Spring事务源码解析
Spring的事务属于逻辑事务。不是物理事务。
Spring并不直接管理事务，而是提供了多种事务管理器，它们将事务管理的职责委托给JDBC或者JTA等持久化机制所提供的相关平台框架的事务来实现。例如JDBC的事物管理器就是DataSourceTransactionManager。
Spring事务管理器的接口是org.springframework.transaction.PlatformTransactionManager：
public interface PlatformTransactionManager {
TransactionStatus getTransaction(TransactionDefinition definition) throws TransactionException;
void commit(TransactionStatus status) throws TransactionException;
void rollback(TransactionStatus status) throws TransactionException;
}
通过这个接口，Spring为各个平台如JDBC、Hibernate等都提供了对应的事务管理器，但是具体的实现就是各个平台自己的事情了。所以Spring事务管理的一个优点就是为不同的事务API提供一致的编程模型。
Spring Boot 使用事务非常简单，需要@EnableTransactionManagement和@Transactional配合使用。首先使用注解@EnableTransactionManagement 开启事务支持后，然后在Service方法上添加注解@Transactional便可。@EnableTransactionManagement，启注解事务管理等同于xml配置方式的
首先是EnableTransactionManagement类：
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import(TransactionManagementConfigurationSelector.class)
public @interface EnableTransactionManagement {
这里会import TransactionManagementConfigurationSelector类：
public class TransactionManagementConfigurationSelector extends AdviceModeImportSelector<EnableTransactionManagement> {
@Override
protected String[] selectImports(AdviceMode adviceMode) {
switch (adviceMode) {
case PROXY:
// 默认就是 PROXY
return new String[] {AutoProxyRegistrar.class.getName(), ProxyTransactionManagementConfiguration.class.getName()};
case ASPECTJ:
return new String[] {TransactionManagementConfigUtils.TRANSACTION_ASPECT_CONFIGURATION_CLASS_NAME};
default:
return null;
}
}
}
selectImports会返回两个类：AutoProxyRegistrar 和 ProxyTransactionManagementConfiguration。
-
AutoProxyRegistrar的作用就是向BeanFactory注册InfrastructureAdvisorAutoProxyCreator.class。而InfrastructureAdvisorAutoProxyCreator继承自AbstractAdvisorAutoProxyCreator，就是让容器支持了AOP。 -
ProxyTransactionManagementConfiguration会加载关键的几个bean：
@Configuration
public class ProxyTransactionManagementConfiguration extends AbstractTransactionManagementConfiguration {
@Bean(name = TransactionManagementConfigUtils.TRANSACTION_ADVISOR_BEAN_NAME)
@Role(BeanDefinition.ROLE_INFRASTRUCTURE)
public BeanFactoryTransactionAttributeSourceAdvisor transactionAdvisor() {
BeanFactoryTransactionAttributeSourceAdvisor advisor = new BeanFactoryTransactionAttributeSourceAdvisor();
advisor.setTransactionAttributeSource(transactionAttributeSource());
advisor.setAdvice(transactionInterceptor());
advisor.setOrder(this.enableTx.<Integer>getNumber("order"));
return advisor;
}
@Bean
@Role(BeanDefinition.ROLE_INFRASTRUCTURE)
public TransactionAttributeSource transactionAttributeSource() {
return new AnnotationTransactionAttributeSource();
}
@Bean
@Role(BeanDefinition.ROLE_INFRASTRUCTURE)
public TransactionInterceptor transactionInterceptor() {
TransactionInterceptor interceptor = new TransactionInterceptor();
interceptor.setTransactionAttributeSource(transactionAttributeSource());
if (this.txManager != null) {
interceptor.setTransactionManager(this.txManager);
}
return interceptor;
}
}
- BeanFactoryTransactionAttributeSourceAdvisor：实现了 PointcutAdvisor 接口，组合了TransactionAttributeSourcePointcut。
- AnnotationTransactionAttributeSource：解析事务类，得到事务配置相关信息；
- TransactionInterceptor：事务拦截器，实现了 Advice、MethodInterceptor 接口。TransactionInterceptor是个环绕增强，在目标方法执行前开启事务，执行完目标方法后，进行事务提交或者回滚；
事务代理类的创建
了解Spring AOP应该会熟悉这段代码：
public static boolean canApply(Advisor advisor, Class<?> targetClass, boolean hasIntroductions) {
if (advisor instanceof IntroductionAdvisor) {
return ((IntroductionAdvisor) advisor).getClassFilter().matches(targetClass);
}
else if (advisor instanceof PointcutAdvisor) {
PointcutAdvisor pca = (PointcutAdvisor) advisor;
return canApply(pca.getPointcut(), targetClass, hasIntroductions);
}
else {
// It doesn't have a pointcut so we assume it applies.
return true;
}
}
在前面 BeanFactoryTransactionAttributeSourceAdvisor 类，该类实现了 PointcutAdvisor 接口，其中的切面 pointcut 便是通过 TransactionAttributeSourcePointcut 来实现的。
public class BeanFactoryTransactionAttributeSourceAdvisor extends AbstractBeanFactoryPointcutAdvisor {
private TransactionAttributeSource transactionAttributeSource;
private final TransactionAttributeSourcePointcut pointcut = new TransactionAttributeSourcePointcut() {
@Override
protected TransactionAttributeSource getTransactionAttributeSource() {
return transactionAttributeSource;
}
};
}
调用路径会到TransactionAttributeSourcePointcut的matches方法，该方法根据能否可以从目标 bean 中得到 TransactionAttribute 来判断是否匹配的。
public boolean matches(Method method, @Nullable Class<?> targetClass) {
if (targetClass != null && TransactionalProxy.class.isAssignableFrom(targetClass)) {
return false;
}
TransactionAttributeSource tas = getTransactionAttributeSource();
return (tas == null || tas.getTransactionAttribute(method, targetClass) != null);
}
上面的tas即AnnotationTransactionAttributeSource。AnnotationTransactionAttributeSource父类AbstractFallbackTransactionAttributeSource实现了getTransactionAttribute方法：
public TransactionAttribute getTransactionAttribute(Method method, @Nullable Class<?> targetClass) {
if (method.getDeclaringClass() == Object.class) {
return null;
}
// First, see if we have a cached value.
Object cacheKey = getCacheKey(method, targetClass);
Object cached = this.attributeCache.get(cacheKey);
if (cached != null) {
// Value will either be canonical value indicating there is no transaction attribute,
// or an actual transaction attribute.
if (cached == NULL_TRANSACTION_ATTRIBUTE) {
return null;
}
else {
return (TransactionAttribute) cached;
}
}
else {
// We need to work it out.
//重点，获取事务属性
TransactionAttribute txAttr = computeTransactionAttribute(method, targetClass);
// Put it in the cache.
if (txAttr == null) {
this.attributeCache.put(cacheKey, NULL_TRANSACTION_ATTRIBUTE);
}
else {
String methodIdentification = ClassUtils.getQualifiedMethodName(method, targetClass);
if (txAttr instanceof DefaultTransactionAttribute) {
((DefaultTransactionAttribute) txAttr).setDescriptor(methodIdentification);
}
if (logger.isDebugEnabled()) {
logger.debug("Adding transactional method '" + methodIdentification + "' with attribute: " + txAttr);
}
this.attributeCache.put(cacheKey, txAttr);
}
return txAttr;
}
}
再看看computeTransactionAttribute方法：
protected TransactionAttribute computeTransactionAttribute(Method method, @Nullable Class<?> targetClass) {
// Don't allow no-public methods as required.
if (allowPublicMethodsOnly() && !Modifier.isPublic(method.getModifiers())) {
return null;
}
// The method may be on an interface, but we need attributes from the target class.
// If the target class is null, the method will be unchanged.
Method specificMethod = AopUtils.getMostSpecificMethod(method, targetClass);
// First try is the method in the target class.
//先查找目标方式是否有事务属性
TransactionAttribute txAttr = findTransactionAttribute(specificMethod);
if (txAttr != null) {
return txAttr;
}
// Second try is the transaction attribute on the target class.
//再查找目标类是否有事务属性
txAttr = findTransactionAttribute(specificMethod.getDeclaringClass());
if (txAttr != null && ClassUtils.isUserLevelMethod(method)) {
return txAttr;
}
if (specificMethod != method) {
// Fallback is to look at the original method.
txAttr = findTransactionAttribute(method);
if (txAttr != null) {
return txAttr;
}
// Last fallback is the class of the original method.
txAttr = findTransactionAttribute(method.getDeclaringClass());
if (txAttr != null && ClassUtils.isUserLevelMethod(method)) {
return txAttr;
}
}
return null;
}
从上面可知，方法级别上的注解会覆盖类级别上的注解，两个findTransactionAttribute方法都在AnnotationTransactionAttributeSource实现，
protected TransactionAttribute findTransactionAttribute(Method method) {
return determineTransactionAttribute(method);
}
protected TransactionAttribute findTransactionAttribute(Class<?> clazz) {
return determineTransactionAttribute(clazz);
}
再看看determineTransactionAttribute方法：
protected TransactionAttribute determineTransactionAttribute(AnnotatedElement ae) {
for (TransactionAnnotationParser annotationParser : this.annotationParsers) {
TransactionAttribute attr = annotationParser.parseTransactionAnnotation(ae);
if (attr != null) {
return attr;
}
}
return null;
}
annotationParsers集合包含了3个类，分别是SpringTransactionAnnotationParser、JtaTransactionAnnotationParser、Ejb3TransactionAnnotationParser。分别解析不同包的注解，这里的解析类是SpringTransactionAnnotationParser，其parseTransactionAnnotation方法：
public TransactionAttribute parseTransactionAnnotation(AnnotatedElement ae) {
AnnotationAttributes attributes = AnnotatedElementUtils.findMergedAnnotationAttributes(
ae, Transactional.class, false, false);
if (attributes != null) {
return parseTransactionAnnotation(attributes);
}
else {
return null;
}
}
以上代码讲解析目标元素上@Transactional注解的相关信息，然后封装成AnnotationAttributes类，其继承LinkedHashMap。
看看parseTransactionAnnotation方法：
protected TransactionAttribute parseTransactionAnnotation(AnnotationAttributes attributes) {
RuleBasedTransactionAttribute rbta = new RuleBasedTransactionAttribute();
Propagation propagation = attributes.getEnum("propagation");
rbta.setPropagationBehavior(propagation.value());
Isolation isolation = attributes.getEnum("isolation");
rbta.setIsolationLevel(isolation.value());
rbta.setTimeout(attributes.getNumber("timeout").intValue());
rbta.setReadOnly(attributes.getBoolean("readOnly"));
rbta.setQualifier(attributes.getString("value"));
ArrayList<RollbackRuleAttribute> rollBackRules = new ArrayList<RollbackRuleAttribute>();
Class<?>[] rbf = attributes.getClassArray("rollbackFor");
for (Class<?> rbRule : rbf) {
RollbackRuleAttribute rule = new RollbackRuleAttribute(rbRule);
rollBackRules.add(rule);
}
String[] rbfc = attributes.getStringArray("rollbackForClassName");
for (String rbRule : rbfc) {
RollbackRuleAttribute rule = new RollbackRuleAttribute(rbRule);
rollBackRules.add(rule);
}
Class<?>[] nrbf = attributes.getClassArray("noRollbackFor");
for (Class<?> rbRule : nrbf) {
NoRollbackRuleAttribute rule = new NoRollbackRuleAttribute(rbRule);
rollBackRules.add(rule);
}
String[] nrbfc = attributes.getStringArray("noRollbackForClassName");
for (String rbRule : nrbfc) {
NoRollbackRuleAttribute rule = new NoRollbackRuleAttribute(rbRule);
rollBackRules.add(rule);
}
rbta.getRollbackRules().addAll(rollBackRules);
return rbta;
}
切面实现
TransactionInterceptor 实现了方法拦截器 MethodInterceptor 接口，用于对业务类进行事务增强。TransactionInterceptor 的 invoke 方法主要是调用了父类 TransactionAspectSupport 的 invokeWithinTransaction 方法。
@Override
public Object invoke(final MethodInvocation invocation) throws Throwable {
Class<?> targetClass = (invocation.getThis() != null ? AopUtils.getTargetClass(invocation.getThis()) : null);
return invokeWithinTransaction(invocation.getMethod(), targetClass, new InvocationCallback() {
@Override
public Object proceedWithInvocation() throws Throwable {
return invocation.proceed();
}
});
}
protected Object invokeWithinTransaction(Method method, Class<?> targetClass, final InvocationCallback invocation) throws Throwable {
// 获取事务配置信息
final TransactionAttribute txAttr = getTransactionAttributeSource().getTransactionAttribute(method, targetClass);
// 获取事务管理器
final PlatformTransactionManager tm = determineTransactionManager(txAttr);
final String joinpointIdentification = methodIdentification(method, targetClass, txAttr);
// 同步操作
if (txAttr == null || !(tm instanceof CallbackPreferringPlatformTransactionManager)) {
// 创建事务类
TransactionInfo txInfo = createTransactionIfNecessary(tm, txAttr, joinpointIdentification);
Object retVal = null;
try {
// 环绕增强：触发链条上的下一个拦截器，最终会调用目标类
retVal = invocation.proceedWithInvocation();
} catch (Throwable ex) {
// 出现异常则回滚
completeTransactionAfterThrowing(txInfo, ex);
throw ex;
} finally {
cleanupTransactionInfo(txInfo);
}
commitTransactionAfterReturning(txInfo);
return retVal;
} else {
// 省略异步操作，原理类似
}
}
invoke 方法里主要做以下几件事：
- 获取事务属性；
- 获取事务管理器；
- 创建事务；
- 执行目标方法；
- 遇到异常则回滚，正常结束则提交。
创建事务的是TransactionAspectSupport#createTransactionIfNecessary()方法，该方法的调用代码比较多，主要摘取比较重要的看一下：
---+TransactionAspectSupport:createTransactionIfNecessary();
`---+AbstractPlatformTransactionManager:getTransaction();
+---AbstractPlatformTransactionManager:doGetTransaction();
`---+AbstractPlatformTransactionManager:doBegin();
+---DataSource:getConnection();
`---TransactionSynchronizationManager.bindResource();
参考：
https://blog.csdn.net/wang704987562/article/details/88913808
https://blog.csdn.net/ai_xiangjuan/article/details/79687560
https://blog.csdn.net/u012562943/article/details/78333153
https://www.coder4.com/archives/5917
https://blog.csdn.net/dslztx/article/details/46636079