@Transactional
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";
static String getTransactionManagerName(Element element) {
return (element.hasAttribute(TRANSACTION_MANAGER_ATTRIBUTE) ?
element.getAttribute(TRANSACTION_MANAGER_ATTRIBUTE) : DEFAULT_TRANSACTION_MANAGER_BEAN_NAME);
}
@Override
public void init() {
registerBeanDefinitionParser("advice", new TxAdviceBeanDefinitionParser());
registerBeanDefinitionParser("annotation-driven", new AnnotationDrivenBeanDefinitionParser());
registerBeanDefinitionParser("jta-transaction-manager", new JtaTransactionManagerBeanDefinitionParser());
}
}
进入AnnotationDrivenBeanDefinitionParser类，重点看parse方法。
@Override
@Nullable
public BeanDefinition parse(Element element, ParserContext parserContext) {
registerTransactionalEventListenerFactory(parserContext);
String mode = element.getAttribute("mode");
if ("aspectj".equals(mode)) {
// mode="aspectj"
registerTransactionAspect(element, parserContext);
if (ClassUtils.isPresent("javax.transaction.Transactional", getClass().getClassLoader())) {
registerJtaTransactionAspect(element, parserContext);
}
}
else {
// mode="proxy"
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。
private static class AopAutoProxyConfigurer {
public static void configureAutoProxyCreator(Element element, ParserContext parserContext) {
AopNamespaceUtils.registerAutoProxyCreatorIfNecessary(parserContext, element);
String txAdvisorBeanName = TransactionManagementConfigUtils.TRANSACTION_ADVISOR_BEAN_NAME;
if (!parserContext.getRegistry().containsBeanDefinition(txAdvisorBeanName)) {
Object eleSource = parserContext.extractSource(element);
// Create the TransactionAttributeSource definition.
RootBeanDefinition sourceDef = new RootBeanDefinition(
"org.springframework.transaction.annotation.AnnotationTransactionAttributeSource");
sourceDef.setSource(eleSource);
sourceDef.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
String sourceName = parserContext.getReaderContext().registerWithGeneratedName(sourceDef);
// Create the TransactionInterceptor definition.
RootBeanDefinition interceptorDef = new RootBeanDefinition(TransactionInterceptor.class);
interceptorDef.setSource(eleSource);
interceptorDef.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
registerTransactionManager(element, interceptorDef);
interceptorDef.getPropertyValues().add("transactionAttributeSource", new RuntimeBeanReference(sourceName));
String interceptorName = parserContext.getReaderContext().registerWithGeneratedName(interceptorDef);
// Create the TransactionAttributeSourceAdvisor definition.
RootBeanDefinition advisorDef = new RootBeanDefinition(BeanFactoryTransactionAttributeSourceAdvisor.class);
advisorDef.setSource(eleSource);
advisorDef.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
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);
}
}
}
进入AopNamespaceUtils.registerAutoProxyCreatorIfNecessary方法。这是流程中的核心方法。
public static void registerAutoProxyCreatorIfNecessary(
ParserContext parserContext, Element sourceElement) {
BeanDefinition beanDefinition = AopConfigUtils.registerAutoProxyCreatorIfNecessary(
parserContext.getRegistry(), parserContext.extractSource(sourceElement));
useClassProxyingIfNecessary(parserContext.getRegistry(), sourceElement);
registerComponentIfNecessary(beanDefinition, parserContext);
}
@Nullable
public static BeanDefinition registerAutoProxyCreatorIfNecessary(
BeanDefinitionRegistry registry, @Nullable Object source) {
return registerOrEscalateApcAsRequired(InfrastructureAdvisorAutoProxyCreator.class, registry, source);
}
上图中的代码向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。
通过上图中的关系，可以发现InfrastructureAdvisorAutoProxyCreator间接实现了BeanPostProcessor接口，从AbstractAutoProxyCreator类中继承了postProcessAfterInitialization方法。Spring容器在初始化每个单例bean的时候，会遍历容器中的所有BeanPostProcessor实现类，并执行其postProcessAfterInitialization方法。
进入AbstractAutoProxyCreator类的postProcessAfterInitialization方法。
@Override
public Object postProcessAfterInitialization(@Nullable Object bean, String beanName) {
if (bean != null) {
Object cacheKey = getCacheKey(bean.getClass(), beanName);
if (this.earlyProxyReferences.remove(cacheKey) != bean) {
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 (StringUtils.hasLength(beanName) && 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;
}
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, @Nullable String beanName,
@Nullable 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);
}
return proxyFactory.getProxy(getProxyClassLoader());
}
进入ProxyFactory.getProxy方法。
public Object getProxy(@Nullable ClassLoader classLoader) {
return createAopProxy().getProxy(classLoader);
}
createAopProxy方法决定使用JDK还是Cglib创建代理。
@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(@Nullable ClassLoader classLoader) {
if (logger.isTraceEnabled()) {
logger.trace("Creating JDK dynamic proxy: " + this.advised.getTargetSource());
}
Class<?>[] proxiedInterfaces = AopProxyUtils.completeProxiedInterfaces(this.advised, true);
findDefinedEqualsAndHashCodeMethods(proxiedInterfaces);
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。
// 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.
Object[] argsToUse = AopProxyUtils.adaptArgumentsIfNecessary(method, args);
retVal = AopUtils.invokeJoinpointUsingReflection(target, method, argsToUse);
}
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 != 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;
this.advised.getInterceptorsAndDynamicInterceptionAdvice获取的是当前目标方法对应的拦截器，里面是根据之前获取到的切面来获取相对应拦截器，这时候会得到TransactionInterceptor实例。如果获取不到拦截器，则不会创建MethodInvocation，直接调用目标方法。这里使用TransactionInterceptor创建一个ReflectiveMethodInvocation实例，调用的时候进入ReflectiveMethodInvocation的proceed方法。
@Override
@Nullable
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;
Class<?> targetClass = (this.targetClass != null ? this.targetClass : this.method.getDeclaringClass());
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.
return ((MethodInterceptor) interceptorOrInterceptionAdvice).invoke(this);
}
}
代码中的interceptorOrInterceptionAdvice就是TransactionInterceptor的实例，执行invoke方法进入TransactionInterceptor的invoke方法。
@Override
@Nullable
public Object invoke(MethodInvocation invocation) throws Throwable {
// Work out the target class: may be {@code null}.
// The TransactionAttributeSource should be passed the target class
// as well as the method, which may be from an interface.
Class<?> targetClass = (invocation.getThis() != null ? AopUtils.getTargetClass(invocation.getThis()) : null);
// Adapt to TransactionAspectSupport's invokeWithinTransaction...
return invokeWithinTransaction(invocation.getMethod(), targetClass, invocation::proceed);
}
TransactionInterceptor从父类TransactionAspectSupport中继承了invokeWithinTransaction方法。
@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 PlatformTransactionManager tm = determineTransactionManager(txAttr);
final String joinpointIdentification = methodIdentification(method, targetClass, txAttr);
if (txAttr == null || !(tm instanceof CallbackPreferringPlatformTransactionManager)) {
// Standard transaction demarcation with getTransaction and commit/rollback calls.
TransactionInfo txInfo = createTransactionIfNecessary(tm, txAttr, joinpointIdentification);
Object retVal;
try {
// 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);
}
commitTransactionAfterReturning(txInfo);
return retVal;
}
else {
final ThrowableHolder throwableHolder = new ThrowableHolder();
// It's a CallbackPreferringPlatformTransactionManager: pass a TransactionCallback in.
try {
Object result = ((CallbackPreferringPlatformTransactionManager) tm).execute(txAttr, status -> {
TransactionInfo txInfo = prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);
try {
return invocation.proceedWithInvocation();
}
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;
}
}
}
可以看到，在需要进行事务操作的时候，Spring会在调用目标类的目标方法之前进行开启事务、调用异常回滚事务、调用完成会提交事务。是否需要开启新事务，是根据@Transactional注解上配置的参数值来判断的。如果需要开启新事务，获取Connection连接，然后将连接的自动提交事务改为false，改为手动提交。当对目标类的目标方法进行调用的时候，若发生异常将会进入completeTransactionAfterThrowing方法。
总结
从上面的分析可以看到，Spring使用AOP实现事务的统一管理，为开发者提供了很大的便利。但是，有部分开发人员会误用这个便利，基本都是下面这两种情况：
-
A类的a1方法没有标注@Transactional，a2方法标注@Transactional，在a1里面调用a2；
-
将@Transactional注解标注在非public方法上。
第一种为什么是错误用法，原因很简单，a1方法是目标类A的原生方法，调用a1的时候即直接进入目标类A进行调用，在目标类A里面只有a2的原生方法，在a1里调用a2，即直接执行a2的原生方法，并不通过创建代理对象进行调用，所以并不会进入TransactionInterceptor的invoke方法，不会开启事务。
@Transactional的工作机制是基于AOP实现的，而AOP是使用动态代理实现的，动态代理要么是JDK方式、要么是Cglib方式。如果是JDK动态代理的方式，根据上面的分析可以知道，目标类的目标方法是在接口中定义的，也就是必须是public修饰的方法才可以被代理。如果是Cglib方式，代理类是目标类的子类，理论上可以代理public和protected方法，但是Spring在进行事务增强是否能够应用到当前目标类判断的时候，遍历的是目标类的public方法，所以Cglib方式也只对public方法有效。
