring 在 bean 初始化时会给使用了事务的对象创建代理。

		public Object intercept(Object proxy, Method method, Object[] args, MethodProxy methodProxy) throws Throwable {
			Object oldProxy = null;
			boolean setProxyContext = false;
			Object target = null;
			TargetSource targetSource = this.advised.getTargetSource();
			try {
				if (this.advised.exposeProxy) {
					// Make invocation available if necessary.
					oldProxy = AopContext.setCurrentProxy(proxy);
					setProxyContext = true;
				}
				// Get as late as possible to minimize the time we "own" the target, in case it comes from a pool...
				target = targetSource.getTarget();
				Class<?> targetClass = (target != null ? target.getClass() : null);
                // 获取interceptor 链
				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.
                    // 没有 interceptChain 则直接调用目标方法
					Object[] argsToUse = AopProxyUtils.adaptArgumentsIfNecessary(method, args);
					retVal = methodProxy.invoke(target, argsToUse);
				}
				else {
                    // 有 interceptChain , 创建 methodInvocation 传到链中，继续调用（proceed）
					// 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 && !targetSource.isStatic()) {
					targetSource.releaseTarget(target);
				}
				if (setProxyContext) {
					// Restore old proxy.
					AopContext.setCurrentProxy(oldProxy);
				}
			}
		}

intercept 方法中，使用了责任链的模式，会获取 interceptorChain，如果没有其他的 interceptor，直接调用目标方法并返回，如果还有其他 interceptor，比如自定义的一些 AOP 操作，则会将调用传到链中，继续递归调用。

CglibMethodInvocation 的 proceed 方法:

public Object proceed() throws Throwable {
        // 所有拦截器调用完，或者压根没有拦截器（方法增强），直接调用joinPoint 并返回
		// We start with an index of -1 and increment early.
		if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1) {
			return invokeJoinpoint();
		}
        // 依次获取aop(interceptor 或者是 自定义的advice)（++this.currentInterceptorIndex）interceptor 并调用方法返回
		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;
			Class<?> targetClass = (this.targetClass != null ? this.targetClass : this.method.getDeclaringClass());
            // 这里的 interceptor 实际上是个 advice:AspectJAroundAdvice （自定义的环绕增强）
			if (dm.methodMatcher.matches(this.method, 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.
            // 非自定义的 interceptor 走 invoke 方法：1. ExposeInvocationInterceptor 2. TransactionInterceptor
			return ((MethodInterceptor) interceptorOrInterceptionAdvice).invoke(this);
		}
	}

按照 chain 依次调用，直到 TransactionInterceptor.invoke 方法：

 invocation::proceed  作为方法参数 InvocationCallback 传递进来，实际调用是在  invokeWithinTransaction 中：

@Nullable
	protected Object invokeWithinTransaction(Method method, @Nullable Class<?> targetClass,
			final InvocationCallback invocation) throws Throwable {

		// If the transaction attribute is null, the method is non-transactional.
		TransactionAttributeSource tas = getTransactionAttributeSource();
		final TransactionAttribute txAttr = (tas != null ? tas.getTransactionAttribute(method, targetClass) : null);
		final TransactionManager tm = determineTransactionManager(txAttr);

		PlatformTransactionManager ptm = asPlatformTransactionManager(tm);
		final String joinpointIdentification = methodIdentification(method, targetClass, txAttr);

		if (txAttr == null || !(ptm instanceof CallbackPreferringPlatformTransactionManager)) {
			// Standard transaction demarcation with getTransaction and commit/rollback calls.
			TransactionInfo txInfo = createTransactionIfNecessary(ptm, txAttr, joinpointIdentification);

			Object retVal;
			try {
                // 这里通过方法参数来调用 proceed 方法，来调用自定义的aop, 责任链模式！！！
				// This is an around advice: Invoke the next interceptor in the chain.
				// This will normally result in a target object being invoked.
				retVal = invocation.proceedWithInvocation();
			}
			catch (Throwable ex) {
				// target invocation exception
				completeTransactionAfterThrowing(txInfo, ex);
				throw ex;
			}
			finally {
				cleanupTransactionInfo(txInfo);
			}

			if (vavrPresent && VavrDelegate.isVavrTry(retVal)) {
				// Set rollback-only in case of Vavr failure matching our rollback rules...
				TransactionStatus status = txInfo.getTransactionStatus();
				if (status != null && txAttr != null) {
					retVal = VavrDelegate.evaluateTryFailure(retVal, txAttr, status);
				}
			}

			commitTransactionAfterReturning(txInfo);
			return retVal;
		}

		else {
			final ThrowableHolder throwableHolder = new ThrowableHolder();

			// It's a CallbackPreferringPlatformTransactionManager: pass a TransactionCallback in.
			try {
				Object result = ((CallbackPreferringPlatformTransactionManager) ptm).execute(txAttr, status -> {
					TransactionInfo txInfo = prepareTransactionInfo(ptm, txAttr, joinpointIdentification, status);
					try {
						Object retVal = invocation.proceedWithInvocation();
						if (vavrPresent && VavrDelegate.isVavrTry(retVal)) {
							// Set rollback-only in case of Vavr failure matching our rollback rules...
							retVal = VavrDelegate.evaluateTryFailure(retVal, txAttr, status);
						}
						return retVal;
					}
					catch (Throwable ex) {
						if (txAttr.rollbackOn(ex)) {
							// A RuntimeException: will lead to a rollback.
							if (ex instanceof RuntimeException) {
								throw (RuntimeException) ex;
							}
							else {
								throw new ThrowableHolderException(ex);
							}
						}
						else {
							// A normal return value: will lead to a commit.
							throwableHolder.throwable = ex;
							return null;
						}
					}
					finally {
						cleanupTransactionInfo(txInfo);
					}
				});

				// Check result state: It might indicate a Throwable to rethrow.
				if (throwableHolder.throwable != null) {
					throw throwableHolder.throwable;
				}
				return result;
			}
			catch (ThrowableHolderException ex) {
				throw ex.getCause();
			}
			catch (TransactionSystemException ex2) {
				if (throwableHolder.throwable != null) {
					logger.error("Application exception overridden by commit exception", throwableHolder.throwable);
					ex2.initApplicationException(throwableHolder.throwable);
				}
				throw ex2;
			}
			catch (Throwable ex2) {
				if (throwableHolder.throwable != null) {
					logger.error("Application exception overridden by commit exception", throwableHolder.throwable);
				}
				throw ex2;
			}
		}
	}

这个时候的 proceed 方法就会走自定义的动态方法拦截器（不是intercept 方法，而是自定义的某个方法，这也许就是被称作“动态”的原因）：

最后回到 invokeWithinTransaction 方法中，继续事务的回滚或提交。

题外话，顺便带大家看下自定义的 AOP 走的源码：AspectJAroundAdvice.java

	public Object invoke(MethodInvocation mi) throws Throwable {
		if (!(mi instanceof ProxyMethodInvocation)) {
			throw new IllegalStateException("MethodInvocation is not a Spring ProxyMethodInvocation: " + mi);
		}
		ProxyMethodInvocation pmi = (ProxyMethodInvocation) mi;
		ProceedingJoinPoint pjp = lazyGetProceedingJoinPoint(pmi);
		JoinPointMatch jpm = getJoinPointMatch(pmi);
		return invokeAdviceMethod(pjp, jpm, null, null);
	}

debug 能看到 aop 里的具体组件，参考《代码详解 AOP 概念》

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