@Transactional

事务的传播特性

什么是事务的传播特性？

指的是当一个事务方法被另一个事务方法调用时，这个事务方法该如何进行？

@Transactional
public void A(){
    // xx
    B();
    C();
    // xx
    // 出现异常
}

@Transactional
public void B(){
    // xx
    // 出现异常
}

@Transactional
public void C(){
    // xx
    // 出现异常
}

如上：事务A内部有事务B,事务C；如果A异常了，那么B,C怎么处理事务；如果B，C事务异常，那么事务A怎么处理？

Spring给出的7种事务的传播类型

/**
 * 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,
     * PROPAGATION_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 it 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 it 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 PROPAGATION_REQUIRED else. 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 when working on a JDBC 3.0 driver.
     * 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;
    }

}

PROPAGATION_REQUIRED

@Transactional(propagation = Propagation.REQUIRED)
public void methodA() {
    methodB();
    // do something
}

@Transactional(propagation = Propagation.REQUIRED)
public void methodB() {
    // do something
}

• 单独调用methodB方法时，因为当前上下文不存在事务，所以会开启一个新的事务。
• 调用methodA方法时，因为当前上下文不存在事务，所以会开启一个新的事务。当执行到methodB时，methodB发现当前上下文有事务，因此就加入到当前事务中来。

PROPAGATION_SUPPORTS

@Transactional(propagation = Propagation.REQUIRED)
public void methodA() {
    methodB();
    // do something
}

// 事务属性为SUPPORTS
@Transactional(propagation = Propagation.SUPPORTS)
public void methodB() {
    // do something
}

• 单纯的调用methodB时，methodB方法是非事务的执行的。
• 当调用methodA时,methodB则加入了methodA的事务中,事务地执行。

Spring的5种事务隔离级别

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

    /**
     * 这是一个PlatformTransactionManager默认的隔离级别，使用数据库默认的事务隔离级别
     *
     * Use the default isolation level of the underlying datastore.
     * All other levels correspond to the JDBC isolation levels.
     * @see java.sql.Connection
     */
    DEFAULT(TransactionDefinition.ISOLATION_DEFAULT),

    /**
     * A constant indicating that dirty reads, non-repeatable reads and phantom reads
     * can occur. This level allows a row changed by one transaction to be read by
     * another transaction before any changes in that row have been committed
     * (a "dirty read"). If any of the changes are rolled back, the second
     * transaction will have retrieved an invalid row.
     * @see java.sql.Connection#TRANSACTION_READ_UNCOMMITTED
     */
    READ_UNCOMMITTED(TransactionDefinition.ISOLATION_READ_UNCOMMITTED),

    /**
     * A constant indicating that dirty reads are prevented; non-repeatable reads
     * and phantom reads can occur. This level only prohibits a transaction
     * from reading a row with uncommitted changes in it.
     * @see java.sql.Connection#TRANSACTION_READ_COMMITTED
     */
    READ_COMMITTED(TransactionDefinition.ISOLATION_READ_COMMITTED),

    /**
     * A constant indicating that dirty reads and non-repeatable reads are
     * prevented; phantom reads can occur. This level prohibits a transaction
     * from reading a row with uncommitted changes in it, and it also prohibits
     * the situation where one transaction reads a row, a second transaction
     * alters the row, and the first transaction rereads the row, getting
     * different values the second time (a "non-repeatable read").
     * @see java.sql.Connection#TRANSACTION_REPEATABLE_READ
     */
    REPEATABLE_READ(TransactionDefinition.ISOLATION_REPEATABLE_READ),

    /**
     * A constant indicating that dirty reads, non-repeatable reads and phantom
     * reads are prevented. This level includes the prohibitions in
     * {@code ISOLATION_REPEATABLE_READ} and further prohibits the situation
     * where one transaction reads all rows that satisfy a {@code WHERE}
     * condition, a second transaction inserts a row that satisfies that
     * {@code WHERE} condition, and the first transaction rereads for the
     * same condition, retrieving the additional "phantom" row in the second read.
     * @see java.sql.Connection#TRANSACTION_SERIALIZABLE
     */
    SERIALIZABLE(TransactionDefinition.ISOLATION_SERIALIZABLE);


    private final int value;


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

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

}

rollbackFor属性

/**
 * Defines zero (0) or more exception {@link Class classes}, which must be
 * subclasses of {@link Throwable}, indicating which exception types must cause
 * a transaction rollback.
 * <p>By default, a transaction will be rolling back on {@link RuntimeException}
 * and {@link Error} but not on checked exceptions (business exceptions). See
 * {@link org.springframework.transaction.interceptor.DefaultTransactionAttribute#rollbackOn(Throwable)}
 * for a detailed explanation.
 * <p>This is the preferred way to construct a rollback rule (in contrast to
 * {@link #rollbackForClassName}), matching the exception class and its subclasses.
 * <p>Similar to {@link org.springframework.transaction.interceptor.RollbackRuleAttribute#RollbackRuleAttribute(Class clazz)}.
 * @see #rollbackForClassName
 * @see org.springframework.transaction.interceptor.DefaultTransactionAttribute#rollbackOn(Throwable)
 */
Class<? extends Throwable>[] rollbackFor() default {};

• roll back默认是Error和RuntimeException

eg1: 不加rollbackFor属性，抛出RuntimeException，正常回滚

@Transactional

eg2: 不加rollbackFor属性，抛出IOException，不回滚

@Transactional

eg3: 加上rollbackFor = Exception.class，抛出IOException，正常回滚

@Transactional(rollbackFor = Exception.class)

eg4: 不加rollbackFor属性，抛出OutOfMemoryError，正常回滚

@Transactional()

throw new OutOfMemoryError();

eg5: 加上rollbackFor = Exception.class，抛出OutOfMemoryError，正常回滚

@Transactional(rollbackFor = Exception.class)

throw new OutOfMemoryError();