关于spring:聊聊spring的TransactionSynchronizationAdapter

序

本文次要钻研一下 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");
    }

应用 TransactionSynchronizationManager.registerSynchronization 注册了一个 TransactionSynchronizationAdapter，在其 afterCommit 办法也就是事务提交胜利之后执行一些额定逻辑

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，申明实现 TransactionSynchronization 及 Ordered 接口

TransactionSynchronization

org/springframework/transaction/support/TransactionSynchronization.java

    /**
     * Invoked after transaction commit. Can perform further operations right
     * <i>after</i> the main transaction has <i>successfully</i> committed.
     * <p>Can e.g. commit further operations that are supposed to follow on a successful
     * commit of the main transaction, like confirmation messages or emails.
     * <p><b>NOTE:</b> 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: <b>Use {@code PROPAGATION_REQUIRES_NEW} for any
     * transactional operation that is called from here.</b>
     * @throws RuntimeException in case of errors; will be <b>propagated to the caller</b>
     * (note: do not throw TransactionException subclasses here!)
     */
    default void afterCommit() {}

留神这里正文阐明了异样不会被捕捉，而且倡议不在这里抛出 TransactionException 的子类；另外对于 afterCommit 有数据库相干操作的倡议应用 PROPAGATION_REQUIRES_NEW 这个事务流传级别，不然 afterCommit 的操作可能不会失效

registerSynchronization

org/springframework/transaction/support/TransactionSynchronizationManager.java

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

    /**
     * Register a new transaction synchronization for the current thread.
     * Typically called by resource management code.
     * <p>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<TransactionSynchronization> synchs = synchronizations.get();
        if (synchs == null) {throw new IllegalStateException("Transaction synchronization is not active");
        }
        synchs.add(synchronization);
    }

TransactionSynchronizationManager 的 registerSynchronization 办法会把 TransactionSynchronization 注册到以后线程的 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 办法，在提交胜利之后触发 triggerAfterCommit，这里调用了 TransactionSynchronizationUtils.triggerAfterCommit()，留神这里没有 try catch，阐明 triggerAfterCommit 的异样最终会抛给调用方

triggerAfterCommit

org/springframework/transaction/support/TransactionSynchronizationUtils.java

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

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

这里遍历 synchronizations 执行 afterCommit 办法，如果其中有一个有异样抛出则中断

小结

应用 TransactionSynchronizationManager.registerSynchronization 能够在以后线程的事务注册一个 TransactionSynchronizationAdapter，能够在 afterCommit 办法也就是事务提交胜利之后执行一些额定逻辑；留神这里抛出的异样不影响事务提交，然而异样不会被 catch 须要由调用方解决，对于 afterCommit 有数据库相干操作的倡议应用 PROPAGATION_REQUIRES_NEW 这个事务流传级别，不然 afterCommit 的 db 操作可能不会失效。

在事务提交之后做一些事件可能不须要 TransactionSynchronizationManager.registerSynchronization 这种形式也能实现，也就是须要额定一层来调用事务操作，有异样会抛出，没有异样则执行事务提交之后的事件，前提就是事务回滚异样不能被吞掉，不然外层调用可能认为事务胜利了
还有一种形式就是应用 TransactionalEventListener，这种形式比 TransactionSynchronizationManager.registerSynchronization 更为优雅一些

doc

• 如何在数据库事务提交胜利后进行异步操作
• Spring 事务 aftercommit 原理及实际