Spring事务抽象的是事务管理和事务策略。而实现则由各种资源方实现的。我们最常用的数据库实现：DataSourceTransactionManager
尝试阅读一下spring 的实现代码，由3个核心类：
1，PlatformTransactionManager
public interface PlatformTransactionManager {
TransactionStatus getTransaction(TransactionDefinition definition) throws TransactionException;
void commit(TransactionStatus status) throws TransactionException;
void rollback(TransactionStatus status) throws TransactionException;
事务管理的抽象，一个获取事务状态，一个提交事务，一个回滚事务。
2，TransactionStatus
public interface TransactionStatus extends SavepointManager, Flushable {
boolean isNewTransaction();
boolean hasSavepoint();
void setRollbackOnly();
boolean isRollbackOnly();
void flush();
boolean isCompleted();
}
事务状态的抽象
3，DefaultTransactionDefinition
public interface TransactionDefinition {
int getPropagationBehavior();
int getIsolationLevel();
int getTimeout();
boolean isReadOnly();
String getName();
}
事务策略抽象，1，事务传播行为，2，事务隔离级别，3，超时时间，4，只读属性
编程的方法实现事务管理使用简单代码：
transactionTemplate.execute(new TransactionCallback<String>() {
@Override
public String doInTransaction(TransactionStatus status) {
updatePayPrice(orderData, offer);
return null;
}
});
类似下面的配置：
<bean id="txManager"
class="org.springframework.jdbc.datasource.DataSourceTransactionManager">
<property name="dataSource" ref="dataSource" />
</bean> <bean id="transactionTemplate"
class="org.springframework.transaction.support.TransactionTemplate"
p:transactionManager-ref="txManager" scope="prototype" />
TransactionTemplate继承了DefaultTransactionDefinition也就是TransactionDefinition默认实现，afterPropertiesSet方法检查transactionManager，
核心方法execute，其中四步：0.获取一个事务状态，1，执行业务逻辑 2，出现异常，3，没有异常事务提交
public class TransactionTemplate extends DefaultTransactionDefinition
implements TransactionOperations, InitializingBean { /** Logger available to subclasses */
protected final Log logger = LogFactory.getLog(getClass()); private PlatformTransactionManager transactionManager;
public TransactionTemplate() {
}
public TransactionTemplate(PlatformTransactionManager transactionManager) {
this.transactionManager = transactionManager;
}
public TransactionTemplate(PlatformTransactionManager transactionManager, TransactionDefinition transactionDefinition) {
super(transactionDefinition);
this.transactionManager = transactionManager;
} public void setTransactionManager(PlatformTransactionManager transactionManager) {
this.transactionManager = transactionManager;
} /**
* Return the transaction management strategy to be used.
*/
public PlatformTransactionManager getTransactionManager() {
return this.transactionManager;
} @Override
public void afterPropertiesSet() {
if (this.transactionManager == null) {
throw new IllegalArgumentException("Property 'transactionManager' is required");
}
} @Override
public <T> T execute(TransactionCallback<T> action) throws TransactionException {
if (this.transactionManager instanceof CallbackPreferringPlatformTransactionManager) {
return ((CallbackPreferringPlatformTransactionManager) this.transactionManager).execute(this, action);
}
else {
// 0.获取一个事务状态
TransactionStatus status = this.transactionManager.getTransaction(this);
T result;
try {
//1.执行业务逻辑
result = action.doInTransaction(status);
}
// 2.异常则回退
catch (RuntimeException ex) {
// Transactional code threw application exception -> rollback
rollbackOnException(status, ex);
throw ex;
}
catch (Error err) {
// Transactional code threw error -> rollback
rollbackOnException(status, err);
throw err;
}
catch (Exception ex) {
// Transactional code threw unexpected exception -> rollback
rollbackOnException(status, ex);
throw new UndeclaredThrowableException(ex, "TransactionCallback threw undeclared checked exception");
}
// 3.没有异常事务提交
this.transactionManager.commit(status);
return result;
}
} private void rollbackOnException(TransactionStatus status, Throwable ex) throws TransactionException {
logger.debug("Initiating transaction rollback on application exception", ex);
try {
this.transactionManager.rollback(status);
}
catch (TransactionSystemException ex2) {
logger.error("Application exception overridden by rollback exception", ex);
ex2.initApplicationException(ex);
throw ex2;
}
catch (RuntimeException ex2) {
logger.error("Application exception overridden by rollback exception", ex);
throw ex2;
}
catch (Error err) {
logger.error("Application exception overridden by rollback error", ex);
throw err;
}
} }
0，获取一个事务状态
this.transactionManager.getTransaction(this);
这里的transactionManager就是前面使用代码的配置中txManager，就是DataSourceTransactionManager，模版类AbstractPlatformTransactionManager。而传参this就是DefaultTransactionDefinition。
下面是AbstractPlatformTransactionManager里的模版方法：
public final TransactionStatus getTransaction(TransactionDefinition definition) throws TransactionException {
// 进来就先获取事务
Object transaction = doGetTransaction();
// Cache debug flag to avoid repeated checks.
// 这里为打日志判断日志级别做的优化，这个在http://www.cnblogs.com/killbug/p/6721047.html 最后有解释。
boolean debugEnabled = logger.isDebugEnabled();
if (definition == null) {
// Use defaults if no transaction definition given.
definition = new DefaultTransactionDefinition();
}
if (isExistingTransaction(transaction)) {
// Existing transaction found -> check propagation behavior to find out how to behave.
return handleExistingTransaction(definition, transaction, debugEnabled);
}
// Check definition settings for new transaction.
if (definition.getTimeout() < TransactionDefinition.TIMEOUT_DEFAULT) {
throw new InvalidTimeoutException("Invalid transaction timeout", definition.getTimeout());
}
// No existing transaction found -> check propagation behavior to find out how to proceed.
// 配置了PROPAGATION_MANDATORY就不能调用这个方法的，这个方法是开始获取事务的地方
if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_MANDATORY) {
throw new IllegalTransactionStateException(
"No existing transaction found for transaction marked with propagation 'mandatory'");
}
else if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRED ||
definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW ||
definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {
// 外层挂起事务时保存的资源
SuspendedResourcesHolder suspendedResources = suspend(null);
if (debugEnabled) {
logger.debug("Creating new transaction with name [" + definition.getName() + "]: " + definition);
}
try {
boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
// 新建事务
DefaultTransactionStatus status = newTransactionStatus(
definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);
doBegin(transaction, definition);
prepareSynchronization(status, definition);
return status;
}
catch (RuntimeException ex) {
resume(null, suspendedResources);
throw ex;
}
catch (Error err) {
resume(null, suspendedResources);
throw err;
}
}
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);
return prepareTransactionStatus(definition, null, true, newSynchronization, debugEnabled, null);
}
}
1，回滚代码：
private void rollbackOnException(TransactionStatus status, Throwable ex) throws TransactionException {
logger.debug("Initiating transaction rollback on application exception", ex);
try {
this.transactionManager.rollback(status);
}
catch (TransactionSystemException ex2) {
logger.error("Application exception overridden by rollback exception", ex);
ex2.initApplicationException(ex);
throw ex2;
}
catch (RuntimeException ex2) {
logger.error("Application exception overridden by rollback exception", ex);
throw ex2;
}
catch (Error err) {
logger.error("Application exception overridden by rollback error", ex);
throw err;
}
}
transactionManager.rollback
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");
}
DefaultTransactionStatus defStatus = (DefaultTransactionStatus) status;
processRollback(defStatus);
}
//模版方法 子类实现自己的会滚操作
private void processRollback(DefaultTransactionStatus status) {
try {
try {
triggerBeforeCompletion(status);
if (status.hasSavepoint()) {
if (status.isDebug()) {
logger.debug("Rolling back transaction to savepoint");
}
status.rollbackToHeldSavepoint();
}
else if (status.isNewTransaction()) {
if (status.isDebug()) {
logger.debug("Initiating transaction rollback");
}
doRollback(status);
}
else if (status.hasTransaction()) {
if (status.isLocalRollbackOnly() || isGlobalRollbackOnParticipationFailure()) {
if (status.isDebug()) {
logger.debug("Participating transaction failed - marking existing transaction as rollback-only");
}
doSetRollbackOnly(status);
}
else {
if (status.isDebug()) {
logger.debug("Participating transaction failed - letting transaction originator decide on rollback");
}
}
}
else {
logger.debug("Should roll back transaction but cannot - no transaction available");
}
}
catch (RuntimeException ex) {
triggerAfterCompletion(status, TransactionSynchronization.STATUS_UNKNOWN);
throw ex;
}
catch (Error err) {
triggerAfterCompletion(status, TransactionSynchronization.STATUS_UNKNOWN);
throw err;
}
triggerAfterCompletion(status, TransactionSynchronization.STATUS_ROLLED_BACK);
}
finally {
cleanupAfterCompletion(status);
}
}
事务提交：
// 模版
public final void commit(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");
} DefaultTransactionStatus defStatus = (DefaultTransactionStatus) status;
if (defStatus.isLocalRollbackOnly()) {
if (defStatus.isDebug()) {
logger.debug("Transactional code has requested rollback");
}
processRollback(defStatus);
return;
}
if (!shouldCommitOnGlobalRollbackOnly() && defStatus.isGlobalRollbackOnly()) {
if (defStatus.isDebug()) {
logger.debug("Global transaction is marked as rollback-only but transactional code requested commit");
}
processRollback(defStatus);
// Throw UnexpectedRollbackException only at outermost transaction boundary
// or if explicitly asked to.
if (status.isNewTransaction() || isFailEarlyOnGlobalRollbackOnly()) {
throw new UnexpectedRollbackException(
"Transaction rolled back because it has been marked as rollback-only");
}
return;
} processCommit(defStatus);
}
AbstractPlatformTransactionManager做为模版类，代码清晰。
我们发现在上面的execute，异常会退时是不没有任何区分什么异常会滚，什么异常不会滚，都是走rollbackOnException。其实我们只要在业务方法中把异常吃掉即可，另外对于回滚后异常抛出，在最外层再把异常做处理。
本篇没有真正深入了解原理实现，写在这里是为了开个头。