Spring 事务
Spring 事务是在数据库事务的基础上完成的，Spring 事务框架就是这样的一种管理事务的框架，它的作用和我们手动地使用mysql命令处理事务没有什么不同，只是做了一些更好的封装。可以思考一下如果我们想要写一个事务管理框架并交给 Spring 容器来管理，那么我们将面临哪些问题？我们得解决哪些问题？显然这个框架的问题域是 mysql 事务，那么 mysql 事务有哪些操作，用法，我们的框架就将有哪些用法，它能向下管理底层的 mysql 事务，那么它就会拥有向上提供mysql事务的能力，实际上这个框架就是mysql事务语句的java版的能力映射。比如，mybatis是mysql的中间件，一段mybatis代码可以映射为一条mysql 增删改查的语句，它们是一一对应的，就像翻译一样，对于其它 java 程序，mybatis就代表了一种数据库能力，所以，所有可能的mysql语句，都得被mybatis所能表达的能力覆盖，这就是问题域，它得解决哪些问题。
有两种事务使用方式，一种是设置本会话的autocommit=0,然后使用commit手动提交，另一种是使用begin 或start transaction开启事务，使用commit进行提交；代码中使用前者。
if (con.getAutoCommit()) {
txObject.setMustRestoreAutoCommit(true);
if (logger.isDebugEnabled()) {
logger.debug("Switching JDBC Connection [" + con + "] to manual commit");
}
con.setAutoCommit(false);
}在嵌套事务的内部，可以支持savepoint的机制设置回滚点，回滚到某个保存点后就不再继续回滚，这就是Spring事务所要支持的；只需要记得，两个@Transactional修饰的方法的嵌套调用，它们其实是两个代理逻辑就行，只不过需要注意要及时维护线程绑定的事务管理环境。
事务失效场景
我们可以从Spring 事务失效的场景来分析spring 事务要注意的方面。
@Transactional修饰的方法是private的而不是public的，或类和方法是被static，final修饰的，那么事务失效，因为Spring 事务是通过Spring Aop来完成的，Spring Aop底层使用了 Cglib 动态代理，而Cglib 是通过子类继承和覆写来进行功能增强，final类不能被继承，final方法无法覆写，static修饰的方法是类的方法，不属于任何对象，代理是通过代理的方式代理某个对象，所以static方法不能被重写，即便写法上是重写，但是并不具备重写的含义，也就是说static方法也不被进行动态代理。
当事务发生异常时，该异常不是Spring 事务支持的异常，事务失效。可以在rollbackFor进行配置。或者catch了异常，但没有抛出异常（比如throw new RuntimeException()），也会导致事务失效，未抛出异常，事务不会回滚。
@Transactional所在类没有被Spring 容器所管理。解决方法，加@Service注解。
this的问题，当它自己调用自己时，要使用代理类去调用。可以通过AopContext.currentProxy()获取本对象的代理类。
多线程导致事务失效，不同线程获取到的数据库连接不一样，SqlSession会话也不一样，属于不同的事务。只有同一个数据库连接，才能进行提交和回滚。
数据源没有配置事务管理器，导致事务失效。
传播类型不支持事务，导致事务失效。这个需要注意。
这些事务失效场景在Spring 事务框架代码是怎么体现的？比如嵌套事务是怎么实现的，回滚和保存点在哪里？这些都需要去查看代码。
事务执行过程
因为Spring 事务是通过Spring Aop来实现的，那么它就会进入到拦截器链去执行。在ReflectiveMethodInvocation#proceed() 打断点。
发现拦截器链里只有一个拦截器，叫TransactionInterceptor，执行完这个拦截器后就会进入invokeJoinpoint() 方法去执行原始的方法。TransactionInterceptor里会完成Spring 事务的封装操作。Spring 事务对事务的封装操作主要依赖于事务管理器，比如我们mybatis就会使用DataSourceTransactionManager，DataSourceTransactionManager的大部分操作都在它的父类AbstractPlatformTransactionManager中完成。
public class TransactionInterceptor extends TransactionAspectSupport implements MethodInterceptor, Serializable {
public Object invoke(MethodInvocation invocation) throws Throwable {
Class targetClass = (invocation.getThis() != null ? AopUtils.getTargetClass(invocation.getThis()) : null);
return invokeWithinTransaction(invocation.getMethod(), targetClass, new CoroutinesInvocationCallback() {
@Override
@Nullable
public Object proceedWithInvocation() throws Throwable {
return invocation.proceed();
}
@Override
public Object getTarget() {
return invocation.getThis();
}
@Override
public Object[] getArguments() {
return invocation.getArguments();
}
});
}
}TransactionAspectSupport#invokeWithinTransaction()方法比较长，我截取它实际运行的逻辑代码。这个方法里调用了很多的子方法。我们可以通过查看这些子方法来了解它的内部实现过程。因为我们的demo 使用了@Transactional 注解，所以是声明式事务，如果是编程式事务会走另一套逻辑，所以这里走声明式事务的逻辑。
protected Object invokeWithinTransaction(Method method, @Nullable Class targetClass,
final InvocationCallback invocation) throws Throwable {
// If the transaction attribute is null, the method is non-transactional.
TransactionAttributeSource tas = getTransactionAttributeSource();
final TransactionAttribute txAttr = (tas != null ? tas.getTransactionAttribute(method, targetClass) : null);
final TransactionManager tm = determineTransactionManager(txAttr);
PlatformTransactionManager ptm = asPlatformTransactionManager(tm);
final String joinpointIdentification = methodIdentification(method, targetClass, txAttr);
if (txAttr == null || !(ptm instanceof CallbackPreferringPlatformTransactionManager)) {
// Standard transaction demarcation with getTransaction and commit/rollback calls.
TransactionInfo txInfo = createTransactionIfNecessary(ptm, txAttr, joinpointIdentification);
Object retVal;
try {
retVal = invocation.proceedWithInvocation();
}
catch (Throwable ex) {
completeTransactionAfterThrowing(txInfo, ex);
throw ex;
}
finally {
cleanupTransactionInfo(txInfo);
}
if (retVal != null && vavrPresent && VavrDelegate.isVavrTry(retVal)) {
TransactionStatus status = txInfo.getTransactionStatus();
if (status != null && txAttr != null) {
retVal = VavrDelegate.evaluateTryFailure(retVal, txAttr, status);
}
}
commitTransactionAfterReturning(txInfo);
return retVal;
}
}里面最重要的方法就是 createTransactionIfNecessary() ,
protected TransactionInfo createTransactionIfNecessary(@Nullable PlatformTransactionManager tm,
@Nullable TransactionAttribute txAttr, final String joinpointIdentification) {
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) {
status = tm.getTransaction(txAttr);
}
else {
...
}
}
return prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);
}getTransaction()获取事务。
public abstract class AbstractPlatformTransactionManager implements PlatformTransactionManager, Serializable {
@Override
public final TransactionStatus getTransaction(@Nullable TransactionDefinition definition)
throws TransactionException {
TransactionDefinition def = (definition != null ? definition : TransactionDefinition.withDefaults());
Object transaction = doGetTransaction();
boolean debugEnabled = logger.isDebugEnabled();
if (isExistingTransaction(transaction)) {
return handleExistingTransaction(def, transaction, debugEnabled);
}
if (def.getTimeout() < TransactionDefinition.TIMEOUT_DEFAULT) {
throw new InvalidTimeoutException("Invalid transaction timeout", def.getTimeout());
}
// No existing transaction found -> check propagation behavior to find out how to proceed.
if (def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_MANDATORY) {
throw new IllegalTransactionStateException(
"No existing transaction found for transaction marked with propagation 'mandatory'");
}
else if (def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRED ||
def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW ||
def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {
SuspendedResourcesHolder suspendedResources = suspend(null);
try {
return startTransaction(def, transaction, debugEnabled, suspendedResources);
}
catch (RuntimeException | Error ex) {
resume(null, suspendedResources);
throw ex;
}
}
else {
...
boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
return prepareTransactionStatus(def, null, true, newSynchronization, debugEnabled, null);
}
}
}如果在准备创建事务时发现已存在事务，那么就会根据事务传播规则创建新事务或抛出异常。旧事务会被suspend()悬挂起来，等到恢复执行时会被resume()，它的信息我们使用SuspendedResourcesHolder 保存起来，所以新事务里会保存上一个事务的所有信息，所以事务可以无限嵌套下去。
private TransactionStatus handleExistingTransaction(
TransactionDefinition definition, Object transaction, boolean debugEnabled)
throws TransactionException {
if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NEVER) {
throw new IllegalTransactionStateException(
"Existing transaction found for transaction marked with propagation 'never'");
}
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);
}
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 {
return startTransaction(definition, transaction, debugEnabled, suspendedResources);
}
catch (RuntimeException | Error beginEx) {
resumeAfterBeginException(transaction, suspendedResources, beginEx);
throw beginEx;
}
}
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() + "]");
}
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.
DefaultTransactionStatus status =
prepareTransactionStatus(definition, transaction, false, false, debugEnabled, null);
status.createAndHoldSavepoint();
return status;
}
else {
// 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.
return startTransaction(definition, transaction, debugEnabled, null);
}
}
// 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() != SYNCHRONIZATION_NEVER);
return prepareTransactionStatus(definition, transaction, false, newSynchronization, debugEnabled, null);
}注意，只有一种情况下会创建保存点，那就是事务传播行为为PROPAGATION_NESTED时。这个可以通过查看代码得知。
最后准备好了一个新事务的所有信息放入TransactionInfo对象中，并把该对象放入ThreadLocal绑定到线程里。
然后开始执行@Transactional修饰的方法。
然后处理回滚信息，这个和保存点有关，遇到保存点就不再继续回滚了。然后执行提交。事务执行完毕。清理事务信息。
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, false);
return;
}
if (!shouldCommitOnGlobalRollbackOnly() && defStatus.isGlobalRollbackOnly()) {
if (defStatus.isDebug()) {
logger.debug("Global transaction is marked as rollback-only but transactional code requested commit");
}
processRollback(defStatus, true);
return;
}
processCommit(defStatus);
}
private void processCommit(DefaultTransactionStatus status) throws TransactionException {
try {
boolean beforeCompletionInvoked = false;
try {
boolean unexpectedRollback = false;
prepareForCommit(status);
triggerBeforeCommit(status);
triggerBeforeCompletion(status);
beforeCompletionInvoked = true;
if (status.hasSavepoint()) {
if (status.isDebug()) {
logger.debug("Releasing transaction savepoint");
}
unexpectedRollback = status.isGlobalRollbackOnly();
status.releaseHeldSavepoint();
}
else if (status.isNewTransaction()) {
if (status.isDebug()) {
logger.debug("Initiating transaction commit");
}
unexpectedRollback = status.isGlobalRollbackOnly();
doCommit(status);
}
else if (isFailEarlyOnGlobalRollbackOnly()) {
unexpectedRollback = status.isGlobalRollbackOnly();
}
// Throw UnexpectedRollbackException if we have a global rollback-only
// marker but still didn't get a corresponding exception from commit.
if (unexpectedRollback) {
throw new UnexpectedRollbackException(
"Transaction silently rolled back because it has been marked as rollback-only");
}
}
catch (UnexpectedRollbackException ex) {
// can only be caused by doCommit
triggerAfterCompletion(status, TransactionSynchronization.STATUS_ROLLED_BACK);
throw ex;
}
catch (TransactionException ex) {
// can only be caused by doCommit
if (isRollbackOnCommitFailure()) {
doRollbackOnCommitException(status, ex);
}
else {
triggerAfterCompletion(status, TransactionSynchronization.STATUS_UNKNOWN);
}
throw ex;
}
catch (RuntimeException | Error ex) {
if (!beforeCompletionInvoked) {
triggerBeforeCompletion(status);
}
doRollbackOnCommitException(status, ex);
throw ex;
}
// Trigger afterCommit callbacks, with an exception thrown there
// propagated to callers but the transaction still considered as committed.
try {
triggerAfterCommit(status);
}
finally {
triggerAfterCompletion(status, TransactionSynchronization.STATUS_COMMITTED);
}
}
finally {
cleanupAfterCompletion(status);
}
}在 cleanupAfterCompletion(status) 方法中，如果本事务之前存在事务，它会通过resume()方法唤醒老事务继续执行。
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resume()中把原来的旧的事务信息再绑定到本线程中去。TransactionSynchronization是事务扩展的钩子。
ps：在使用Spring 嵌套事务时，如果两个方法都操作同一张表，很可能会导致发生死锁，我刚好就遇到过。
未完待续。。。