Spring事务传播机制和数据库隔离级别
在标准SQL规范中定义了4个事务隔离级别，不同隔离级别对事务处理不同 。
未授权读取（Read Uncommitted）： 也称 未提交读。允许脏读取但不允许更新丢失，如果一个事务已经开始写数据则另外一个数据则不允许同时进行写操作但允许其他事务读此行数据。该隔离级别可以通过 “排他写锁”实现。事务隔离的最低级别，仅可保证不读取物理损坏的数据。与READ COMMITTED 隔离级相反，它允许读取已经被其它用户修改但尚未提交确定的数据。
授权读取（Read Committed）： 也称提交 读。允许不可重复读取但不允许脏读取。这可以通过“瞬间共享读锁”和“排他写锁”实现，读取数据的事务允许其他事务继续访问该行数据，但是未提交写事务将 会禁止其他事务访问该行。SQL Server 默认的级别。在此隔离级下，SELECT 命令不会返回尚未提交（Committed） 的数据，也不能返回脏数据。
可重复读取（Repeatable Read）： 禁止 不可重复读取和脏读取。但是有时可能出现幻影数据，这可以通过“共享读锁”和“排他写锁”实现，读取数据事务将会禁止写事务（但允许读事务），写事务则禁 止任何其他事务。在此隔离级下，用SELECT 命令读取的数据在整个命令执行过程中不会被更改。此选项会影响系统的效能，非必要情况最好不用此隔离级。
串行（Serializable）： 也称可串行读。提 供严格的事务隔离，它要求事务序列化执行，事务只能一个接着一个地执行，但不能并发执行。如果仅仅通过“行级锁”是无法实现事务序列化的，必须通过其他机 制保证新插入的数据不会被刚执行查询操作事务访问到。事务隔离的最高级别，事务之间完全隔离。如果事务在可串行读隔离级别上运行，则可以保证任何并发重叠 事务均是串行的。
| 隔离级别 | 更新丢失 | 脏读取 | 重复读取 | 幻读 |
|---|---|---|---|---|
| 未授权读取 | N | Y | Y | Y |
| 授权读取 | N | N | Y | Y |
| 可重复读取 | N | N | N | Y |
| 串行 | N | N | N | N |
Spring在TransactionDefinition接口中规定了7种类型的事务传播行为，它们规定了事务方法和事务方法发生嵌套调用时事务如何进行传播：
package org.springframework.transaction.annotation;
import org.springframework.transaction.TransactionDefinition;
/**
* 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.
* 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.
*
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.
*
Note: Actual transaction suspension will not work out-of-the-box on
* all transaction managers. This in particular applies to JtaTransactionManager,
* which requires the {@code javax.transaction.TransactionManager} to be
* made available it to it (which is server-specific in standard J2EE).
* @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.
*
Note: Actual transaction suspension will not work on out-of-the-box
* on all transaction managers. This in particular applies to JtaTransactionManager,
* which requires the {@code javax.transaction.TransactionManager} to be
* made available it to it (which is server-specific in standard J2EE).
* @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.
*
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
通过创建Savepoint实现嵌套事务，达到内层事务若抛出异常(unchecked exception)则回滚到savepoint处，但不影响外层事务；外层事务的回滚会一起回滚内层事务；
*/
NESTED(TransactionDefinition.PROPAGATION_NESTED);
private final int value;
Propagation(int value) { this.value = value; }
public int value() { return this.value; }
}
/*
* Copyright 2002-2012 the original author or authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.springframework.transaction;
import java.sql.Connection;
/**
* Interface that defines Spring-compliant transaction properties.
* Based on the propagation behavior definitions analogous to EJB CMT attributes.
*
* Note that isolation level and timeout settings will not get applied unless
* an actual new transaction gets started. As only {@link #PROPAGATION_REQUIRED},
* {@link #PROPAGATION_REQUIRES_NEW} and {@link #PROPAGATION_NESTED} can cause
* that, it usually doesn't make sense to specify those settings in other cases.
* Furthermore, be aware that not all transaction managers will support those
* advanced features and thus might throw corresponding exceptions when given
* non-default values.
*
*
The {@link #isReadOnly() read-only flag} applies to any transaction context,
* whether backed by an actual resource transaction or operating non-transactionally
* at the resource level. In the latter case, the flag will only apply to managed
* resources within the application, such as a Hibernate {@code Session}.
*
* @author Juergen Hoeller
* @since 08.05.2003
* @see PlatformTransactionManager#getTransaction(TransactionDefinition)
* @see org.springframework.transaction.support.DefaultTransactionDefinition
* @see org.springframework.transaction.interceptor.TransactionAttribute
*/
public interface TransactionDefinition {
/**
* Support a current transaction; create a new one if none exists.
* Analogous to the EJB transaction attribute of the same name.
*
This is typically the default setting of a transaction definition,
* and typically defines a transaction synchronization scope.
*/
int PROPAGATION_REQUIRED = 0;
/**
* Support a current transaction; execute non-transactionally if none exists.
* Analogous to the EJB transaction attribute of the same name.
*
NOTE: For transaction managers with transaction synchronization,
* {@code PROPAGATION_SUPPORTS} is slightly different from no transaction
* at all, as it defines a transaction scope that synchronization might apply to.
* As a consequence, the same resources (a JDBC {@code Connection}, a
* Hibernate {@code Session}, etc) will be shared for the entire specified
* scope. Note that the exact behavior depends on the actual synchronization
* configuration of the transaction manager!
*
In general, use {@code PROPAGATION_SUPPORTS} with care! In particular, do
* not rely on {@code PROPAGATION_REQUIRED} or {@code PROPAGATION_REQUIRES_NEW}
* within a {@code PROPAGATION_SUPPORTS} scope (which may lead to
* synchronization conflicts at runtime). If such nesting is unavoidable, make sure
* to configure your transaction manager appropriately (typically switching to
* "synchronization on actual transaction").
* @see org.springframework.transaction.support.AbstractPlatformTransactionManager#setTransactionSynchronization
* @see org.springframework.transaction.support.AbstractPlatformTransactionManager#SYNCHRONIZATION_ON_ACTUAL_TRANSACTION
*/
int PROPAGATION_SUPPORTS = 1;
/**
* Support a current transaction; throw an exception if no current transaction
* exists. Analogous to the EJB transaction attribute of the same name.
*
Note that transaction synchronization within a {@code PROPAGATION_MANDATORY}
* scope will always be driven by the surrounding transaction.
*/
int PROPAGATION_MANDATORY = 2;
/**
* Create a new transaction, suspending the current transaction if one exists.
* Analogous to the EJB transaction attribute of the same name.
*
NOTE: 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 J2EE).
*
A {@code PROPAGATION_REQUIRES_NEW} scope always defines its own
* transaction synchronizations. Existing synchronizations will be suspended
* and resumed appropriately.
* @see org.springframework.transaction.jta.JtaTransactionManager#setTransactionManager
*/
int PROPAGATION_REQUIRES_NEW = 3;
/**
* Do not support a current transaction; rather always execute non-transactionally.
* Analogous to the EJB transaction attribute of the same name.
*
NOTE: 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 J2EE).
*
Note that transaction synchronization is not available within a
* {@code PROPAGATION_NOT_SUPPORTED} scope. Existing synchronizations
* will be suspended and resumed appropriately.
* @see org.springframework.transaction.jta.JtaTransactionManager#setTransactionManager
*/
int PROPAGATION_NOT_SUPPORTED = 4;
/**
* Do not support a current transaction; throw an exception if a current transaction
* exists. Analogous to the EJB transaction attribute of the same name.
*
Note that transaction synchronization is not available within a
* {@code PROPAGATION_NEVER} scope.
*/
int PROPAGATION_NEVER = 5;
/**
* Execute within a nested transaction if a current transaction exists,
* behave like {@link #PROPAGATION_REQUIRED} else. There is no analogous
* feature in EJB.
*
NOTE: Actual creation of a nested transaction will only work on
* specific transaction managers. Out of the box, this only applies to the JDBC
* {@link org.springframework.jdbc.datasource.DataSourceTransactionManager}
* when working on a JDBC 3.0 driver. Some JTA providers might support
* nested transactions as well.
* @see org.springframework.jdbc.datasource.DataSourceTransactionManager
*/
int PROPAGATION_NESTED = 6;
/**
* Use the default isolation level of the underlying datastore.
* All other levels correspond to the JDBC isolation levels.
* @see java.sql.Connection
*/
int ISOLATION_DEFAULT = -1;
/**
* Indicates 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
*/
int ISOLATION_READ_UNCOMMITTED = Connection.TRANSACTION_READ_UNCOMMITTED;
/**
* Indicates 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
*/
int ISOLATION_READ_COMMITTED = Connection.TRANSACTION_READ_COMMITTED;
/**
* Indicates 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 re-reads the row,
* getting different values the second time (a "non-repeatable read").
* @see java.sql.Connection#TRANSACTION_REPEATABLE_READ
*/
int ISOLATION_REPEATABLE_READ = Connection.TRANSACTION_REPEATABLE_READ;
/**
* Indicates that dirty reads, non-repeatable reads and phantom reads
* are prevented.
*
This level includes the prohibitions in {@link #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
* re-reads for the same condition, retrieving the additional "phantom" row
* in the second read.
* @see java.sql.Connection#TRANSACTION_SERIALIZABLE
*/
int ISOLATION_SERIALIZABLE = Connection.TRANSACTION_SERIALIZABLE;
/**
* Use the default timeout of the underlying transaction system,
* or none if timeouts are not supported.
*/
int TIMEOUT_DEFAULT = -1;
/**
* Return the propagation behavior.
*
Must return one of the {@code PROPAGATION_XXX} constants
* defined on {@link TransactionDefinition this interface}.
* @return the propagation behavior
* @see #PROPAGATION_REQUIRED
* @see org.springframework.transaction.support.TransactionSynchronizationManager#isActualTransactionActive()
*/
int getPropagationBehavior();
/**
* Return the isolation level.
*
Must return one of the {@code ISOLATION_XXX} constants
* defined on {@link TransactionDefinition this interface}.
*
Only makes sense in combination with {@link #PROPAGATION_REQUIRED}
* or {@link #PROPAGATION_REQUIRES_NEW}.
*
Note that a transaction manager that does not support custom isolation levels
* will throw an exception when given any other level than {@link #ISOLATION_DEFAULT}.
* @return the isolation level
*/
int getIsolationLevel();
/**
* Return the transaction timeout.
*
Must return a number of seconds, or {@link #TIMEOUT_DEFAULT}.
*
Only makes sense in combination with {@link #PROPAGATION_REQUIRED}
* or {@link #PROPAGATION_REQUIRES_NEW}.
*
Note that a transaction manager that does not support timeouts will throw
* an exception when given any other timeout than {@link #TIMEOUT_DEFAULT}.
* @return the transaction timeout
*/
int getTimeout();
/**
* Return whether to optimize as a read-only transaction.
*
The read-only flag applies to any transaction context, whether
* backed by an actual resource transaction
* ({@link #PROPAGATION_REQUIRED}/{@link #PROPAGATION_REQUIRES_NEW}) or
* operating non-transactionally at the resource level
* ({@link #PROPAGATION_SUPPORTS}). In the latter case, the flag will
* only apply to managed resources within the application, such as a
* Hibernate {@code Session}.
<< *
This just serves as a hint for the actual transaction subsystem;
* it will not necessarily cause failure of write access attempts.
* A transaction manager which cannot interpret the read-only hint will
* not throw an exception when asked for a read-only transaction.
* @return {@code true} if the transaction is to be optimized as read-only
* @see org.springframework.transaction.support.TransactionSynchronization#beforeCommit(boolean)
* @see org.springframework.transaction.support.TransactionSynchronizationManager#isCurrentTransactionReadOnly()
*/
boolean isReadOnly();
/**
* Return the name of this transaction. Can be {@code null}.
*
This will be used as the transaction name to be shown in a
* transaction monitor, if applicable (for example, WebLogic's).
*
In case of Spring's declarative transactions, the exposed name will be
* the {@code fully-qualified class name + "." + method name} (by default).
* @return the name of this transaction
* @see org.springframework.transaction.interceptor.TransactionAspectSupport
* @see org.springframework.transaction.support.TransactionSynchronizationManager#getCurrentTransactionName()
*/
String getName();
}
PROPAGATION_REQUIRES_NEW ：
启动一个新的, 不依赖于环境的 "内部" 事务.
这个事务将被完全 commited 或 rolled back 而不依赖于外部事务, 它拥有自己的隔离范围, 自己的锁, 等等. 当内部事务开始执行时, 外部事务将被挂起, 内务事务结束时, 外部事务将继续执行.
PROPAGATION_NESTED ：
如果外部事务 commit, 嵌套事务也会被 commit；
如果外部事务 roll back, 嵌套事务也会被 roll back 。
开始一个 "嵌套的" 事务, 它是已经存在事务的一个真正的子事务. 嵌套事务开始执行时, 它将取得一个 savepoint. 如果这个嵌套事务失败, 我们将回滚到此 savepoint. 嵌套事务是外部事务的一部分, 只有外部事务结束后它才会被提交.
代码例子：
@Transactional(propagation=Propagation.NESTED)
@Transactional(propagation=Propagation.PROPAGATION_REQUIRES_NEW)
ServiceA{
@Autowired
ServiceB serviceB;
@Transactional(propagation=Propagation.NESTED)
public void method1(){
serviceB.method2();
int i = 1/0;
}
}
ServiceB{
@Transactional(propagation=Propagation.NESTED)
public void method2(){
xxxxxx
}
}
```
因为method1使用 @Transactional(propagation=Propagation.NESTED)，当执行method1时，会抛出异常，method2()也会被回滚；
如果method2()用PROPAGATION_REQUIRES_NEW：
```
ServiceB{
@Transactional(propagation=Propagation.PROPAGATION_REQUIRES_NEW)
public void method2(){
xxxxxx
}
}
```
那么method2不会因为method1抛出异常而回滚。
不管是什么类型的嵌套事务，一个线程只有一个事务，线程结束的时候才提交事务，包括嵌套事务，即使嵌套事务是REQUIRES_NEW，也不是嵌套事务的方法结束就提交事务的，一定是等到外部事务方法结束，整个线程结束才一起提交的。
在相同线程中进行相互嵌套调用的事务方法工作于相同的事务中。如果这些相互嵌套调用的方法工作在不同的线程中，则不同线程下的事务方法工作在独立的事务中。
而锁存在于事务里，锁的生命周期也是一个线程，在一个线程里可多次取得同一个锁。
如果事务加在外部方法A，在内部方法里面有synchronized代码块B，那么当B执行完时，事务还未提交，其他线程进入synchronized代码块B后，读取的库存数据不是最新的。