聊聊spring事务的propagation

序

本文主要研究一下spring事务的propagation

Propagation

org/springframework/transaction/annotation/Propagation.java

/**
 * Enumeration that represents transaction propagation behaviors for use
 * with the {@link Transactional} annotation, corresponding to the
 * {@link TransactionDefinition} interface.
 *
 * @author Colin Sampaleanu
 * @author Juergen Hoeller
 * @since 1.2
 */
public enum Propagation {

    /**
     * Support a current transaction, create a new one if none exists.
     * Analogous to EJB transaction attribute of the same name.
     * <p>This is the default setting of a transaction annotation.
     */
    REQUIRED(TransactionDefinition.PROPAGATION_REQUIRED),

    /**
     * Support a current transaction, execute non-transactionally if none exists.
     * Analogous to EJB transaction attribute of the same name.
     * <p>Note: For transaction managers with transaction synchronization,
     * {@code SUPPORTS} is slightly different from no transaction at all,
     * as it defines a transaction scope that synchronization will apply for.
     * As a consequence, the same resources (JDBC Connection, Hibernate Session, etc)
     * will be shared for the entire specified scope. Note that this depends on
     * the actual synchronization configuration of the transaction manager.
     * @see org.springframework.transaction.support.AbstractPlatformTransactionManager#setTransactionSynchronization
     */
    SUPPORTS(TransactionDefinition.PROPAGATION_SUPPORTS),

    /**
     * Support a current transaction, throw an exception if none exists.
     * Analogous to EJB transaction attribute of the same name.
     */
    MANDATORY(TransactionDefinition.PROPAGATION_MANDATORY),

    /**
     * Create a new transaction, and suspend the current transaction if one exists.
     * Analogous to the EJB transaction attribute of the same name.
     * <p><b>NOTE:</b> Actual transaction suspension will not work out-of-the-box
     * on all transaction managers. This in particular applies to
     * {@link org.springframework.transaction.jta.JtaTransactionManager},
     * which requires the {@code javax.transaction.TransactionManager} to be
     * made available to it (which is server-specific in standard Java EE).
     * @see org.springframework.transaction.jta.JtaTransactionManager#setTransactionManager
     */
    REQUIRES_NEW(TransactionDefinition.PROPAGATION_REQUIRES_NEW),

    /**
     * Execute non-transactionally, suspend the current transaction if one exists.
     * Analogous to EJB transaction attribute of the same name.
     * <p><b>NOTE:</b> Actual transaction suspension will not work out-of-the-box
     * on all transaction managers. This in particular applies to
     * {@link org.springframework.transaction.jta.JtaTransactionManager},
     * which requires the {@code javax.transaction.TransactionManager} to be
     * made available to it (which is server-specific in standard Java EE).
     * @see org.springframework.transaction.jta.JtaTransactionManager#setTransactionManager
     */
    NOT_SUPPORTED(TransactionDefinition.PROPAGATION_NOT_SUPPORTED),

    /**
     * Execute non-transactionally, throw an exception if a transaction exists.
     * Analogous to EJB transaction attribute of the same name.
     */
    NEVER(TransactionDefinition.PROPAGATION_NEVER),

    /**
     * Execute within a nested transaction if a current transaction exists,
     * behave like {@code REQUIRED} otherwise. There is no analogous feature in EJB.
     * <p>Note: Actual creation of a nested transaction will only work on specific
     * transaction managers. Out of the box, this only applies to the JDBC
     * DataSourceTransactionManager. Some JTA providers might support nested
     * transactions as well.
     * @see org.springframework.jdbc.datasource.DataSourceTransactionManager
     */
    NESTED(TransactionDefinition.PROPAGATION_NESTED);


    private final int value;


    Propagation(int value) {
        this.value = value;
    }

    public int value() {
        return this.value;
    }

}

spring事务定义了Propagation枚举，主要有REQUIRED、SUPPORTS、MANDATORY、REQUIRES_NEW、NOT_SUPPORTED、NEVER、NESTED

AbstractPlatformTransactionManager

org/springframework/transaction/support/AbstractPlatformTransactionManager.java

    /**
     * Create a TransactionStatus for an existing transaction.
     */
    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 {
                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);
    }

AbstractPlatformTransactionManager的handleExistingTransaction方法对各个传播级别进行了处理，比如针对PROPAGATION_REQUIRES_NEW，它会执行suspend当前事务，然后newTransactionStatus

小结

spring事务提供了各种传播级别可以设置，它主要是为了方便处理事务嵌套的场景，可以支持里外共用一个事务，或者里头新开事务等等，这样子可以解决里外重复开事务等问题。但是这个用多了也要注意，在调用的时候需要设置什么传播级别。其实其他语言可能没有事务传播这一说，一般就是尽量把代码写简单点，少用事务嵌套。