深入理解Spring系列之十二：@Transactional是如何工作的

转载 https://mp.weixin.qq.com/s/ZwhkUQF1Nun9pNrFI-3a6w

首先从tx:annotation-driven/说起。配置了tx:annotation-driven/，就必定有对应的标签解析器类，查看NamespaceHandler接口的实现类，可以看到一个TxNamespaceHandler，它注册了AnnotationDrivenBeanDefinitionParser对annotation-driven元素进行解析。

public class TxNamespaceHandler extends NamespaceHandlerSupport {
    static final String TRANSACTION_MANAGER_ATTRIBUTE = "transaction-manager";
    static final String DEFAULT_TRANSACTION_MANAGER_BEAN_NAME = "transactionManager";

    public TxNamespaceHandler() {
    }

    static String getTransactionManagerName(Element element) {
        return element.hasAttribute("transaction-manager") ? element.getAttribute("transaction-manager") : "transactionManager";
    }

    public void init() {
        this.registerBeanDefinitionParser("advice", new TxAdviceBeanDefinitionParser());
        
        // <tx:annotation-driven/> 标签解析器
        this.registerBeanDefinitionParser("annotation-driven", new AnnotationDrivenBeanDefinitionParser());
        this.registerBeanDefinitionParser("jta-transaction-manager", new JtaTransactionManagerBeanDefinitionParser());
    }
}

进入AnnotationDrivenBeanDefinitionParser类，重点看parse方法。

public BeanDefinition parse(Element element, ParserContext parserContext) {
        this.registerTransactionalEventListenerFactory(parserContext);
        String mode = element.getAttribute("mode");
        if ("aspectj".equals(mode)) {
            this.registerTransactionAspect(element, parserContext);
        } else {
            AnnotationDrivenBeanDefinitionParser.AopAutoProxyConfigurer.configureAutoProxyCreator(element, parserContext);
        }

        return null;
    }

从代码中可以看出，如果<tx:annotation-driven/>中没有配置mode参数，则默认使用代理模式进行后续处理；如果配置了mode=aspectj，则使用aspectj代码织入模式进行后续处理。
本篇分析使用代理模式的代码，进入AopAutoProxyConfigurer.configureAutoProxyCreator方法。

public static void configureAutoProxyCreator(Element element, ParserContext parserContext) {
            
            // 重点代码
            AopNamespaceUtils.registerAutoProxyCreatorIfNecessary(parserContext, element);
            
            String txAdvisorBeanName = "org.springframework.transaction.config.internalTransactionAdvisor";
            if (!parserContext.getRegistry().containsBeanDefinition(txAdvisorBeanName)) {
                Object eleSource = parserContext.extractSource(element);
                RootBeanDefinition sourceDef = new RootBeanDefinition("org.springframework.transaction.annotation.AnnotationTransactionAttributeSource");
                sourceDef.setSource(eleSource);
                sourceDef.setRole(2);
                String sourceName = parserContext.getReaderContext().registerWithGeneratedName(sourceDef);
                RootBeanDefinition interceptorDef = new RootBeanDefinition(TransactionInterceptor.class);
                interceptorDef.setSource(eleSource);
                interceptorDef.setRole(2);
                AnnotationDrivenBeanDefinitionParser.registerTransactionManager(element, interceptorDef);
                interceptorDef.getPropertyValues().add("transactionAttributeSource", new RuntimeBeanReference(sourceName));
                String interceptorName = parserContext.getReaderContext().registerWithGeneratedName(interceptorDef);
                RootBeanDefinition advisorDef = new RootBeanDefinition(BeanFactoryTransactionAttributeSourceAdvisor.class);
                advisorDef.setSource(eleSource);
                advisorDef.setRole(2);
                advisorDef.getPropertyValues().add("transactionAttributeSource", new RuntimeBeanReference(sourceName));
                advisorDef.getPropertyValues().add("adviceBeanName", interceptorName);
                if (element.hasAttribute("order")) {
                    advisorDef.getPropertyValues().add("order", element.getAttribute("order"));
                }

                parserContext.getRegistry().registerBeanDefinition(txAdvisorBeanName, advisorDef);
                CompositeComponentDefinition compositeDef = new CompositeComponentDefinition(element.getTagName(), eleSource);
                compositeDef.addNestedComponent(new BeanComponentDefinition(sourceDef, sourceName));
                compositeDef.addNestedComponent(new BeanComponentDefinition(interceptorDef, interceptorName));
                compositeDef.addNestedComponent(new BeanComponentDefinition(advisorDef, txAdvisorBeanName));
                parserContext.registerComponent(compositeDef);
            }

        }

上面的代码中标出了一行核心代码，容易被忽略。进入 AopNamespaceUtils.registerAutoProxyCreatorIfNecessary 方法。

public static void registerAutoProxyCreatorIfNecessary(
			ParserContext parserContext, Element sourceElement) {

        // 重点代码
		BeanDefinition beanDefinition = AopConfigUtils.registerAutoProxyCreatorIfNecessary(
				parserContext.getRegistry(), parserContext.extractSource(sourceElement));
				
		useClassProxyingIfNecessary(parserContext.getRegistry(), sourceElement);
		registerComponentIfNecessary(beanDefinition, parserContext);
	}

重点关注上面标出的代码，进入AopConfigUtils.registerAutoProxyCreatorIfNecessary方法。

	public static BeanDefinition registerAutoProxyCreatorIfNecessary(BeanDefinitionRegistry registry, Object source) {
		return registerOrEscalateApcAsRequired(InfrastructureAdvisorAutoProxyCreator.class, registry, source);
	}

上面中的代码向Spring容器中注册了一个InfrastructureAdvisorAutoProxyCreator类。可能会疑问为什么要注册这个类，有什么作用？
查看InfrastructureAdvisorAutoProxyCreator类继承关系。

	@Override
	public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
		if (bean != null) {
			Object cacheKey = getCacheKey(bean.getClass(), beanName);
			if (!this.earlyProxyReferences.contains(cacheKey)) {
			   
			    // 重点代码: 是创建代理对象的核心方法
				return wrapIfNecessary(bean, beanName, cacheKey);
			}
		}
		return bean;
	}

其中 wrapIfNecessary 方法 是创建代理对象的核心方法。

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.
		//重点代码：获取事务属性切面　
		Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
		
		if (specificInterceptors != DO_NOT_PROXY) {
			this.advisedBeans.put(cacheKey, Boolean.TRUE);
			
			// 重点代码: 创建一个AOP代理
			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;
	}

getAdvicesAndAdvisorsForBean方法会遍历容器中所有的切面，查找与当前实例化bean匹配的切面，这里就是获取事务属性切面，查找@Transactional注解及其属性值，具体实现比较复杂，这里暂不深入分析，最终会得到 BeanFactoryTransactionAttributeSourceAdvisor 实例，然后根据得到的切面进入createProxy方法，创建一个AOP代理。

protected Object createProxy(
			Class<?> beanClass, String beanName, 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);
		}
        
        //重点代码
		return proxyFactory.getProxy(getProxyClassLoader());
	}

进入ProxyFactory.getProxy方法。

public Object getProxy(ClassLoader classLoader) {
        // 重点代码：进入 createAopProxy() 方法
		return createAopProxy().getProxy(classLoader);
	}

createAopProxy方法决定使用JDK还是Cglib创建代理。createAopProxy方法代码如下：

	protected final synchronized AopProxy createAopProxy() {
		if (!this.active) {
			activate();
		}
		// 重点代码： 进入 createAopProxy 方法
		return getAopProxyFactory().createAopProxy(this);
	}

    @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 {
			return new JdkDynamicAopProxy(config);
		}
	}

可以看出默认是使用JDK动态代理创建代理，如果目标类是接口，则使用JDK动态代理，否则使用Cglib。这里分析使用JDK动态代理的方式，进入JdkDynamicAopProxy.getProxy方法。

    @Override
	public Object getProxy() {
		return getProxy(ClassUtils.getDefaultClassLoader());
	}

	@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, true);
		findDefinedEqualsAndHashCodeMethods(proxiedInterfaces);
		
		//重点代码
		return Proxy.newProxyInstance(classLoader, proxiedInterfaces, this);
	}

可以看到很熟悉的创建代理的代码Proxy.newProxyInstance。这里要注意的是，newProxyInstance方法的最后一个参数是JdkDynamicAopProxy类本身，也就是说在对目标类进行调用的时候，会进入JdkDynamicAopProxy的invoke方法。这里只关注JdkDynamicAopProxy的invoke方法的重点代码。

@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]);
			}
			else if (!this.hashCodeDefined && AopUtils.isHashCodeMethod(method)) {
				// The target does not implement the hashCode() method itself.
				return hashCode();
			}
			else if (method.getDeclaringClass() == DecoratingProxy.class) {
				// There is only getDecoratedClass() declared -> dispatch to proxy config.
				return AopProxyUtils.ultimateTargetClass(this.advised);
			}
			else 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.
			// 重点代码
			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.
				Object[] argsToUse = AopProxyUtils.adaptArgumentsIfNecessary(method, args);
				
				// 重点代码
				retVal = AopUtils.invokeJoinpointUsingReflection(target, method, argsToUse);
			}
			else {
				// We need to create a method invocation...
			    // 重点代码
				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 != Object.class && 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);
			}
		}
	}