Spring源码剖析-Transactional 事务执行流程
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前言
上一篇《Transactional源码解析》我们介绍了Spring对Transactional的解析，也就是事务的初始化工作，这一篇我们接着来分析事务的执行流程。
事务拦截器：TransactionInterceptor
TransactionInterceptor是事务拦截器，该类实现了TransactionAspectSupport ， TransactionAspectSupport 中持有 TransactionManager ,拥有处理事务的能力。同时该类还实现了 MethodInterceptor 接口 ，它也作为AOP的拦截器。拦截器链中每个拦截器都有一个invoke方法，该方法就是对某个方法进行事务增强的入口，因此主要看invoke方法的实现逻辑！ 源码如下：
public class TransactionInterceptor extends TransactionAspectSupport implements MethodInterceptor, Serializable {
public TransactionInterceptor() {
}
//PlatformTransactionManager：事务管理器
public TransactionInterceptor(PlatformTransactionManager ptm, Properties attributes) {
setTransactionManager(ptm);
setTransactionAttributes(attributes);
}
//根据事务管理器PlatformTransactionManager，和事务注解源构造一个TransactionInterceptor
public TransactionInterceptor(PlatformTransactionManager ptm, TransactionAttributeSource tas) {
setTransactionManager(ptm);
setTransactionAttributeSource(tas);
}
//当程序执行事务方法的时候会走invoke ，MethodInvocation：方法调用的描述，在方法调用时提供给拦截器
@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.
//获取目标类：也就是被代理的那个原生类
Class<?> targetClass = (invocation.getThis() != null ? AopUtils.getTargetClass(invocation.getThis()) : null);
// Adapt to TransactionAspectSupport's invokeWithinTransaction...
//调用方法，有事务支持
return invokeWithinTransaction(invocation.getMethod(), targetClass, invocation::proceed);
}
}
该类首先会通过 MethodInvocation(方法调用描述) 得到目标类 ，然后以事务的方式执行方法 invokeWithinTransaction。该方法是其父类的方法
public abstract class TransactionAspectSupport implements BeanFactoryAware, InitializingBean {
/**
* General delegate for around-advice-based subclasses, delegating to several other template
* methods on this class. Able to handle {@link CallbackPreferringPlatformTransactionManager}
* as well as regular {@link PlatformTransactionManager} implementations.
* @param method the Method being invoked
* @param targetClass the target class that we're invoking the method on
* @param invocation the callback to use for proceeding with the target invocation
* @return the return value of the method, if any
* @throws Throwable propagated from the target invocation
*/
@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.
//获取事务属性源：即 @Transactional注解的属性
TransactionAttributeSource tas = getTransactionAttributeSource();
//获取事务属性
final TransactionAttribute txAttr = (tas != null ? tas.getTransactionAttribute(method, targetClass) : null);
//事务管理器
final PlatformTransactionManager tm = determineTransactionManager(txAttr);
//方法名，类名+方法名：如 cn.xx.UserServiceImpl.save
final String joinpointIdentification = methodIdentification(method, targetClass, txAttr);
//声明式事务处理 @Transactional
if (txAttr == null || !(tm instanceof CallbackPreferringPlatformTransactionManager)) {
// 【标记1】Standard transaction demarcation with getTransaction and commit/rollback calls.
//创建TransactionInfo，事务详情对象，其中包括事务管理器（transactionManager），
//事务属性（transactionAttribute），方法名（joinpointIdentification），事务状态(transactionStatus)
TransactionInfo txInfo = createTransactionIfNecessary(tm, txAttr, joinpointIdentification);
Object retVal = null;
try {
// This is an around advice: Invoke the next interceptor in the chain.
// This will normally result in a target object being invoked.
//【标记2】执行方法，这是一个环绕通知，通常会导致目标对象被调用
retVal = invocation.proceedWithInvocation();
}
catch (Throwable ex) {
// target invocation exception
//回滚事务：AfterThrowing
completeTransactionAfterThrowing(txInfo, ex);
throw ex;
}
finally {
//清理事务
cleanupTransactionInfo(txInfo);
}
//AfterReturning：后置通知，提交事务
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;
}
}
}
上面方法是事务处理的宏观流程，支持编程时和声明式的事务处理，这里是使用了模板模式，细节交给子类去实现，这里我们总结一下上面方法的流程
- 获取事务属性源TransactionAttributeSource 在上一章节有说道该类
- 加载TransactionManager 事务管理器
- 对声明式或者编程式的事务处理
- 创建 TransactionInfo 事务信息对象，其中包括事务管理器（transactionManager），事务属性（transactionAttribute），方法唯一标识（joinpointIdentification），事务状态(transactionStatus)
- 执行目标方法：invocation.proceedWithInvocation
- 出现异常，则回滚事务：completeTransactionAfterThrowing，默认是对RuntimeException异常回滚
- 清理事务信息：cleanupTransactionInfo
- 提交事务：commitTransactionAfterReturning
下面来分析具体的每个步骤流程
创建事务信息：createTransactionIfNecessary
createTransactionIfNecessary 是事务流程中的第一个方法，目的是根据给定的 TransactionAttribute 创建一个事务，其中包括事务实例的创建，事务传播行为处理，开启事务等。
protected TransactionInfo createTransactionIfNecessary(@Nullable PlatformTransactionManager tm,
@Nullable TransactionAttribute txAttr, final String joinpointIdentification) {
//如果未指定名称，则应用方法标识作为事务名称
// If no name specified, apply method identification as transaction name.
if (txAttr != null && txAttr.getName() == null) {
txAttr = new DelegatingTransactionAttribute(txAttr) {
@Override
public String getName() {
return joinpointIdentification;
}
};
}
TransactionStatus status = null;
if (txAttr != null) {
if (tm != null) {
//根据TransactionAttribute 事务属性创建一个事务状态对象
status = tm.getTransaction(txAttr);
}
else {
if (logger.isDebugEnabled()) {
logger.debug("Skipping transactional joinpoint [" + joinpointIdentification +
"] because no transaction manager has been configured");
}
}
}
return prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);
}
获取事务状态：tm.getTransaction
见：AbstractPlatformTransactionManager#getTransaction
@Override
public final TransactionStatus getTransaction(@Nullable TransactionDefinition definition) throws TransactionException {
//开启一个事务，创建了一个DataSourceTransactionObject对象，其中绑定了ConnectionHolder
Object transaction = doGetTransaction();
// Cache debug flag to avoid repeated checks.
boolean debugEnabled = logger.isDebugEnabled();
if (definition == null) {
// Use defaults if no transaction definition given.
definition = new DefaultTransactionDefinition();
}
//判断是否已经存在事务，会进行事务传播机制的判断
//判断连接不为空且连接(connectionHolder)中的 transactionActive 不为空
if (isExistingTransaction(transaction)) {
//如果存在事务，走这里
// Existing transaction found -> check propagation behavior to find out how to behave.
return handleExistingTransaction(definition, transaction, debugEnabled);
}
//事务超时时间判断
// Check definition settings for new transaction.
if (definition.getTimeout() < TransactionDefinition.TIMEOUT_DEFAULT) {
throw new InvalidTimeoutException("Invalid transaction timeout", definition.getTimeout());
}
//【PROPAGATION_MANDATORY处理】 如果当前事务不存在，PROPAGATION_MANDATORY要求必须已有事务，则抛出异常
// No existing transaction found -> check propagation behavior to find out how to proceed.
if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_MANDATORY) {
throw new IllegalTransactionStateException(
"No existing transaction found for transaction marked with propagation 'mandatory'");
}
//【如果没有事务，对于下面三种事务传播行为都需要新开事务】
else if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRED ||
definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW ||
definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {
//这三种事务传播机制都需要新开事务，先挂起事务
SuspendedResourcesHolder suspendedResources = suspend(null);
if (debugEnabled) {
logger.debug("Creating new transaction with name [" + definition.getName() + "]: " + definition);
}
try {
boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
//创建一个新的TransactionStatus
DefaultTransactionStatus status = newTransactionStatus(
definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);
//开始事务，创建一个DataSourceTransactionObject
//设置 ConnectionHolder，设置隔离级别、设置timeout， 切换事务手动提交
//如果是新连接，绑定到当前线程
doBegin(transaction, definition);
//针对当期线程的新事务同步设置
prepareSynchronization(status, definition);
return status;
}
catch (RuntimeException | Error ex) {
resume(null, suspendedResources);
throw ex;
}
}
else {
// Create "empty" transaction: no actual transaction, but potentially synchronization.
if (definition.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT && logger.isWarnEnabled()) {
logger.warn("Custom isolation level specified but no actual transaction initiated; " +
"isolation level will effectively be ignored: " + definition);
}
boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
//创建一个newTransactionStatus
return prepareTransactionStatus(definition, null, true, newSynchronization, debugEnabled, null);
}
}
该方法主要做了如下事情：
- 开启一个事务 创建了一个DataSourceTransactionObject 事务实例对象，其中绑定了ConnectionHolder，ConnectionHolder底层是ThreadLocal保存了当前线程的数据库连接信息。
- 如果当前线程存在事务，则转向嵌套事务的处理。
- 校验事务超时时间
- 如果事务传播机制是 PROPAGATION_MANDATORY ，如果不存在事务就抛异常
- 创建一个新的TransactionStatus：DefaultTransactionStatus。
- 完善事务信息设置ConnectionHolder、设置隔离级别、设置timeout，连接绑定到当前线程。
回顾一下事务传播行为：
| 事务传播行为类型 | 说明 |
|---|---|
| PROPAGATION_REQUIRED | 如果当前没有事务，就新建一个事务，如果已经存在一个事务中，加入到这个事务中。这是最常见的选择。 |
| PROPAGATION_SUPPORTS | 支持当前事务，如果当前没有事务，就以非事务方式执行。 |
| PROPAGATION_MANDATORY | 使用当前的事务，如果当前没有事务，就抛出异常。 |
| PROPAGATION_REQUIRES_NEW | 新建事务，如果当前存在事务，把当前事务挂起。 |
| PROPAGATION_NOT_SUPPORTED | 以非事务方式执行操作，如果当前存在事务，就把当前事务挂起。 |
| PROPAGATION_NEVER | 以非事务方式执行，如果当前存在事务，则抛出异常。 |
| PROPAGATION_NESTED | 如果当前存在事务，则在嵌套事务内执行。如果当前没有事务，则执行与PROPAGATION_REQUIRED类似的操作。 |
处理嵌套事务 ：handleExistingTransaction
//为现有事务创建 TransactionStatus。
private TransactionStatus handleExistingTransaction(
TransactionDefinition definition, Object transaction, boolean debugEnabled)
throws TransactionException {
//以非事务方式执行，如果当前存在事务，则抛出异常。
if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NEVER) {
throw new IllegalTransactionStateException(
"Existing transaction found for transaction marked with propagation 'never'");
}
//以非事务方式执行操作，如果当前存在事务，就把当前事务挂起。并把事务信息设置为null
if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NOT_SUPPORTED) {
if (debugEnabled) {
logger.debug("Suspending current transaction");
}
Object suspendedResources = suspend(transaction);
boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
return prepareTransactionStatus(
definition, null, false, newSynchronization, debugEnabled, suspendedResources);
}
//新建事务，如果当前存在事务，把当前事务挂起。当然会创建新的连接，让业务在新的事务中完成，之后恢复挂起的事务。
if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW) {
if (debugEnabled) {
logger.debug("Suspending current transaction, creating new transaction with name [" +
definition.getName() + "]");
}
SuspendedResourcesHolder suspendedResources = suspend(transaction);
try {
boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
//新开一个事务
DefaultTransactionStatus status = newTransactionStatus(
definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);
doBegin(transaction, definition);
//初始化事务同步
prepareSynchronization(status, definition);
return status;
}
catch (RuntimeException | Error beginEx) {
resumeAfterBeginException(transaction, suspendedResources, beginEx);
throw beginEx;
}
}
//如果当前存在事务，则在嵌套事务内执行。如果当前没有事务，则执行与PROPAGATION_REQUIRED类似的操作。
if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {
//是否运行嵌套事务
if (!isNestedTransactionAllowed()) {
throw new NestedTransactionNotSupportedException(
"Transaction manager does not allow nested transactions by default - " +
"specify 'nestedTransactionAllowed' property with value 'true'");
}
if (debugEnabled) {
logger.debug("Creating nested transaction with name [" + definition.getName() + "]");
}
//对嵌套事务使用保存点
if (useSavepointForNestedTransaction()) {
// Create savepoint within existing Spring-managed transaction,
// through the SavepointManager API implemented by TransactionStatus.
// Usually uses JDBC 3.0 savepoints. Never activates Spring synchronization.
//如果是JDBC，使用保存点方式支持事务回滚
DefaultTransactionStatus status =
prepareTransactionStatus(definition, transaction, false, false, debugEnabled, null);
status.createAndHoldSavepoint();
return status;
}
else {
//如果是类似于JTA这种还无法使用保存点，处理方式如同PROPAGATION_REQUIRES_NEW
// Nested transaction through nested begin and commit/rollback calls.
// Usually only for JTA: Spring synchronization might get activated here
// in case of a pre-existing JTA transaction.
boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
DefaultTransactionStatus status = newTransactionStatus(
definition, transaction, true, newSynchronization, debugEnabled, null);
doBegin(transaction, definition);
prepareSynchronization(status, definition);
return status;
}
}
// Assumably PROPAGATION_SUPPORTS or PROPAGATION_REQUIRED.
if (debugEnabled) {
logger.debug("Participating in existing transaction");
}
if (isValidateExistingTransaction()) {
if (definition.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT) {
Integer currentIsolationLevel = TransactionSynchronizationManager.getCurrentTransactionIsolationLevel();
if (currentIsolationLevel == null || currentIsolationLevel != definition.getIsolationLevel()) {
Constants isoConstants = DefaultTransactionDefinition.constants;
throw new IllegalTransactionStateException("Participating transaction with definition [" +
definition + "] specifies isolation level which is incompatible with existing transaction: " +
(currentIsolationLevel != null ?
isoConstants.toCode(currentIsolationLevel, DefaultTransactionDefinition.PREFIX_ISOLATION) :
"(unknown)"));
}
}
if (!definition.isReadOnly()) {
if (TransactionSynchronizationManager.isCurrentTransactionReadOnly()) {
throw new IllegalTransactionStateException("Participating transaction with definition [" +
definition + "] is not marked as read-only but existing transaction is");
}
}
}
boolean newSynchronization = (getTransactionSynchronization(Spring AOP源码剖析：Spring声明式事务控制