【Spring】事务的执行原理（三）
事务的回滚
-
如果获取事务属性不为空，并且抛出的异常是RuntimeException或者Error类型，调用事务管理器中的rollback方法进行回滚
-
如果事务属性为空或者抛出的异常不是RuntimeException，也不是Error，将继续提交事务
public abstract class TransactionAspectSupport implements BeanFactoryAware, InitializingBean {
/**
* 处理抛出异常下的事务
*/
protected void completeTransactionAfterThrowing(@Nullable TransactionInfo txInfo, Throwable ex) {
// 判空
if (txInfo != null && txInfo.getTransactionStatus() != null) {
if (logger.isTraceEnabled()) {
logger.trace("Completing transaction for [" + txInfo.getJoinpointIdentification() +
"] after exception: " + ex);
}
// 如果事务属性不为空并且异常是是RuntimeException或者Error
if (txInfo.transactionAttribute != null && txInfo.transactionAttribute.rollbackOn(ex)) {
try {
// 获取事务管理器，调用rollback方法进行回滚
txInfo.getTransactionManager().rollback(txInfo.getTransactionStatus());
}
catch (TransactionSystemException ex2) {
logger.error("Application exception overridden by rollback exception", ex);
ex2.initApplicationException(ex);
throw ex2;
}
catch (RuntimeException | Error ex2) {
logger.error("Application exception overridden by rollback exception", ex);
throw ex2;
}
}
else {
// 如果事务属性为空或者异常不是RuntimeException或者Error，继续提交事务
try {
// 提交
txInfo.getTransactionManager().commit(txInfo.getTransactionStatus());
}
catch (TransactionSystemException ex2) {
logger.error("Application exception overridden by commit exception", ex);
ex2.initApplicationException(ex);
throw ex2;
}
catch (RuntimeException | Error ex2) {
logger.error("Application exception overridden by commit exception", ex);
throw ex2;
}
}
}
}
}
// DefaultTransactionAttribute中实现了rollbackOn方法
public class DefaultTransactionAttribute extends DefaultTransactionDefinition implements TransactionAttribute {
/**
* 判断是否是RuntimeException或者Error
*/
@Override
public boolean rollbackOn(Throwable ex) {
return (ex instanceof RuntimeException || ex instanceof Error);
}
}
rollback方法在AbstractPlatformTransactionManager中实现，主要分为以下三种情况：
- 判断事务是否设置了保存点，如果设置了将事务回滚到保存点
- 如果是一个独立的新事务，直接回滚即可
- 如果既没有设置保存点，也不是一个新事务，说明可能处于嵌套事务中，此时只设置回滚状态rollbackOnly为true，当它的外围事务进行提交时，如果发现回滚状态为true，则不提交
以上步骤执行完毕，调用cleanupAfterCompletion方法进行资源的清理已经挂起事务的恢复。
public abstract class AbstractPlatformTransactionManager implements PlatformTransactionManager, Serializable {
/*
* 回滚
*/
@Override
public final void rollback(TransactionStatus status) throws TransactionException {
if (status.isCompleted()) {
throw new IllegalTransactionStateException(
"Transaction is already completed - do not call commit or rollback more than once per transaction");
}
// 转为DefaultTransactionStatus
DefaultTransactionStatus defStatus = (DefaultTransactionStatus) status;
// 处理回滚
processRollback(defStatus, false);
}
/**
* 处理回滚
*/
private void processRollback(DefaultTransactionStatus status, boolean unexpected) {
try {
boolean unexpectedRollback = unexpected;
try {
// 回滚之前的触发器
triggerBeforeCompletion(status);
// 是否有保存点
if (status.hasSavepoint()) {
if (status.isDebug()) {
logger.debug("Rolling back transaction to savepoint");
}
// 回滚至保存点
status.rollbackToHeldSavepoint();
}
else if (status.isNewTransaction()) { // 如果是一个独立的新事务
if (status.isDebug()) {
logger.debug("Initiating transaction rollback");
}
// 直接回滚
doRollback(status);
}
else {
// 如果处于一个嵌套的事务汇总
if (status.hasTransaction()) {
// 如果本地的回滚状态置为true 或者 事务失败进行全局回滚
if (status.isLocalRollbackOnly() || isGlobalRollbackOnParticipationFailure()) {
if (status.isDebug()) {
logger.debug("Participating transaction failed - marking existing transaction as rollback-only");
}
// 设置事务rollbackOnly状态为true
doSetRollbackOnly(stat);
}
else {
// 打印日志，意思是由事务的组织者决定是否回滚
if (status.isDebug()) {
logger.debug("Participating transaction failed - letting transaction originator decide on rollback");
}
}
}
else {
// 打印DEBUG日志，应该回滚但是没有获取到事务
logger.debug("Should roll back transaction but cannot - no transaction available");
}
// Unexpected rollback only matters here if we're asked to fail early
if (!isFailEarlyOnGlobalRollbackOnly()) {
unexpectedRollback = false;
}
}
}
catch (RuntimeException | Error ex) {
triggerAfterCompletion(status, TransactionSynchronization.STATUS_UNKNOWN);
throw ex;
}
// 回滚之后的触发器
triggerAfterCompletion(status, TransactionSynchronization.STATUS_ROLLED_BACK);
// Raise UnexpectedRollbackException if we had a global rollback-only marker
if (unexpectedRollback) {
throw new UnexpectedRollbackException(
"Transaction rolled back because it has been marked as rollback-only");
}
}
finally {
// 清除相关资源并恢复挂起的事务
cleanupAfterCompletion(status);
}
}
/**
* 当一个事务失败以后，是否全局的标进行事务回滚
*/
public final boolean isGlobalRollbackOnParticipationFailure() {
return this.globalRollbackOnParticipationFailure;
}
}
回滚处理
rollbackToHeldSavepoint回滚至保存点
rollbackToHeldSavepoint方法在AbstractTransactionStatus中实现，它调用了getSavepointManager方法获取保存点管理器，调用SavepointManager的rollbackToSavepoint方法进行回滚的：
public abstract class AbstractTransactionStatus implements TransactionStatus {
public void rollbackToHeldSavepoint() throws TransactionException {
Object savepoint = getSavepoint();
if (savepoint == null) {
throw new TransactionUsageException(
"Cannot roll back to savepoint - no savepoint associated with current transaction");
}
// getSavepointManager方法在DefaultTransactionStatus中实现
getSavepointManager().rollbackToSavepoint(savepoint);
getSavepointManager().releaseSavepoint(savepoint);
setSavepoint(null);
}
}
SavepointManager是一个接口，它的继承关系如下：
DefaultTransactionStatus中获取SavepointManager的方法：
- 获取transaction对象，前面的知识可知这里是一个DataSourceTransactionObject
- 由继承关系可知DataSourceTransactionObject也是SavepointManager子类，所以将DataSourceTransactionObject转为SavepointManager返回
public class DefaultTransactionStatus extends AbstractTransactionStatus {
@Override
protected SavepointManager getSavepointManager() {
// 前面的知识可知这里是一个DataSourceTransactionObject
Object transaction = this.transaction;
if (!(transaction instanceof SavepointManager)) {
throw new NestedTransactionNotSupportedException(
"Transaction object [" + this.transaction + "] does not support savepoints");
}
// 将DataSourceTransactionObject转为SavepointManager
return (SavepointManager) transaction;
}
}
DataSourceTransactionObject是DataSourceTransactionManager的内部类，它继承了JdbcTransactionObjectSupport,rollbackToSavepoint方法在JdbcTransactionObjectSupport中实现：
- 获取ConnectionHolder，ConnectionHolder持有数据库连接
- 调用底层的rollback方法将事务回滚至保存点
public class DataSourceTransactionManager extends AbstractPlatformTransactionManager
implements ResourceTransactionManager, InitializingBean {
// 内部类DataSourceTransactionObject
private static class DataSourceTransactionObject extends JdbcTransactionObjectSupport {
}
}
// JdbcTransactionObjectSupport
public abstract class JdbcTransactionObjectSupport implements SavepointManager, SmartTransactionObject {
/**
* 回滚至保存点
*/
@Override
public void rollbackToSavepoint(Object savepoint) throws TransactionException {
// 获取ConnectionHolder
ConnectionHolder conHolder = getConnectionHolderForSavepoint();
try {
// 调用底层的rollback方法将事务回滚至保存点
conHolder.getConnection().rollback((Savepoint) savepoint);
conHolder.resetRollbackOnly();
}
catch (Throwable ex) {
throw new TransactionSystemException("Could not roll back to JDBC savepoint", ex);
}
}
/**
* 释放保存点
*/
@Override
public void releaseSavepoint(Object savepoint) throws TransactionException {
ConnectionHolder conHolder = getConnectionHolderForSavepoint();
try {
// 调用底层的方法释放保存点
conHolder.getConnection().releaseSavepoint((Savepoint) savepoint);
}
catch (Throwable ex) {
logger.debug("Could not explicitly release JDBC savepoint", ex);
}
}
}
doRollback事务回滚
回滚事务时先获取数据库连接，然后调用底层的rollback进行回滚：
public class DataSourceTransactionManager extends AbstractPlatformTransactionManager
implements ResourceTransactionManager, InitializingBean {
@Override
protected void doRollback(DefaultTransactionStatus status) {
// 获取数据源事务对象
DataSourceTransactionObject txObject = (DataSourceTransactionObject) status.getTransaction();
// 获取数据库连接
Connection con = txObject.getConnectionHolder().getConnection();
if (status.isDebug()) {
logger.debug("Rolling back JDBC transaction on Connection [" + con + "]");
}
try {
// 调用底层的回滚方法
con.rollback();
}
catch (SQLException ex) {
throw new TransactionSystemException("Could not roll back JDBC transaction", ex);
}
}
}
doSetRollbackOnly设置回滚状态
doSetRollbackOnly方法在DataSourceTransactionManager中实现：
-
将事务转为DataSourceTransactionObject对象，前面讲过DataSourceTransactionObject持有了数据库连接对象ConnectionHolder
-
将ConnectionHolder的rollbackOnly属性置为true，先标记事务的回滚状态，交由外围事务进行判断统一进行回滚
public class DataSourceTransactionManager extends AbstractPlatformTransactionManager
implements ResourceTransactionManager, InitializingBean {
@Override
protected void doSetRollbackOnly(DefaultTransactionStatus status) {
// 获取数据源事务对象
DataSourceTransactionObject txObject = (DataSourceTransactionObject) status.getTransaction();
if (status.isDebug()) {
logger.debug("Setting JDBC transaction [" + txObject.getConnectionHolder().getConnection() +
"] rollback-only");
}
// 设置回滚状态
txObject.setRollbackOnly();
}
/**
* 内部类DataSourceTransactionObject
*/
private static class DataSourceTransactionObject extends JdbcTransactionObjectSupport {
// 省略其他方法
// 设置回滚状态
public void setRollbackOnly() {
// 将ConnectionHolder的rollbackOnly属性置为true，在ConnectionHolder的父类ResourceHolderSupport中实现
getConnectionHolder().setRollbackOnly();
}
}
}
// ConnectionHolder的父类ResourceHolderSupport
public abstract class ResourceHolderSupport implements ResourceHolder {
private boolean rollbackOnly = false;
/**
* 标记事务回滚状态为true
*/
public void setRollbackOnly() {
this.rollbackOnly = true;
}
}
资源清理
在事务回滚之后，需要清理相关的资源以及恢复被挂起的事务：
- 如果事务的newSynchronization状态为true，清除当前线程绑定的事务相关信息
- 在TransactionSynchronizationManager的clear方法中实现，清理了当前线程绑定的事务名称、事务隔离级别等信息
- 如果是一个新事务，清除当前线程与数据库连接的绑定关系，在DataSourceTransactionManager的doCleanupAfterCompletion方法中实现
- 如果挂起的事务不为空，恢复挂起的事务
- 获取数据源，恢复数据源与挂起事务的绑定关系
- 恢复挂起事务与当前线程的同步信息
public abstract class AbstractPlatformTransactionManager implements PlatformTransactionManager, Serializable {
/**
* 回滚之后的清除操作
* @see #doCleanupAfterCompletion
*/
private void cleanupAfterCompletion(DefaultTransactionStatus status) {
status.setCompleted();
if (status.isNewSynchronization()) {
// 清除当前线程绑定的信息
TransactionSynchronizationManager.clear();
}
// 如果是一个新事务
if (status.isNewTransaction()) {
// 清除当前线程与数据库连接的绑定关系
doCleanupAfterCompletion(status.getTransaction());
}
// 如果挂起的事务不为空
if (status.getSuspendedResources() != null) {
if (status.isDebug()) {
logger.debug("Resuming suspended transaction after completion of inner transaction");
}
Object transaction = (status.hasTransaction() ? status.getTransaction() : null);
// 恢复挂起的事务
resume(transaction, (SuspendedResourcesHolder) status.getSuspendedResources());
}
}
/**
* 恢复挂起的事务
* @see #doResume
* @see #suspend
*/
protected final void resume(@Nullable Object transaction, @Nullable SuspendedResourcesHolder resourcesHolder)
throws TransactionException {
if (resourcesHolder != null) {
// 获取挂起的事务
Object suspendedResources = resourcesHolder.suspendedResources;
if (suspendedResources != null) {
// 获取数据源，并与挂起的事务进行绑定
doResume(transaction, suspendedResources);
}
// 挂起事务的同步信息
List<TransactionSynchronization> suspendedSynchronizations = resourcesHolder.suspendedSynchronizations;
if (suspendedSynchronizations != null) {
// 恢复事务与线程的同步信息
TransactionSynchronizationManager.setActualTransactionActive(resourcesHolder.wasActive);
TransactionSynchronizationManager.setCurrentTransactionIsolationLevel(resourcesHolder.isolationLevel);
TransactionSynchronizationManager.setCurrentTransactionReadOnly(resourcesHolder.readOnly);
TransactionSynchronizationManager.setCurrentTransactionName(resourcesHolder.name);
doResumeSynchronization(suspendedSynchronizations);
}
}
}
}
// DataSourceTransactionManager
public class DataSourceTransactionManager extends AbstractPlatformTransactionManager
implements ResourceTransactionManager, InitializingBean {
// 主要是清除当前线程与数据库连接的绑定关系
protected void doCleanupAfterCompletion(Object transaction) {
DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
// Remove the connection holder from the thread, if exposed.
if (txObject.isNewConnectionHolder()) {
TransactionSynchronizationManager.unbindResource(obtainDataSource());
}
// 获取数据库连接
Connection con = txObject.getConnectionHolder().getConnection();
try {
if (txObject.isMustRestoreAutoCommit()) {
// 自动提交置为true
con.setAutoCommit(true);
}
// 重置数据库连接的相关设置
DataSourceUtils.resetConnectionAfterTransaction(
con, txObject.getPreviousIsolationLevel(), txObject.isReadOnly());
}
catch (Throwable ex) {
logger.debug("Could not reset JDBC Connection after transaction", ex);
}
if (txObject.isNewConnectionHolder()) {
if (logger.isDebugEnabled()) {
logger.debug("Releasing JDBC Connection [" + con + "] after transaction");
}
// 释放连接
DataSourceUtils.releaseConnection(con, this.dataSource);
}
// 清除当前线程与数据库连接的绑定关系
txObject.getConnectionHolder().clear();
}
@Override
protected void doResume(@Nullable Object transaction, Object suspendedResources) {
// 获取数据源，并与挂起的事务进行绑定
TransactionSynchronizationManager.bindResource(obtainDataSource(), suspendedResources);
}
}
// TransactionSynchronizationManager
public abstract class TransactionSynchronizationManager {
// 保存了线程绑定的数据库资源信息，Map中Key为数据源构建的KEY，value为对应的ConnectionHolder
private static final ThreadLocal<Map<Object, Object>> resources =
new NamedThreadLocal<>("Transactional resources");
// 保存了线程绑定的事务同步信息TransactionSynchronization
private static final ThreadLocal<Set<TransactionSynchronization>> synchronizations =
new NamedThreadLocal<>("Transaction synchronizations");
// 保存了线程绑定的事务名称
private static final ThreadLocal<String> currentTransactionName =
new NamedThreadLocal<>("Current transaction name");
// 保存了线程绑定的事务只读状态
private static final ThreadLocal<Boolean> currentTransactionReadOnly =
new NamedThreadLocal<>("Current transaction read-only status");
// 保存了线程绑定的事务隔离级别
private static final ThreadLocal<Integer> currentTransactionIsolationLevel =
new NamedThreadLocal<>("Current transaction isolation level");
// 保存了线程绑定的事务活跃状态
private static final ThreadLocal<Boolean> actualTransactionActive =
new NamedThreadLocal<>("Actual transaction active");
/**
* 清理事务与当前线程的各种同步状态
*/
public static void clear() {
// 清除当前线程绑定的事务同步信息TransactionSynchronization
synchronizations.remove();
// 清除当前线程绑定的事务名称
currentTransactionName.remove();
// 清除线程绑定的事务只读状态
currentTransactionReadOnly.remove();
// 清除线程绑定的事务隔离级别
currentTransactionIsolationLevel.remove();
// 清除线程绑定的事务活跃状态
actualTransactionActive.remove();
}
}
总结
参考
Spring版本：5.2.5.RELEASE
