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从源码分析 Spring 基于注解的事务
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在spring引入基于注解的事务(@Transactional)之前，我们一般都是如下这样进行拦截事务的配置：
<!-- 拦截器方式配置事务 -->
<tx:advice id="transactionAdvice" transaction-manager="transactionManager">
<tx:attributes>
<tx:method name="add*" propagation="REQUIRED" />
<tx:method name="append*" propagation="REQUIRED" />
<tx:method name="insert*" propagation="REQUIRED" />
<tx:method name="save*" propagation="REQUIRED" />
<tx:method name="update*" propagation="REQUIRED" />
<tx:method name="modify*" propagation="REQUIRED" />
<tx:method name="edit*" propagation="REQUIRED" />
<tx:method name="delete*" propagation="REQUIRED" />
<tx:method name="remove*" propagation="REQUIRED" />
<tx:method name="repair" propagation="REQUIRED" />
<tx:method name="delAndRepair" propagation="REQUIRED" /> <tx:method name="get*" propagation="SUPPORTS" />
<tx:method name="find*" propagation="SUPPORTS" />
<tx:method name="load*" propagation="SUPPORTS" />
<tx:method name="search*" propagation="SUPPORTS" />
<tx:method name="datagrid*" propagation="SUPPORTS" /> <tx:method name="*" propagation="SUPPORTS" />
</tx:attributes>
</tx:advice>
<aop:config>
<aop:pointcut id="transactionPointcut" expression="execution(* net.aazj.service..*Impl.*(..))" />
<aop:advisor pointcut-ref="transactionPointcut" advice-ref="transactionAdvice" />
</aop:config>
这种方式明显的缺点是，不太容易理解，并且限定了service层的方法名称的前缀，没有模板的话写起来也很难，很容易写错。
因此在spring中引入了基于注解的事务配置方法之后，我们应该抛弃这种事务配置方法了。基于注解 @Transactional 的事务配置具有简单，灵活的优点。下面看一个例子：
@Service("userService")
@Transactional
public class UserServiceImpl implements UserService{
@Autowired
private UserMapper userMapper;
@Transactional (propagation=Propagation.REQUIRED,isolation=Isolation.DEFAULT,readOnly=true)
public User getUser(int userId) {
return userMapper.getUser(userId);
}
@Transactional
public void addUser(String username){
userMapper.addUser(username);
// int i = 1/0; // 测试事务的回滚
}
@Transactional (rollbackFor = BaseBusinessException.class)
public void addAndDeleteUser(String username, int id) throws BaseBusinessException{
userMapper.addUser(username);
this.m1();
userMapper.deleteUserById(id);
}
private void m1() throws BaseBusinessException {
throw new BaseBusinessException("xxx");
}
}
首先在service类上声明了@Transactional，表明类中的所有方法都需要运行在事务中，然后在方法中可以指定具体的事务特性，方法中的@Transactional会覆盖类上的@Transactional。
下面我们从源码的角度( 从源码的学习可以给我们实打实的比较深入理解，而且不会出错，二手资料总是会有时延的 )来探究一下它们：
public @interface Transactional {
/**
* A qualifier value for the specified transaction.
* <p>May be used to determine the target transaction manager,
* matching the qualifier value (or the bean name) of a specific
* {@link org.springframework.transaction.PlatformTransactionManager}
* bean definition.
*/
String value() default "";
/**
* The transaction propagation type.
* Defaults to {@link Propagation#REQUIRED}.
* @see org.springframework.transaction.interceptor.TransactionAttribute#getPropagationBehavior()
*/
Propagation propagation() default Propagation.REQUIRED;
/**
* The transaction isolation level.
* Defaults to {@link Isolation#DEFAULT}.
* @see org.springframework.transaction.interceptor.TransactionAttribute#getIsolationLevel()
*/
Isolation isolation() default Isolation.DEFAULT;
/**
* The timeout for this transaction.
* Defaults to the default timeout of the underlying transaction system.
* @see org.springframework.transaction.interceptor.TransactionAttribute#getTimeout()
*/
int timeout() default TransactionDefinition.TIMEOUT_DEFAULT;
/**
* {@code true} if the transaction is read-only.
* Defaults to {@code false}.
* <p>This just serves as a hint for the actual transaction subsystem;
* it will <i>not necessarily</i> cause failure of write access attempts.
* A transaction manager which cannot interpret the read-only hint will
* <i>not</i> throw an exception when asked for a read-only transaction.
* @see org.springframework.transaction.interceptor.TransactionAttribute#isReadOnly()
*/
boolean readOnly() default false;
/**
* Defines zero (0) or more exception {@link Class classes}, which must be a
* subclass of {@link Throwable}, indicating which exception types must cause
* a transaction rollback.
* <p>This is the preferred way to construct a rollback rule, matching the
* exception class and subclasses.
* <p>Similar to {@link org.springframework.transaction.interceptor.RollbackRuleAttribute#RollbackRuleAttribute(Class clazz)}
*/
Class<? extends Throwable>[] rollbackFor() default {};
/**
* Defines zero (0) or more exception names (for exceptions which must be a
* subclass of {@link Throwable}), indicating which exception types must cause
* a transaction rollback.
* <p>This can be a substring, with no wildcard support at present.
* A value of "ServletException" would match
* {@link javax.servlet.ServletException} and subclasses, for example.
* <p><b>NB: </b>Consider carefully how specific the pattern is, and whether
* to include package information (which isn't mandatory). For example,
* "Exception" will match nearly anything, and will probably hide other rules.
* "java.lang.Exception" would be correct if "Exception" was meant to define
* a rule for all checked exceptions. With more unusual {@link Exception}
* names such as "BaseBusinessException" there is no need to use a FQN.
* <p>Similar to {@link org.springframework.transaction.interceptor.RollbackRuleAttribute#RollbackRuleAttribute(String exceptionName)}
*/
String[] rollbackForClassName() default {};
/**
* Defines zero (0) or more exception {@link Class Classes}, which must be a
* subclass of {@link Throwable}, indicating which exception types must <b>not</b>
* cause a transaction rollback.
* <p>This is the preferred way to construct a rollback rule, matching the
* exception class and subclasses.
* <p>Similar to {@link org.springframework.transaction.interceptor.NoRollbackRuleAttribute#NoRollbackRuleAttribute(Class clazz)}
*/
Class<? extends Throwable>[] noRollbackFor() default {};
/**
* Defines zero (0) or more exception names (for exceptions which must be a
* subclass of {@link Throwable}) indicating which exception types must <b>not</b>
* cause a transaction rollback.
* <p>See the description of {@link #rollbackForClassName()} for more info on how
* the specified names are treated.
* <p>Similar to {@link org.springframework.transaction.interceptor.NoRollbackRuleAttribute#NoRollbackRuleAttribute(String exceptionName)}
*/
String[] noRollbackForClassName() default {};
}
public @interface Transactional
注解@Transactional的属性有：propagation, isolation, timeout, readOnly, rollbackFor, rollbackForClassName, noRollbackFor, noRollbackForClassName
propagation, isolation, timeout, readOnly都有默认值，而rollbackFor, rollbackForClassName, noRollbackFor, noRollbackForClassName默认值都是空的。
我们具体看下我们可能会用到的属性：propagation, isolation, readOnly, rollbackFor
1）propagation指定事务的传播属性 ：
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>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.
* <p>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.
* <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; }
}
public enum Propagation
propagation可以取如下的值：REQUIRED, SUPPORTS, MANDATORY, REQUIRES_NEW, NOT_SUPPORTED, NEVER, NESTED
我们一般会只用：REQUIRED（默认值）, SUPPORTS，其它的取值基本上不使用(具体可以参考上面源码中的注释，已经很详细了)。
propagation=Propagation.REQUIRED：表示该方法或类必须要事务的支持，如果已经是在一个事务中被调用，那么就使用该事务，如果没有在一个事务中，那么就新建一个事务。
propagation=Propagation.SUPPORTS：表示该方法或类支持事务，如果已经是在一个事务中被调用，那么就使用该事务，如果没有在一个事务中，也可以。
2）isolation指定事务的隔离级别 ：
public enum Isolation {
/**
* 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; }
}
public enum Isolation
isolation可以取如下的值：DEFAULT, READ_UNCOMMITTED, READ_COMMITTED, REPEATABLE_READ, SERIALIZABLE
我们一般只使用：DEFAULT（默认值），因为我们一般是直接在数据库的层面上来设置事务的隔离级别，很少会在应用层来设置隔离基本。
isolation=Isolation.DEFAULT：表示使用下层数据库指定的隔离级别
3）readOnly为只读 ，利于数据库优化器进行优化。默认值为 false。所以对于只对数据库进行读取的方法，我们可以如下指定：
@Transactional (propagation=Propagation.SUPPORTS)
public User getUser(int userId) {
return userMapper.getUser(userId);
}
表示：有事务则使用当前事务，没有事务则不使用事务。最大限度的利于数据库的优化器进行优化。
如果一定要使用事务的话，也可以这样使用readOnly=true来优化：
@Transactional (propagation=Propagation.REQUIRED,readOnly=true)
public User getUser(int userId) {
return userMapper.getUser(userId);
}
这就是 readOnly 的作用。表示有事务则使用当前事务，如果没有事务，则新建一个只读事务。其实 上面的 propagation=Propagation.REQUIRED也是可以去掉的，因为他是默认值！
@Transactional (readOnly=true)
public User getUser(int userId) {
return userMapper.getUser(userId);
}
表示强制事务，并且是 只读事务。readOnly 要注意有点的是，readOnly只有在 有事务时，才会生效，如果没有事务，那么 readOnly 是会被忽略的。比如:
@Transactional(propagation=Propagation.SUPPORTS, readonly=true)
表示的就是 没有事务 也是可以的，有事务的话，就一定是 只读事务。
4）rollbackFor ，其实该属性也很少使用，而且经常被误用。表示抛出什么异常时，会回滚事务。异常分为受检异常(必须进行处理或者重新抛出)和非受检异常(可以不进行处理)。在遇到非受检异常时，事务是一定会进行回滚的。rollbackFor用于指定对于何种受检异常发生时，进行回滚。因为受检异常，我们必须进行处理或者重新抛出，所以只有一种情况下我们要使用rollbackFor来指定，就是我们不处理异常，直接抛出受检异常，并且我们需要方法在抛出该异常时，进行回滚，如上面的例子：
@Transactional (rollbackFor = BaseBusinessException.class)
public void addAndDeleteUser(String username, int id) throws BaseBusinessException{
userMapper.addUser(username);
this.m1();
userMapper.deleteUserById(id);
}
private void m1() throws BaseBusinessException {
throw new BaseBusinessException("xxx");
}
因为 m1 方法抛出受检异常，我们在 addAndDeleteUser 方法中不对该异常进行处理，而是直接抛出，如果我们希望 userMapper.addUser(username) 和 userMapper.deleteUserById(id) 要么都成功，要么都失败，此时我们则应该指定：rollbackFor = BaseBusinessException.class ，进行回滚。
所以只有我们的方法声明要抛出一个受检异常时，我们才应该使用 rollbackFor 属性来进行处理。如果我们在 addAndDeleteUser 方法中对 m1 方法的受检异常进行了处理，那么就没有必要使用 rollbackFor 了：
public void addAndDeleteUser(String username, int id){
userMapper.addUser(username);
try{
this.m1();
}catch(BaseBusinessException e){
// 处理异常，比如记录进日志文件等
}
userMapper.deleteUserById(id);
}
因为我们处理了 m1 方法的异常，那么就不会 有受检异常 导致 userMapper.addUser(username) 和 userMapper.deleteUserById(id) 这两个方法一个执行成功，一个没有执行。而非受检异常默认就会回滚。
受检异常是必须进行处理或者重新声明抛出的。只有声明重新抛出受检异常时，才会需要使用 rollbackFor 属性。所以下面的方式就属于滥用 rollbackFor 了：
@Transactional (rollbackFor=Exception.class)
public void addAndDeleteUser2(String username, int id){
userMapper.addUser(username);
userMapper.deleteUserById(id);
}
因为：受检异常是必须进行处理或者重新声明抛出，而我们既没有进行处理，也没有重新抛出，就说明他绝对不可能会抛出受检异常了。而只会抛出未受检异常，而未受检异常，默认就会回滚，所以上面的 @Transactional (rollbackFor=Exception.class) 完全是多余的。
总结 ：
1） @Transactional 的默认值为：@Transactional (propagation=Propagation.REQUIRED,isolation=Isolation.DEFAULT,readOnly=false,
timeout=TransactionDefinition.TIMEOUT_DEFAULT)，默认值已经适合绝大多数情况，所以我们一般使用 @Transactional 进行注解就够了。
2） 只有当默认值不符合我们的需要时才给@Transactional的属性指定值，一般也就指定：propagation=Propagation.SUPPORTS 和 readOnly=true，其它的属性和值一般很少使用非默认值。所以我们前面的UserServiceImpl类可以重构如下：
@Service("userService")
@Transactional
public class UserServiceImpl implements UserService{
@Autowired
private UserMapper userMapper;
@Transactional (readOnly=true)
public User getUser(int userId) {
return userMapper.getUser(userId);
}
public void addUser(String username){
userMapper.addUser(username);
int i = 1/0; // 测试事务的回滚
}
public void deleteUser(int id){
userMapper.deleteUserById(id);
// int i = 1/0; // 测试事务的回滚
}
@Transactional (rollbackFor = BaseBusinessException.class)
public void addAndDeleteUser(String username, int id) throws BaseBusinessException{
userMapper.addUser(username);
this.m1();
userMapper.deleteUserById(id);
}
private void m1() throws BaseBusinessException {
throw new BaseBusinessException("xxx");
}
}
addUser 和 deleteUser 因为会继承上的 @Transactional ，所以无需另外指定了，只有当类上指定的 @Transactional 不适合时，才需要另外在方法上进行指定。
3） 所以我们只实际情况中，我们只需要使用下面三者来进行注解事务的配置：
@Transactional，
@Transactional (readOnly=true)，
@Transactional (propagation=Propagation.SUPPORTS, readOnly=true)，
@Transactional (rollbackFor = xxException.class)，其它都可以保持默认值，其它的非默认值极少使用。
另外 ：
要使用@Transactional来进行注解事务配置，必须要在spring的配置文件中加入下面的配置说明，启用基于注解的配置：
<bean id="transactionManager" class="org.springframework.jdbc.datasource.DataSourceTransactionManager">
<property name="dataSource" ref="dataSource" />
</bean>
<!-- 使用annotation定义事务 -->
<tx:annotation-driven transaction-manager="transactionManager" />
补充说明 ：
也许你会觉得奇怪，userMapper.getUser(userId); 这些涉及到数据库访问的方法，为什么不会抛出 SQLException 受检异常呢？如果手动注册驱动，然后获取链接，进行数据库操作时，是会有许多的受检异常要处理的：
try {
Class.forName("com.mysql.jdbc.Driver");
} catch (ClassNotFoundException e) {
// ...
}
String url="jdbc:mysql://localhost:3306/databasename";
try {
Connection conn = (Connection) DriverManager.getConnection(url,"username","password");
} catch (SQLException e) {
// ...
}
哪为什么在这里userMapper.getUser(userId); 就不需要处理受检异常呢 ？其实这是spring的功能， spring为了将我们从那些十分麻烦的受检异常比如SQLException中解救处理，将所有的数据库访问层的受检异常转嫁到 Spring 的 非受检异常RuntimeException 体系中来——DataAccessException 。
