上一篇: [spring事务控制的原理解析1](https://www.cnblogs.com/chengxuxiaoyuan/p/16581334.html)
上一篇中总结了在spring中事务控制的基本原理,这一篇来记录下在spring源码中是如何实现的。
一、TransactionInterceptor
spring中的事务控制是通过aop实现的，提到aop肯定会有一个切面类,spring中处理事务的切面被封装成了
TransactionInterceptor
public class TransactionInterceptor extends TransactionAspectSupport implements MethodInterceptor, Serializable {
//事务切面执行时就会执行这个方法
public Object invoke(MethodInvocation invocation) throws Throwable {
Class<?> targetClass = (invocation.getThis() != null ? AopUtils.getTargetClass(invocation.getThis()) : null);
//这个方法定义在父类TransactionAspectSupport中
return invokeWithinTransaction(invocation.getMethod(), targetClass, invocation::proceed);
}
}
二、TransactionAspectSupport
public abstract class TransactionAspectSupport implements BeanFactoryAware, InitializingBean {
//带事务执行目标方法
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);
//获取事务管理器
final PlatformTransactionManager tm = determineTransactionManager(txAttr);
final String joinpointIdentification = methodIdentification(method, targetClass, txAttr);
if (txAttr == null || !(tm instanceof CallbackPreferringPlatformTransactionManager)) {
// Standard transaction demarcation with getTransaction and commit/rollback calls.
// 这个createTransactionIfNecessary就是开启事务的方法,开启事务其实就是获取数据库连接
//需要重点看下这个方法
TransactionInfo txInfo = createTransactionIfNecessary(tm, txAttr, joinpointIdentification);
Object retVal;
try {
// This is an around advice: Invoke the next interceptor in the chain.
// This will normally result in a target object being invoked.
//执行目标方法
retVal = invocation.proceedWithInvocation();
}
catch (Throwable ex) {
// target invocation exception
//回滚事务
completeTransactionAfterThrowing(txInfo, ex);
throw ex;
}
finally {
cleanupTransactionInfo(txInfo);
}
//提交事务
commitTransactionAfterReturning(txInfo);
return retVal;
}
else {
final ThrowableHolder throwableHolder = new ThrowableHolder();
// It's a CallbackPreferringPlatformTransactionManager: pass a TransactionCallback in.
try {
Object result = ((CallbackPreferringPlatformTransactionManager) tm).execute(txAttr, status -> {
TransactionInfo txInfo = prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);
try {
return invocation.proceedWithInvocation();
}
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);
}
});
// Check result state: It might indicate a Throwable to rethrow.
if (throwableHolder.throwable != null) {
throw throwableHolder.throwable;
}
return result;
}
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;
}
}
}
//开启事务的方法
protected TransactionInfo createTransactionIfNecessary(@Nullable PlatformTransactionManager tm,
@Nullable TransactionAttribute txAttr, final String joinpointIdentification) {
// If no name specified, apply method identification as transaction name.
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) {
//这里就是在开启事务，可以看到是调用了事务管理器
// PlatformTransactionManager#getTransaction方法，这和在spring中手动控制事务时
// 是一致的
status = tm.getTransaction(txAttr);
}
else {
if (logger.isDebugEnabled()) {
logger.debug("Skipping transactional joinpoint [" + joinpointIdentification +
"] because no transaction manager has been configured");
}
}
}
return prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);
}
}
所以根据上边的源码分析可以知道spring事务控制的关键是调用事务管理器的方法
PlatformTransactionManager#getTransaction 获取一个TransactionStatus 对象。
三、PlatformTransactionManager
这是一个接口,spring用抽象类 AbstractPlatformTransactionManager 给出了getTransaction方法的实现。
public abstract class AbstractPlatformTransactionManager implements PlatformTransactionManager, Serializable {
//这个方法就是在开启事务
public final TransactionStatus getTransaction(@Nullable TransactionDefinition definition) throws TransactionException {
//先获取下事务
Object transaction = doGetTransaction();
// Cache debug flag to avoid repeated checks.
boolean debugEnabled = logger.isDebugEnabled();
if (definition == null) {
// Use defaults if no transaction definition given.
definition = new DefaultTransactionDefinition();
}
//如果上边doGetTransaction获取到事务了就要处理事务已经存在的情况
if (isExistingTransaction(transaction)) {
// Existing transaction found -> check propagation behavior to find out how to behave.
// 处理事务已经存在的情况,这种情况肯定是事务方法之间互相调用，也就是事务的传播行为的处理
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());
}
//走到这里表示当前没有事务，需要新建事务，在这之前还是需要对传播行为做下校验
// No existing transaction found -> check propagation behavior to find out how to proceed.
//如果传播行为是PROPAGATION_MANDATORY(必要的),当前没有事务就抛出异常
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) {
//以上这几种传播行为时需要开启一个新的事务
//这个suspend方法是为了挂起就的事务,是为了处理A调用B，B需要在新事务中运行时这种情况要把
//旧的事务保存下来等B的事务提交后再恢复这个事务,
//由前一篇博文可以知道spring中的事务本质上就是一个数据库连接,这里其实就是把数据库连接
//保存起来
SuspendedResourcesHolder suspendedResources = suspend(null);
if (debugEnabled) {
logger.debug("Creating new transaction with name [" + definition.getName() + "]: " + definition);
}
try {
boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
//创建TransactionStatus对象
DefaultTransactionStatus status = newTransactionStatus(
definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);
//开启事务,这是一个抽象方法，由子类DataSourceTransactionManager实现
doBegin(transaction, definition);
prepareSynchronization(status, definition);
return status;
}
catch (RuntimeException | Error ex) {
resume(null, suspendedResources);
throw ex;
}
}
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);
}
}
//根据传播行为处理当前已存在事务的情况
private TransactionStatus handleExistingTransaction(
TransactionDefinition definition, Object transaction, boolean debugEnabled)
throws TransactionException {
//传播行为是PROPAGATION_NEVER就抛出异常
if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NEVER) {
throw new IllegalTransactionStateException(
"Existing transaction found for transaction marked with propagation 'never'");
}
//以下是传播行为是其他值时的处理,其中suspend方法就是在挂起旧事务,doBegin是开启新事务
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 {
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;
}
}
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.
boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
DefaultTransactionStatus status = newTransactionStatus(
definition, transaction, true, newSynchronization, debugEnabled, null);
doBegin(transaction, definition);
prepareSynchronization(status, definition);
return status;
}
}
// 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);
}
}
注意到开启事务其实就是得到了一个TransactionStatus对象,它是一个接口，有一个默认实现DefaultTransactionStatus,这个类中有一个属性suspendedResources用来存储被挂起的事务，可以理解成当需要开启新事务时旧的事务对象会被保存在新事务对象上，为了在新事务提交后还原它。
public class DefaultTransactionStatus extends AbstractTransactionStatus {
@Nullable
private final Object transaction;
private final boolean newTransaction;
private final boolean newSynchronization;
private final boolean readOnly;
private final boolean debug;
@Nullable
private final Object suspendedResources;//用来存储被挂起的事务
}
需要继续看下DataSourceTransactionManager#doBgein方法来看下事务到底是怎么开启的
四、DataSourceTransactionManager
public class DataSourceTransactionManager extends AbstractPlatformTransactionManager implements ResourceTransactionManager, InitializingBean {
// 这是开启事务的方法
protected void doBegin(Object transaction, TransactionDefinition definition) {
DataSourceTransactionManager.DataSourceTransactionObject txObject = (DataSourceTransactionManager.DataSourceTransactionObject)transaction;
Connection con = null;
try {
if (!txObject.hasConnectionHolder() || txObject.getConnectionHolder().isSynchronizedWithTransaction()) {
//注意这里从DataSource中获取到了一个数据库连接
Connection newCon = this.obtainDataSource().getConnection();
if (this.logger.isDebugEnabled()) {
this.logger.debug("Acquired Connection [" + newCon + "] for JDBC transaction");
}
//根据数据库连接创建Holder对象并设置到txObject
txObject.setConnectionHolder(new ConnectionHolder(newCon), true);
}
txObject.getConnectionHolder().setSynchronizedWithTransaction(true);
//这个连接和上边新建的那个是一样的
con = txObject.getConnectionHolder().getConnection();
Integer previousIsolationLevel = DataSourceUtils.prepareConnectionForTransaction(con, definition);
txObject.setPreviousIsolationLevel(previousIsolationLevel);
if (con.getAutoCommit()) {
txObject.setMustRestoreAutoCommit(true);
if (this.logger.isDebugEnabled()) {
this.logger.debug("Switching JDBC Connection [" + con + "] to manual commit");
}
//设置数据库连接的事务为手动提交
con.setAutoCommit(false);
}
this.prepareTransactionalConnection(con, definition);
txObject.getConnectionHolder().setTransactionActive(true);
int timeout = this.determineTimeout(definition);
if (timeout != -1) {
txObject.getConnectionHolder().setTimeoutInSeconds(timeout);
}
if (txObject.isNewConnectionHolder()) {
//注意这里,TransactionSynchronizationManager是spring提供的一个同步器,
//里边有多个ThreadLocal可以在线程范围内共享变量,
//这里以datasource为key把持有上边新建的connection的ConnectionHolder存到了这个
//同步器里，当dao层方法需要执行时就可以从这个同步器里再把数据库连接拿出来使用,
//这样就保证了多个dao方法用同一个数据库连接执行，就可以控制事务
TransactionSynchronizationManager.bindResource(this.obtainDataSource(), txObject.getConnectionHolder());
}
} catch (Throwable var7) {
if (txObject.isNewConnectionHolder()) {
DataSourceUtils.releaseConnection(con, this.obtainDataSource());
txObject.setConnectionHolder((ConnectionHolder)null, false);
}
throw new CannotCreateTransactionException("Could not open JDBC Connection for transaction", var7);
}
}
}
通过上边的分析可以知道事务控制的本质是spring把数据库连接存在TransactionSynchronizationManager
中,多个dao方法需要执行时都会从其中获取数据库连接，保证了使用同一个数据库连接，这就是事务控制的关键。
四、TransactionSynchronizationManager
这是spring提供的用来在线程范围内共享变量的工具类,spring控制事务就是用它在多个dao方法之间共享数据库连接
public abstract class TransactionSynchronizationManager {
//这里一堆的ThreadLocal就是用来在同一个线程范围内共享变量的。
private static final Log logger = LogFactory.getLog(TransactionSynchronizationManager.class);
private static final ThreadLocal<Map<Object, Object>> resources =
new NamedThreadLocal<>("Transactional resources");
private static final ThreadLocal<Set<TransactionSynchronization>> synchronizations =
new NamedThreadLocal<>("Transaction synchronizations");
private static final ThreadLocal<String> currentTransactionName =
new NamedThreadLocal<>("Current transaction name");
private static final ThreadLocal<Boolean> currentTransactionReadOnly =
new NamedThreadLocal<>("Current transaction read-only status");
private static final ThreadLocal<Integer> currentTransactionIsolationLevel =
new NamedThreadLocal<>("Current transaction isolation level");
private static final ThreadLocal<Boolean> actualTransactionActive =
new NamedThreadLocal<>("Actual transaction active");
...省略
}
}