一、spring事务简介
spring中有两种事务实现方式：
1）编程式事务
使用TransactionTemplate，实现更加细粒度的事务控制。
2）声明式事务
使用@Transactional，无代码侵入的事务使用方式。通过AOP实现，本质是在方法前后进行拦截，简单描述就是在方法开始前开启事务，结束后进行提交或者回滚。
二、编程式事务TransactionTemplate
下面看下类图：
image.png
如上图所示，发现其实现了TransactionOperrations和InitializingBean两个接口，继承自DefaultTransactionDefinition。分别看下都是干嘛的。
TransactionOperations内部是执行事务回调的方法，分别提供有返回值和没有返回值的两个方法。
image.png
InitializingBean是spring在bean初始化时预留的接口，专门用来在bean属性加载完成后用来执行的方法。
下面我们看看TransactionTemplate分别实现的这两个接口的哪些方法：
@Override
public void afterPropertiesSet() {
//事务管理器是否为空
if (this.transactionManager == null) {
throw new IllegalArgumentException("Property 'transactionManager' is required");
}
}
@Override
@Nullable
public <T> T execute(TransactionCallback<T> action) throws TransactionException {
Assert.state(this.transactionManager != null, "No PlatformTransactionManager set");
//内部封装的事务管理器
if (this.transactionManager instanceof CallbackPreferringPlatformTransactionManager) {
return ((CallbackPreferringPlatformTransactionManager) this.transactionManager).execute(this, action);
}
else {
//手动获取事务，执行方法，提交事务管理器
//1、获取事务
TransactionStatus status = this.transactionManager.getTransaction(this);
T result;
try {
//2、执行业务逻辑
result = action.doInTransaction(status);
}
catch (RuntimeException | Error ex) {
// 应用运行时异常-》回滚
rollbackOnException(status, ex);
throw ex;
}
catch (Throwable ex) {
// 未知异常-》回滚
rollbackOnException(status, ex);
throw new UndeclaredThrowableException(ex, "TransactionCallback threw undeclared checked exception");
}
//、事务提交
this.transactionManager.commit(status);
return result;
}
}
三、声明式事务@Transactional
声明式事务的核心就是TransactionInterceptor。
image.png
如上图所示，事务拦截器的拦截功能就是依靠实现了MethodInterceptor接口，这个是spring的方法拦截器。一个函数式接口，只有一个invoke方法。
@FunctionalInterface
public interface MethodInterceptor extends Interceptor {
/**
* Implement this method to perform extra treatments before and
* after the invocation. Polite implementations would certainly
* like to invoke {@link Joinpoint#proceed()}.
* @param invocation the method invocation joinpoint
* @return the result of the call to {@link Joinpoint#proceed()};
* might be intercepted by the interceptor
* @throws Throwable if the interceptors or the target object
* throws an exception
*/
@Nullable
Object invoke(@Nonnull MethodInvocation invocation) throws Throwable;
}
看下其方法的实现：
@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.
Class<?> targetClass = (invocation.getThis() != null ? AopUtils.getTargetClass(invocation.getThis()) : null);
// 调用TransactionAspectSupport的invokeWithinTransaction方法
return invokeWithinTransaction(invocation.getMethod(), targetClass, invocation::proceed);
}
下面主要关注invokeWithinTransaction方法，只注释主要的方法：
@Nullable
protected Object invokeWithinTransaction(Method method, @Nullable Class<?> targetClass,
final InvocationCallback invocation) throws Throwable {
// 如果事务属性为空，则该方法是非事务性的。
TransactionAttributeSource tas = getTransactionAttributeSource();
final TransactionAttribute txAttr = (tas != null ? tas.getTransactionAttribute(method, targetClass) : null);
final TransactionManager tm = determineTransactionManager(txAttr);
if (this.reactiveAdapterRegistry != null && tm instanceof ReactiveTransactionManager) {
ReactiveTransactionSupport txSupport = this.transactionSupportCache.computeIfAbsent(method, key -> {
if (KotlinDetector.isKotlinType(method.getDeclaringClass()) && KotlinDelegate.isSuspend(method)) {
throw new TransactionUsageException(
"Unsupported annotated transaction on suspending function detected: " + method +
". Use TransactionalOperator.transactional extensions instead.");
}
ReactiveAdapter adapter = this.reactiveAdapterRegistry.getAdapter(method.getReturnType());
if (adapter == null) {
throw new IllegalStateException("Cannot apply reactive transaction to non-reactive return type: " +
method.getReturnType());
}
return new ReactiveTransactionSupport(adapter);
});
return txSupport.invokeWithinTransaction(
method, targetClass, invocation, txAttr, (ReactiveTransactionManager) tm);
}
PlatformTransactionManager ptm = asPlatformTransactionManager(tm);
final String joinpointIdentification = methodIdentification(method, targetClass, txAttr);
// 声明式事务
if (txAttr == null || !(ptm instanceof CallbackPreferringPlatformTransactionManager)) {
// 使用 getTransaction 和 commit/rollback 调用进行标准事务划分。
TransactionInfo txInfo = createTransactionIfNecessary(ptm, txAttr, joinpointIdentification);
Object retVal;
try {
// 业务代码
retVal = invocation.proceedWithInvocation();
}
catch (Throwable ex) {
// 捕获异常，回滚或提交事务
completeTransactionAfterThrowing(txInfo, ex);
throw ex;
}
finally {
//清空缓存的事务信心，并设置当前线程的事务信息为老的，即首次进入方法获取的事务
cleanupTransactionInfo(txInfo);
}
if (retVal != null && vavrPresent && VavrDelegate.isVavrTry(retVal)) {
// Set rollback-only in case of Vavr failure matching our rollback rules...
TransactionStatus status = txInfo.getTransactionStatus();
if (status != null && txAttr != null) {
retVal = VavrDelegate.evaluateTryFailure(retVal, txAttr, status);
}
}
// 返回结果之前，进行事务提交
commitTransactionAfterReturning(txInfo);
return retVal;
}
//编程式事务
else {
Object result;
final ThrowableHolder throwableHolder = new ThrowableHolder();
// It's a CallbackPreferringPlatformTransactionManager: pass a TransactionCallback in.
try {
result = ((CallbackPreferringPlatformTransactionManager) ptm).execute(txAttr, status -> {
TransactionInfo txInfo = prepareTransactionInfo(ptm, txAttr, joinpointIdentification, status);
try {
Object retVal = invocation.proceedWithInvocation();
if (retVal != null && vavrPresent && VavrDelegate.isVavrTry(retVal)) {
// Set rollback-only in case of Vavr failure matching our rollback rules...
retVal = VavrDelegate.evaluateTryFailure(retVal, txAttr, status);
}
return retVal;
}
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);
}
});
}
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;
}
// Check result state: It might indicate a Throwable to rethrow.
if (throwableHolder.throwable != null) {
throw throwableHolder.throwable;
}
return result;
}
}
3.1 createTransactionIfNecessary
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 (txAttr != null && txAttr.getName() == null) {
txAttr = new DelegatingTransactionAttribute(txAttr) {
@Override
public String getName() {
return joinpointIdentification;
}
};
}
//初始化事务的状态
TransactionStatus status = null;
if (txAttr != null) {
if (tm != null) {
// 获取事务状态
status = tm.getTransaction(txAttr);
}
else {
if (logger.isDebugEnabled()) {
logger.debug("Skipping transactional joinpoint [" + joinpointIdentification +
"] because no transaction manager has been configured");
}
}
}
// 事务为空，创建新事务
return prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);
}
prepareTransactionInfo
protected TransactionInfo prepareTransactionInfo(@Nullable PlatformTransactionManager tm,
@Nullable TransactionAttribute txAttr, String joinpointIdentification,
@Nullable TransactionStatus status) {
// 创建一个事务
TransactionInfo txInfo = new TransactionInfo(tm, txAttr, joinpointIdentification);
if (txAttr != null) {
// We need a transaction for this method...
if (logger.isTraceEnabled()) {
logger.trace("Getting transaction for [" + txInfo.getJoinpointIdentification() + "]");
}
// The transaction manager will flag an error if an incompatible tx already exists.
txInfo.newTransactionStatus(status);
}
else {
// The TransactionInfo.hasTransaction() method will return false. We created it only
// to preserve the integrity of the ThreadLocal stack maintained in this class.
if (logger.isTraceEnabled()) {
logger.trace("No need to create transaction for [" + joinpointIdentification +
"]: This method is not transactional.");
}
}
// 将事务绑定到线程
txInfo.bindToThread();
return txInfo;
}
bindToThread
private void bindToThread() {
// 获取当前事务信息并保存为旧的，以便日后进行恢复
this.oldTransactionInfo = transactionInfoHolder.get();
// 将当前事务绑定到当前持有者，transactionInfoHolder是一个ThreadLocal
transactionInfoHolder.set(this);
}
3.2 invocation.proceedWithInvocation() 回调业务代码
@FunctionalInterface
protected interface InvocationCallback {
@Nullable
Object proceedWithInvocation() throws Throwable;
}
上面的接口实现其实是下面的方法，最终实现类是ReflectiveMethodInvocation：
image.png
image.png
如上图，ReflectiveMethodInvocation类实现了ProxyMethodInvocation接口，但是ProxyMethodInvocation继承了3层接口ProxyMethodInvocation -> MethodInvocation -> Invocation -> Joinpoint
Joinpoint：连接点接口，定义了执行接口：Object proceed() throws Throwable;
执行当前连接点，并跳到拦截器链上的下一个拦截器。
Invocation：调用接口，继承自Joinpoint，定义了获取参数接口： Object[] getArguments();
是一个带参数的、可被拦截器拦截的连接点。
MethodInvocation：方法调用接口，继承自Invocation，定义了获取方法接口：Method getMethod();
是一个带参数的可被拦截的连接点方法。
ProxyMethodInvocation：代理方法调用接口，继承自MethodInvocation，定义了获取代理对象接口：Object getProxy();
是一个由代理类执行的方法调用连接点方法。
ReflectiveMethodInvocation：实现了ProxyMethodInvocation接口，自然就实现了父类接口的的所有接口。
获取代理类，获取方法，获取参数，用代理类执行这个方法并且自动跳到下一个连接点。
**proceed() **
@Override
@Nullable
public Object proceed() throws Throwable {
// 我们从 -1 的索引开始并提前增加。
if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1) {
return invokeJoinpoint();
}
Object interceptorOrInterceptionAdvice =
this.interceptorsAndDynamicMethodMatchers.get(++this.currentInterceptorIndex);
if (interceptorOrInterceptionAdvice instanceof InterceptorAndDynamicMethodMatcher) {
// 这里进行动态方法匹配校验，静态的方法匹配早已经校验过了
InterceptorAndDynamicMethodMatcher dm =
(InterceptorAndDynamicMethodMatcher) interceptorOrInterceptionAdvice;
Class<?> targetClass = (this.targetClass != null ? this.targetClass : this.method.getDeclaringClass());
if (dm.methodMatcher.matches(this.method, targetClass, this.arguments)) {
return dm.interceptor.invoke(this);
}
else {
// 动态匹配失败，跳过当前拦截，进入下一个拦截器
return proceed();
}
}
else {
// 它是一个拦截器，所以我们只需调用它：在构造这个对象之前，切入点将被静态计算
// 这就是回调我们业务方法
return ((MethodInterceptor) interceptorOrInterceptionAdvice).invoke(this);
}
}
3.3 completeTransactionAfterThrowing方法
protected void completeTransactionAfterThrowing(@Nullable TransactionInfo txInfo, Throwable ex) {
if (txInfo != null && txInfo.getTransactionStatus() != null) {
if (logger.isTraceEnabled()) {
logger.trace("Completing transaction for [" + txInfo.getJoinpointIdentification() +
"] after exception: " + ex);
}
if (txInfo.transactionAttribute != null && txInfo.transactionAttribute.rollbackOn(ex)) {
try {
// PlatformTransactionManager的rollback方法
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 {
// 我们不会回滚这个异常
// 如果 TransactionStatus.isRollbackOnly() 为真，仍将回滚
try {
// PlatformTransactionManager的commit方法
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;
}
}
}
}
其实无论是声明式事务还是编程式事务，都是走的PlatformTransactionManager的getTranscation()，commit()，rockback()。
四、事务的核心源码
基于前面的简单分析，我们能够得出结论，PlatformTransactionManager就是整个spring事务的核心接口：
public interface PlatformTransactionManager extends TransactionManager {
/**
* 根据指定的传播行为，返回当前活动的事务或创建一个新事务。
* 请注意，隔离级别或超时等参数仅适用于新事务，因此在参与活动事务时会被忽略。
* 此外，并非每个事务管理器都支持所有事务定义设置：当遇到不支持的设置时，正确的事务管理器实现应该抛出异常。
* 上述规则的一个例外是只读标志，如果不支持显式只读模式，则应忽略该标志。 本质上，只读标志只是潜在优化的提示。
*/
TransactionStatus getTransaction(@Nullable TransactionDefinition definition)
throws TransactionException;
/**
* 提交给定的事务根据其状态。
* 如果事务以编程方式标记为仅回滚，则执行回滚。
* 如果事务不是新事务，则省略提交以正确参与周围的事务。
* 如果先前的事务已暂停以便能够创建新事务，则在提交新事务后恢复先前的事务。
* 注意，当提交调用完成时，无论是正常还是抛出异常，都必须完全完成并清理事务
*/
void commit(TransactionStatus status) throws TransactionException;
/**
* 执行给定事务的回滚。
* 如果事务不是新事务，只需将其设置为仅回滚，以便正确参与周围的事务。
* 如果先前的事务已暂停以能够创建新事务，则在回滚新事务后恢复先前的事务。
* 如果提交引发异常，则不要在事务上调用回滚。
* 即使在提交异常的情况下，事务也将在提交返回时已经完成并清理。
* 因此，提交失败后的回滚调用将导致 IllegalTransactionStateException
*/
void rollback(TransactionStatus status) throws TransactionException;
4.1 getTransaction
大概的调用流程如下：
spring事务-getTransaction.png
代码如下：
/**
* This implementation handles propagation behavior. Delegates to
* {@code doGetTransaction}, {@code isExistingTransaction}
* and {@code doBegin}.
* 此实现处理传播行为。 委托doGetTransaction 、 isExistingTransaction和doBegin
* @see #doGetTransaction
* @see #isExistingTransaction
* @see #doBegin
*/
@Override
public final TransactionStatus getTransaction(@Nullable TransactionDefinition definition)
throws TransactionException {
// 如果没有给出事务定义，则使用默认值。
TransactionDefinition def = (definition != null ? definition : TransactionDefinition.withDefaults());
// DataSourceTransactionManager实现doGetTransaction方法，获取事务
Object transaction = doGetTransaction();
boolean debugEnabled = logger.isDebugEnabled();
// 找到事务
if (isExistingTransaction(transaction)) {
// 找到现有事务 -> 检查传播行为以了解行为方式
return handleExistingTransaction(def, transaction, debugEnabled);
}
// Check definition settings for new transaction.
if (def.getTimeout() < TransactionDefinition.TIMEOUT_DEFAULT) {
throw new InvalidTimeoutException("Invalid transaction timeout", def.getTimeout());
}
// 没找到现有事务 -> 检查传播行为以了解行为方式
if (def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_MANDATORY) {
throw new IllegalTransactionStateException(
"No existing transaction found for transaction marked with propagation 'mandatory'");
}
//如果事务传播机制是以下三种：required，requires_new，nested，则新建事务
else if (def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRED ||
def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW ||
def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {
SuspendedResourcesHolder suspendedResources = suspend(null);
if (debugEnabled) {
logger.debug("Creating new transaction with name [" + def.getName() + "]: " + def);
}
try {
//开启一个事务
return startTransaction(def, transaction, debugEnabled, suspendedResources);
}
catch (RuntimeException | Error ex) {
resume(null, suspendedResources);
throw ex;
}
}
else {
// 当前不存在事务，且传播机制=PROPAGATION_SUPPORTS/PROPAGATION_NOT_SUPPORTED/PROPAGATION_NEVER，这三种情况
if (def.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT && logger.isWarnEnabled()) {
logger.warn("Custom isolation level specified but no actual transaction initiated; " +
"isolation level will effectively be ignored: " + def);
}
boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
return prepareTransactionStatus(def, null, true, newSynchronization, debugEnabled, null);
}
}
doGetTransaction:
@Override
protected Object doGetTransaction() {
// 创建一个数据源事务管理对象
DataSourceTransactionObject txObject = new DataSourceTransactionObject();
// 设置是否允许嵌套事务，默认是false
txObject.setSavepointAllowed(isNestedTransactionAllowed());
// 获取jdbc连接
ConnectionHolder conHolder =
(ConnectionHolder) TransactionSynchronizationManager.getResource(obtainDataSource());
txObject.setConnectionHolder(conHolder, false);
return txObject;
}
isExistingTransaction:
@Override
protected boolean isExistingTransaction(Object transaction) {
DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
//是否持有链接 和 是否存在事务
return (txObject.hasConnectionHolder() && txObject.getConnectionHolder().isTransactionActive());
}
startTransaction：
/**
* Start a new transaction.
*/
private TransactionStatus startTransaction(TransactionDefinition definition, Object transaction,
boolean debugEnabled, @Nullable SuspendedResourcesHolder suspendedResources) {
// 默认不开启事务同步
boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
// 根据给定参数创建一个事务
DefaultTransactionStatus status = newTransactionStatus(
definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);
// 开始一个新事务
doBegin(transaction, definition);
// 设置事务的名称，只读、隔离级别等等
prepareSynchronization(status, definition);
return status;
}
doBegin：
@Override
protected void doBegin(Object transaction, TransactionDefinition definition) {
DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
Connection con = null;
try {
// 如果没有获取数据库连接 或者 是个同步事务
if (!txObject.hasConnectionHolder() ||
txObject.getConnectionHolder().isSynchronizedWithTransaction()) {
// 这里在获取一次数据连接
Connection newCon = obtainDataSource().getConnection();
if (logger.isDebugEnabled()) {
logger.debug("Acquired Connection [" + newCon + "] for JDBC transaction");
}
// 设置数据连接
txObject.setConnectionHolder(new ConnectionHolder(newCon), true);
}
// 开启事务同步
txObject.getConnectionHolder().setSynchronizedWithTransaction(true);
// 获取connection连接
con = txObject.getConnectionHolder().getConnection();
// 获取事务隔离级别
Integer previousIsolationLevel = DataSourceUtils.prepareConnectionForTransaction(con, definition);
// 设置事务隔离级别
txObject.setPreviousIsolationLevel(previousIsolationLevel);
txObject.setReadOnly(definition.isReadOnly());
// 如有必要，切换到手动提交。这在某些 JDBC 驱动程序中非常昂贵
// 所以我们不想做不必要的事情（例如，如果我们已经明确配置连接池来设置它）
if (con.getAutoCommit()) {
txObject.setMustRestoreAutoCommit(true);
if (logger.isDebugEnabled()) {
logger.debug("Switching JDBC Connection [" + con + "] to manual commit");
}
// 关闭自动提交
con.setAutoCommit(false);
}
prepareTransactionalConnection(con, definition);
//设置此持有者是代表活动的、由 JDBC 管理的事务。
txObject.getConnectionHolder().setTransactionActive(true);
int timeout = determineTimeout(definition);
if (timeout != TransactionDefinition.TIMEOUT_DEFAULT) {
txObject.getConnectionHolder().setTimeoutInSeconds(timeout);
}
// 将连接绑定到线程上
if (txObject.isNewConnectionHolder()) {
TransactionSynchronizationManager.bindResource(obtainDataSource(), txObject.getConnectionHolder());
}
}
catch (Throwable ex) {
if (txObject.isNewConnectionHolder()) {
//关闭数据链接
DataSourceUtils.releaseConnection(con, obtainDataSource());
txObject.setConnectionHolder(null, false);
}
throw new CannotCreateTransactionException("Could not open JDBC Connection for transaction", ex);
}
}
到此为止上面一连串的流程都是基于事务的传播机制是required，我们除此之外还必须要明白requires_new和nested的过程。
在getTransaction()方法中，由于首次创建，三种方式都是一样的流程。当事务方法内部的方法仍然使用事务的时候，存在三种不同的情况，主要看getTransaction中下面的方法：handleExistingTransaction()，这个方法我们主要关注required，requires_nes,nested:
requires_new
//如果事务时requires_new
if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW) {
if (debugEnabled) {
logger.debug("Suspending current transaction, creating new transaction with name [" +
definition.getName() + "]");
}
//暂停当前的事务
SuspendedResourcesHolder suspendedResources = suspend(transaction);
try {
//创建新事务
return startTransaction(definition, transaction, debugEnabled, suspendedResources);
}
catch (RuntimeException | Error beginEx) {
resumeAfterBeginException(transaction, suspendedResources, beginEx);
throw beginEx;
}
}
suspend方法：
@Nullable
protected final SuspendedResourcesHolder suspend(@Nullable Object transaction) throws TransactionException {
//存在同步
if (TransactionSynchronizationManager.isSynchronizationActive()) {
List<TransactionSynchronization> suspendedSynchronizations = doSuspendSynchronization();
try {
Object suspendedResources = null;
if (transaction != null) {
//事务不为空，挂起事务
suspendedResources = doSuspend(transaction);
}
//获取当前事务的属性
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);
//创建一个挂起资源持有者
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) {
// 存在事务但是没有同步，挂起事务
Object suspendedResources = doSuspend(transaction);
// 返回挂起资源持有者
return new SuspendedResourcesHolder(suspendedResources);
}
else {
// 既没有事务，也没有同步
return null;
}
}
doSuspend方法：
@Override
protected Object doSuspend(Object transaction) {
//获取当前事务的数据库连接对象，并置为空
DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
txObject.setConnectionHolder(null);
//从当前线程解除给定键的资源绑定。
return TransactionSynchronizationManager.unbindResource(obtainDataSource());
}
nested:
// nested
if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {
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() + "]");
}
// 使用保存点嵌套事务,非JTA走这
if (useSavepointForNestedTransaction()) {
// 在现有 Spring 管理的事务中创建保存点，
// 通过 TransactionStatus 实现的 SavepointManager API。
// 通常使用 JDBC 3.0 保存点。从不激活 Spring 同步。
DefaultTransactionStatus status =
prepareTransactionStatus(definition, transaction, false, false, debugEnabled, null);
//创建一个事务保存点
status.createAndHoldSavepoint();
return status;
}
else {
//JTA从这走，开启一个新事务
return startTransaction(definition, transaction, debugEnabled, null);
}
}
在handleExistingTransaction()存在每种传播机制的判断，不满足的会走最后一行代码：
// 不符合上面的传播行为，所以走默认的，包含required
// 关注第三个参数，newTransaction：false，不创建新事务
return prepareTransactionStatus(definition, transaction, false, newSynchronization, debugEnabled, null);
4.2 commit
这里关注invokeWithinTransaction中的commitTransactionAfterReturning方法：
protected void commitTransactionAfterReturning(@Nullable TransactionInfo txInfo) {
// 存在事务就提交，否则什么都不做
if (txInfo != null && txInfo.getTransactionStatus() != null) {
if (logger.isTraceEnabled()) {
logger.trace("Completing transaction for [" + txInfo.getJoinpointIdentification() + "]");
}
// 提交
txInfo.getTransactionManager().commit(txInfo.getTransactionStatus());
}
}
public final void commit(TransactionStatus status) throws TransactionException {
// 事务已完成
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;
// 如果事务明确标记位回滚
if (defStatus.isLocalRollbackOnly()) {
if (defStatus.isDebug()) {
logger.debug("Transactional code has requested rollback");
}
// 回滚
processRollback(defStatus, false);
return;
}
// 如果不需要全局回滚则应该提交 并且 全局回滚
if (!shouldCommitOnGlobalRollbackOnly() && defStatus.isGlobalRollbackOnly()) {
if (defStatus.isDebug()) {
logger.debug("Global transaction is marked as rollback-only but transactional code requested commit");
}
// 回滚
processRollback(defStatus, true);
return;
}
// 提交
processCommit(defStatus);
}
接下来只关注提交：processCommit(defStatus);
private void processCommit(DefaultTransactionStatus status) throws TransactionException {
try {
boolean beforeCompletionInvoked = false;
try {
boolean unexpectedRollback = false;
//三个前置操作
// 没有实现
prepareForCommit(status);
// 提交之前回调
triggerBeforeCommit(status);
// 完成前回调
triggerBeforeCompletion(status);
//设定前置操作完成
beforeCompletionInvoked = true;
//如果有保存点,即嵌套事务
if (status.hasSavepoint()) {
if (status.isDebug()) {
logger.debug("Releasing transaction savepoint");
}
// 是否是全局回滚
unexpectedRollback = status.isGlobalRollbackOnly();
// 释放保存点
status.releaseHeldSavepoint();
}
// 新事务
else if (status.isNewTransaction()) {
if (status.isDebug()) {
logger.debug("Initiating transaction commit");
}
// 是否是全局回滚
unexpectedRollback = status.isGlobalRollbackOnly();
// 提交
doCommit(status);
}
else if (isFailEarlyOnGlobalRollbackOnly()) {
// 是否是全局回滚
unexpectedRollback = status.isGlobalRollbackOnly();
}
// 抛出 UnexpectedRollbackException 如果我们有一个全局仅回滚
//标记但仍然没有从提交中获得相应的异常，手动抛出
if (unexpectedRollback) {
throw new UnexpectedRollbackException(
"Transaction silently rolled back because it has been marked as rollback-only");
}
}
catch (UnexpectedRollbackException ex) {
// 触发完成后，同步状态设置为回滚
triggerAfterCompletion(status, TransactionSynchronization.STATUS_ROLLED_BACK);
throw ex;
}
catch (TransactionException ex) {
// 提交失败则回滚
if (isRollbackOnCommitFailure()) {
doRollbackOnCommitException(status, ex);
}
else {
// 触发完成后，同步状态是未知
triggerAfterCompletion(status, TransactionSynchronization.STATUS_UNKNOWN);
}
throw ex;
}
catch (RuntimeException | Error ex) {
// 如果前三步未完成，调用前置第三个操作
if (!beforeCompletionInvoked) {
triggerBeforeCompletion(status);
}
// 提交失败回滚
doRollbackOnCommitException(status, ex);
throw ex;
}
try {
// 触发后置回调
triggerAfterCommit(status);
}
finally {
// 事务状态设置为已提交
triggerAfterCompletion(status, TransactionSynchronization.STATUS_COMMITTED);
}
}
finally {
// 完成后处理事务状态
cleanupAfterCompletion(status);
}
}
4.3 rollback
前面多次出现completeTransactionAfterThrowing方法，我们进入其内存看看毁掉方法的实现：
// PlatformTransactionManager的rollback方法
txInfo.getTransactionManager().rollback(txInfo.getTransactionStatus());
AbstractPlatformTransactionManager实现回调
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;
// 回调
processRollback(defStatus, false);
}
processRollback:
private void processRollback(DefaultTransactionStatus status, boolean unexpected) {
try {
// 默认false
boolean unexpectedRollback = unexpected;
try {
//触发前置回调
triggerBeforeCompletion(status);
//嵌套事务
if (status.hasSavepoint()) {
if (status.isDebug()) {
logger.debug("Rolling back transaction to savepoint");
}
//回滚保存点
status.rollbackToHeldSavepoint();
}
//新事务
else if (status.isNewTransaction()) {
if (status.isDebug()) {
logger.debug("Initiating transaction rollback");
}
//回滚
doRollback(status);
}
else {
// Participating in larger transaction
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");
}
// 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;
}
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 {
cleanupAfterCompletion(status);
}
}
4.4 cleanupAfterCompletion
这个方法无论是提交还是回滚，都是最后一步需要做的，我们看下其源码：
protected void doCleanupAfterCompletion(Object transaction) {
DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
// 清除线程的资源绑定
if (txObject.isNewConnectionHolder()) {
TransactionSynchronizationManager.unbindResource(obtainDataSource());
}
// 重置链接
Connection con = txObject.getConnectionHolder().getConnection();
try {
// 恢复自动提交
if (txObject.isMustRestoreAutoCommit()) {
con.setAutoCommit(true);
}
//重置链接的只读和隔离级别
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");
}
// 关闭数据链接
DataSourceUtils.releaseConnection(con, this.dataSource);
}
//清除持有者的属性
txObject.getConnectionHolder().clear();
}
-----------结束：源码很枯燥，文章写的一般，感谢大家支持--------------
@Transactional声明式事务，有什么短板？
不能在内部使用远程服务调用，当网络发生超时，会持续占用数据库连接池，不被释放，持续侵占连接池资源。
