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本文主要研究一下spring事务的REQUIRES_NEW
TransactionDefinition
org/springframework/transaction/TransactionDefinition.java
代码语言：javascript复制 /**
* Create a new transaction, suspending the current transaction if one exists.
* Analogous to the EJB transaction attribute of the same name.
* <p><b>NOTE:</b> Actual transaction suspension will not work out-of-the-box
* on all transaction managers. This in particular applies to
* {@link org.springframework.transaction.jta.JtaTransactionManager},
* which requires the {@code javax.transaction.TransactionManager} to be
* made available it to it (which is server-specific in standard Java EE).
* <p>A {@code PROPAGATION_REQUIRES_NEW} scope always defines its own
* transaction synchronizations. Existing synchronizations will be suspended
* and resumed appropriately.
* @see org.springframework.transaction.jta.JtaTransactionManager#setTransactionManager
*/
int PROPAGATION_REQUIRES_NEW = 3;PROPAGATION_REQUIRES_NEW在有事务的场景下会suspend当前事务，然后创建新事务
AbstractPlatformTransactionManager
org/springframework/transaction/support/AbstractPlatformTransactionManager.java
代码语言：javascript复制 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);
DefaultTransactionStatus status = newTransactionStatus(
definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);
doBegin(transaction, definition);
prepareSynchronization(status, definition);
return status;
}
catch (RuntimeException | Error beginEx) {
resumeAfterBeginException(transaction, suspendedResources, beginEx);
throw beginEx;
}
}handleExistingTransaction方法在判断是PROPAGATION_REQUIRES_NEW，会执行suspend方法，然后newTransactionStatus，执行doBegin及prepareSynchronization
suspend
org/springframework/transaction/support/AbstractPlatformTransactionManager.java
代码语言：javascript复制 /**
* Suspend the given transaction. Suspends transaction synchronization first,
* then delegates to the {@code doSuspend} template method.
* @param transaction the current transaction object
* (or {@code null} to just suspend active synchronizations, if any)
* @return an object that holds suspended resources
* (or {@code null} if neither transaction nor synchronization active)
* @see #doSuspend
* @see #resume
*/
@Nullable
protected final SuspendedResourcesHolder suspend(@Nullable Object transaction) throws TransactionException {
if (TransactionSynchronizationManager.isSynchronizationActive()) {
List<TransactionSynchronization> suspendedSynchronizations = doSuspendSynchronization();
try {
Object suspendedResources = null;
if (transaction != null) {
suspendedResources = doSuspend(transaction);
}
String name = TransactionSynchronizationManager.getCurrentTransactionName();
TransactionSynchronizationManager.setCurrentTransactionName(null);
boolean readOnly = TransactionSynchronizationManager.isCurrentTransactionReadOnly();
TransactionSynchronizationManager.setCurrentTransactionReadOnly(false);
Integer isolationLevel = TransactionSynchronizationManager.getCurrentTransactionIsolationLevel();
TransactionSynchronizationManager.setCurrentTransactionIsolationLevel(null);
boolean wasActive = TransactionSynchronizationManager.isActualTransactionActive();
TransactionSynchronizationManager.setActualTransactionActive(false);
return new SuspendedResourcesHolder(
suspendedResources, suspendedSynchronizations, name, readOnly, isolationLevel, wasActive);
}
catch (RuntimeException | Error ex) {
// doSuspend failed - original transaction is still active...
doResumeSynchronization(suspendedSynchronizations);
throw ex;
}
}
else if (transaction != null) {
// Transaction active but no synchronization active.
Object suspendedResources = doSuspend(transaction);
return new SuspendedResourcesHolder(suspendedResources);
}
else {
// Neither transaction nor synchronization active.
return null;
}
}suspend方法主要是执行doSuspendSynchronization方法返回suspendedSynchronizations，执行doSuspend返回suspendedResources，最后根据这两个创建SuspendedResourcesHolder
doSuspendSynchronization
代码语言：javascript复制 /**
* Suspend all current synchronizations and deactivate transaction
* synchronization for the current thread.
* @return the List of suspended TransactionSynchronization objects
*/
private List<TransactionSynchronization> doSuspendSynchronization() {
List<TransactionSynchronization> suspendedSynchronizations =
TransactionSynchronizationManager.getSynchronizations();
for (TransactionSynchronization synchronization : suspendedSynchronizations) {
synchronization.suspend();
}
TransactionSynchronizationManager.clearSynchronization();
return suspendedSynchronizations;
}doSuspendSynchronization这个遍历suspendedSynchronizations，挨个执行suspend，然后clearSynchronization
ResourceHolderSynchronization
org/springframework/transaction/support/ResourceHolderSynchronization.java
代码语言：javascript复制 public void suspend() {
if (this.holderActive) {
TransactionSynchronizationManager.unbindResource(this.resourceKey);
}
}ResourceHolderSynchronization的suspend执行的是TransactionSynchronizationManager.unbindResource
unbindResource
org/springframework/transaction/support/TransactionSynchronizationManager.java
代码语言：javascript复制 /**
* Unbind a resource for the given key from the current thread.
* @param key the key to unbind (usually the resource factory)
* @return the previously bound value (usually the active resource object)
* @throws IllegalStateException if there is no value bound to the thread
* @see ResourceTransactionManager#getResourceFactory()
*/
public static Object unbindResource(Object key) throws IllegalStateException {
Object actualKey = TransactionSynchronizationUtils.unwrapResourceIfNecessary(key);
Object value = doUnbindResource(actualKey);
if (value == null) {
throw new IllegalStateException(
"No value for key [" actualKey "] bound to thread [" Thread.currentThread().getName() "]");
}
return value;
}
/**
* Actually remove the value of the resource that is bound for the given key.
*/
@Nullable
private static Object doUnbindResource(Object actualKey) {
Map<Object, Object> map = resources.get();
if (map == null) {
return null;
}
Object value = map.remove(actualKey);
// Remove entire ThreadLocal if empty...
if (map.isEmpty()) {
resources.remove();
}
// Transparently suppress a ResourceHolder that was marked as void...
if (value instanceof ResourceHolder && ((ResourceHolder) value).isVoid()) {
value = null;
}
if (value != null && logger.isTraceEnabled()) {
logger.trace("Removed value [" value "] for key [" actualKey "] from thread ["
Thread.currentThread().getName() "]");
}
return value;
}unbindResource主要是执行doUnbindResource，从resources中移除
cleanupAfterCompletion
org/springframework/transaction/support/AbstractPlatformTransactionManager.java
代码语言：javascript复制 /**
* Clean up after completion, clearing synchronization if necessary,
* and invoking doCleanupAfterCompletion.
* @param status object representing the transaction
* @see #doCleanupAfterCompletion
*/
private void cleanupAfterCompletion(DefaultTransactionStatus status) {
status.setCompleted();
if (status.isNewSynchronization()) {
TransactionSynchronizationManager.clear();
}
if (status.isNewTransaction()) {
doCleanupAfterCompletion(status.getTransaction());
}
if (status.getSuspendedResources() != null) {
if (status.isDebug()) {
logger.debug("Resuming suspended transaction after completion of inner transaction");
}
Object transaction = (status.hasTransaction() ? status.getTransaction() : null);
resume(transaction, (SuspendedResourcesHolder) status.getSuspendedResources());
}
}在内嵌事务执行完之后，会判断是否有suspendedResources，如果有则执行resume，恢复之前suspend的事务
resume
org/springframework/transaction/support/AbstractPlatformTransactionManager.java
代码语言：javascript复制 /**
* Resume the given transaction. Delegates to the {@code doResume}
* template method first, then resuming transaction synchronization.
* @param transaction the current transaction object
* @param resourcesHolder the object that holds suspended resources,
* as returned by {@code suspend} (or {@code null} to just
* resume synchronizations, if any)
* @see #doResume
* @see #suspend
*/
protected final void resume(@Nullable Object transaction, @Nullable SuspendedResourcesHolder resourcesHolder)
throws TransactionException {
if (resourcesHolder != null) {
Object suspendedResources = resourcesHolder.suspendedResources;
if (suspendedResources != null) {
doResume(transaction, suspendedResources);
}
List<TransactionSynchronization> suspendedSynchronizations = resourcesHolder.suspendedSynchronizations;
if (suspendedSynchronizations != null) {
TransactionSynchronizationManager.setActualTransactionActive(resourcesHolder.wasActive);
TransactionSynchronizationManager.setCurrentTransactionIsolationLevel(resourcesHolder.isolationLevel);
TransactionSynchronizationManager.setCurrentTransactionReadOnly(resourcesHolder.readOnly);
TransactionSynchronizationManager.setCurrentTransactionName(resourcesHolder.name);
doResumeSynchronization(suspendedSynchronizations);
}
}
}resume方法执行doResume，然后恢复之前的TransactionSynchronizationManager的一些设置
doResume
org/springframework/orm/jpa/JpaTransactionManager.java
代码语言：javascript复制 @Override
protected void doResume(@Nullable Object transaction, Object suspendedResources) {
SuspendedResourcesHolder resourcesHolder = (SuspendedResourcesHolder) suspendedResources;
TransactionSynchronizationManager.bindResource(
obtainEntityManagerFactory(), resourcesHolder.getEntityManagerHolder());
if (getDataSource() != null && resourcesHolder.getConnectionHolder() != null) {
TransactionSynchronizationManager.bindResource(getDataSource(), resourcesHolder.getConnectionHolder());
}
}doResume这里就是给bind回来
小结
spring事务的REQUIRES_NEW传播级别的实现就是对当前事务进行suspend，底层是unbind，然后创建新事务，执行完毕判断是否有suspend的事务，有则执行resume，底层是bind。具体对于mysql来讲，它不感知这些嵌套事务，它先接收到的是内嵌的新事务的sql，然后提交，最后接收到了外层resume回来的事务。
