Spring事务源码分析
Posted 张小贱1987
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首先看例子，这例子摘抄自开涛的跟我学spring3。
@Test public void testPlatformTransactionManager() { DefaultTransactionDefinition def = new DefaultTransactionDefinition(); def.setIsolationLevel(TransactionDefinition.ISOLATION_READ_COMMITTED); def.setPropagationBehavior(TransactionDefinition.PROPAGATION_REQUIRED); TransactionStatus status = txManager.getTransaction(def); try { jdbcTemplate.update(INSERT_SQL, "test"); txManager.commit(status); } catch (RuntimeException e) { txManager.rollback(status); } } |
重要的代码在上面高亮处。
在执行jdbcTemplate.update的时候使用的是datasource.getConection获取连接。
实际上，
- 在执行txManager.getTransaction(def);的时候，应该会设置：conection.setAutoConmmit(false)。
- 在执行txManager.commit(status);的时候，应该是执行conection.commit();
- 在执行txManager. rollback (status);的时候，应该是执行conection. rollback ();
但是，Spring是如何保证，txManager中的conn就是jdbcTemplate中的conn的呢。从这点出发，开始看源代码。
因为是执行的jdbc操作，这里的txManager是DataSourceTransactionManager。我们来看代码：
getTransaction方法：
getTransaction方法在DataSourceTransactionManager的超类中，也就是AbstractPlatformTransactionManager，我们来看方法：
public final TransactionStatus getTransaction(TransactionDefinition definition) throws TransactionException { 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(); }
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. 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) { SuspendedResourcesHolder suspendedResources = suspend(null); if (debugEnabled) { logger.debug("Creating new transaction with name [" + definition.getName() + "]: " + definition); } 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 ex) { resume(null, suspendedResources); throw ex; } catch (Error err) { resume(null, suspendedResources); throw err; } } else { // Create "empty" transaction: no actual transaction, but potentially synchronization. boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS); return prepareTransactionStatus(definition, null, true, newSynchronization, debugEnabled, null); } } |
先看第一句，
Object transaction = doGetTransaction();
方法在AbstractPlatformTransactionManager中，方法为：
protected abstract Object doGetTransaction() throws TransactionException;
这是典型的模板方法设计模式，AbstractPlatformTransactionManager作为抽象类，定义了getTransaction方法，并且设置为final，然后方法内部调用的部分方法是protected abstract的，交给子类去实现。
我们来看在DataSourceTransactionManager类中的doGetTransaction方法的定义：
@Override protected Object doGetTransaction() { DataSourceTransactionObject txObject = new DataSourceTransactionObject(); txObject.setSavepointAllowed(isNestedTransactionAllowed()); ConnectionHolder conHolder = (ConnectionHolder) TransactionSynchronizationManager.getResource(this.dataSource); txObject.setConnectionHolder(conHolder, false); return txObject; } |
注意这里，是new了一个DataSourceTransactionObject对象，重要的是高亮的两句。txObject中有一个ConnectionHolder对象，这么说来，在这一步的时候有可能已经在事务对象（DataSourceTransactionObject）中，保存了一个ConnectionHolder对象，顾名思义，ConnectionHolder中必然有Connection。如果是这样，我们只要确定，在执行jdbc操作的时候使用的Connection和这个ConnectionHolder中的是同一个就可以了。我们先看ConnectionHolder的结构。
确实如我们所想。
我们再看TransactionSynchronizationManager.getResource(this.dataSource);代码如何获取ConnectionHolder的。
TransactionSynchronizationManager这个名字，应该是支持多线程并发读取的。我们看代码。
public static Object getResource(Object key) { Object actualKey = TransactionSynchronizationUtils.unwrapResourceIfNecessary(key); Object value = doGetResource(actualKey); if (value != null && logger.isTraceEnabled()) { logger.trace("Retrieved value [" + value + "] for key [" + actualKey + "] bound to thread [" + Thread.currentThread().getName() + "]"); } return value; } |
看Object value = doGetResource(actualKey);代码：
private static Object doGetResource(Object actualKey) { Map<Object, Object> map = resources.get(); if (map == null) { return null; } Object value = map.get(actualKey); // Transparently remove ResourceHolder that was marked as void... if (value instanceof ResourceHolder && ((ResourceHolder) value).isVoid()) { map.remove(actualKey); // Remove entire ThreadLocal if empty... if (map.isEmpty()) { resources.remove(); } value = null; } return value; } |
高亮代码，看起来就是从一个map中获取了返回的结果，获取的时候使用的key是上一个方法传入的datasource。
看看这个map是什么。
private static final ThreadLocal<Map<Object, Object>> resources = new NamedThreadLocal<Map<Object, Object>>("Transactional resources"); |
看来是ThreadLocal对象。
那么这个对象是在什么时候初始化的呢。
经过查看是在这个方法：
public static void bindResource(Object key, Object value) throws IllegalStateException { |
那么那个地方调了这个方法呢？
经过查看，又回到了DataSourceTransactionManager类：
@Override protected void doResume(Object transaction, Object suspendedResources) { ConnectionHolder conHolder = (ConnectionHolder) suspendedResources; TransactionSynchronizationManager.bindResource(this.dataSource, conHolder); } |
但是这个是在事务执行完毕的时候执行的，所以如果我们是第一次在当前线程执行事务，那么回到最初的代码：
public final TransactionStatus getTransaction(TransactionDefinition definition) throws TransactionException { 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(); }
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. 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) { SuspendedResourcesHolder suspendedResources = suspend(null); if (debugEnabled) { logger.debug("Creating new transaction with name [" + definition.getName() + "]: " + definition); } 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 ex) { resume(null, suspendedResources); throw ex; } catch (Error err) { resume(null, suspendedResources); throw err; } } else { // Create "empty" transactio 以上是关于Spring事务源码分析的主要内容，如果未能解决你的问题，请参考以下文章 |