聊聊spring的TransactionSynchronizationAdapter

序

本文主要研究一下spring的TransactionSynchronizationAdapter

示例代码

public void insert(TechBook techBook){
        bookMapper.insert(techBook);
       // send after tx commit but is async
        TransactionSynchronizationManager.registerSynchronization(new TransactionSynchronizationAdapter() {
            @Override
            public void afterCommit() {
                System.out.println("send email after transaction commit...");
            }
        }
       );
        System.out.println("service end");
    }

使用TransactionSynchronizationManager.registerSynchronization注册了一个TransactionSynchronizationAdapter，在其afterCommit方法也就是事务提交成功之后执行一些额外逻辑

TransactionSynchronizationAdapter

org/springframework/transaction/support/TransactionSynchronizationAdapter.java

public abstract class TransactionSynchronizationAdapter implements TransactionSynchronization, Ordered {

	@Override
	public int getOrder() {
		return Ordered.LOWEST_PRECEDENCE;
	}

	@Override
	public void suspend() {
	}

	@Override
	public void resume() {
	}

	@Override
	public void flush() {
	}

	@Override
	public void beforeCommit(boolean readOnly) {
	}

	@Override
	public void beforeCompletion() {
	}

	@Override
	public void afterCommit() {
	}

	@Override
	public void afterCompletion(int status) {
	}

}

TransactionSynchronizationAdapter是个抽象类，声明实现TransactionSynchronization及Ordered接口

TransactionSynchronization

org/springframework/transaction/support/TransactionSynchronization.java

	/**
	 * Invoked after transaction commit. Can perform further operations right
	 * after the main transaction has successfully committed.
	 * 
Can e.g. commit further operations that are supposed to follow on a successful
	 * commit of the main transaction, like confirmation messages or emails.
	 * 
NOTE: The transaction will have been committed already, but the
	 * transactional resources might still be active and accessible. As a consequence,
	 * any data access code triggered at this point will still "participate" in the
	 * original transaction, allowing to perform some cleanup (with no commit following
	 * anymore!), unless it explicitly declares that it needs to run in a separate
	 * transaction. Hence: Use {@code PROPAGATION_REQUIRES_NEW} for any
	 * transactional operation that is called from here.
	 * @throws RuntimeException in case of errors; will be propagated to the caller
	 * (note: do not throw TransactionException subclasses here!)
	 */
	default void afterCommit() {
	}

注意这里注释说明了异常不会被捕获，而且建议不在这里抛出TransactionException的子类；另外对于afterCommit有数据库相关操作的建议使用PROPAGATION_REQUIRES_NEW这个事务传播级别，不然afterCommit的操作可能不会生效

registerSynchronization

org/springframework/transaction/support/TransactionSynchronizationManager.java

	private static final ThreadLocal> synchronizations =
			new NamedThreadLocal<>("Transaction synchronizations");

	/**
	 * Register a new transaction synchronization for the current thread.
	 * Typically called by resource management code.
	 * 
Note that synchronizations can implement the
	 * {@link org.springframework.core.Ordered} interface.
	 * They will be executed in an order according to their order value (if any).
	 * @param synchronization the synchronization object to register
	 * @throws IllegalStateException if transaction synchronization is not active
	 * @see org.springframework.core.Ordered
	 */
	public static void registerSynchronization(TransactionSynchronization synchronization)
			throws IllegalStateException {

		Assert.notNull(synchronization, "TransactionSynchronization must not be null");
		Set synchs = synchronizations.get();
		if (synchs == null) {
			throw new IllegalStateException("Transaction synchronization is not active");
		}
		synchs.add(synchronization);
	}

TransactionSynchronizationManager的registerSynchronization方法会把TransactionSynchronization注册到当前线程的synchronizations

processCommit

org/springframework/transaction/support/AbstractPlatformTransactionManager.java

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);
		}
	}

	private void triggerAfterCommit(DefaultTransactionStatus status) {
		if (status.isNewSynchronization()) {
			if (status.isDebug()) {
				logger.trace("Triggering afterCommit synchronization");
			}
			TransactionSynchronizationUtils.triggerAfterCommit();
		}
	}

AbstractPlatformTransactionManager的processCommit方法，在提交成功之后触发triggerAfterCommit，这里调用了TransactionSynchronizationUtils.triggerAfterCommit()，注意这里没有try catch，说明triggerAfterCommit的异常最终会抛给调用方

triggerAfterCommit

org/springframework/transaction/support/TransactionSynchronizationUtils.java

	public static void triggerAfterCommit() {
		invokeAfterCommit(TransactionSynchronizationManager.getSynchronizations());
	}

	public static void invokeAfterCommit(@Nullable List synchronizations) {
		if (synchronizations != null) {
			for (TransactionSynchronization synchronization : synchronizations) {
				synchronization.afterCommit();
			}
		}
	}

这里遍历synchronizations执行afterCommit方法，如果其中有一个有异常抛出则中断

小结

使用TransactionSynchronizationManager.registerSynchronization可以在当前线程的事务注册一个TransactionSynchronizationAdapter，可以在afterCommit方法也就是事务提交成功之后执行一些额外逻辑；注意这里抛出的异常不影响事务提交，但是异常不会被catch需要由调用方处理，对于afterCommit有数据库相关操作的建议使用PROPAGATION_REQUIRES_NEW这个事务传播级别，不然afterCommit的db操作可能不会生效。

在事务提交之后做一些事情可能不需要TransactionSynchronizationManager.registerSynchronization这种方式也能实现，也就是需要额外一层来调用事务操作，有异常会抛出，没有异常则执行事务提交之后的事情，前提就是事务回滚异常不能被吞掉，不然外层调用可能以为事务成功了
还有一种方式就是使用TransactionalEventListener，这种方式比TransactionSynchronizationManager.registerSynchronization更为优雅一些

doc

• 如何在数据库事务提交成功后进行异步操作
• Spring事务aftercommit原理及实践