了解一下事务的概念

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什么是事务

事务（Transaction），一般是指要做的或所做的事情。在计算机术语中是指访问并可能更新数据库中各种数据项的一个程序执行单元(unit)。要么全部执行，要么全部不执行。

事务的特性

事务有4个特性，简称ACID。

原子性（Atomicity）：一个事务是一个不可分割的工作单位，事务中包括的操作要么都做，要么都不做。
一致性（Consistency）：事务必须是使数据库从一个一致性状态变到另一个一致性状态。一致性与原子性是密切相关的。
隔离性（Isolation）：一个事务的执行不能被其他事务干扰。即一个事务内部的操作及使用的数据对并发的其他事务是隔离的，并发执行的各个事务之间不能互相干扰。
持久性（Durability）：一个事务一旦提交，它对数据库中数据的改变就应该是永久性的。接下来的其他操作或故障不应该对其有任何影响。

事务的传播机制

用@Transactional注解的Propagation属性可以指定事务的传播机制。

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 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 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;
	}

}

REQUIRED	当前存在事务，则加入这个事务；当前不存事务，则新建一个事务；
SUPPORTS	当前存在事务，则加入这个事务；当前不存在事务，则以非事务的方式执行；
MANDATORY	当前存在事务，则加入这个事务；当前不存事务，则抛出异常；
REQUIRES_NEW	创建一个新事务，如果当前存在事务，则暂停当前事务；
NOT_SUPPORTED	以非事务方式执行，如果存在则暂停当前事务；
NEVER	以非事务方式执行，如果存在事务则抛出异常。
NESTED	如果当前事务存在，则在嵌套事务中执行，类似于PROPAGATION_REQUIRED；