Spring @Transactional 如何开启事务

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Spring @Transactional 如何开启事务

java.lang.Object

org.springframework.transaction.support.TransactionSynchronizationManager

public abstract class TransactionSynchronizationManager

extends Object

Central helper that manages resourcesand transaction synchronizations per thread.

为每个线程管理资源和事务的中心helper

hibernate.current_session_context_class=org.springframework.orm.hibernate4.SpringSessionContext时,

@Transactional,Spring的事务管理器HibernateTransactionManager.doBegin()方法开启的Session和事务就是绑定到TransactionSynchronizationManager的上下文(ThreadLocal的Map)中的..

SpringSessionContext.currentSesssion()方法就是在TransactionSynchronizationManager的上下文中查找的..

上文回顾:

现在对于hibernate.current_session_context_class= org.springframework.orm.hibernate4.SpringSessionContext时的getCurrentSession()就很清楚了:

1:@Transactional声明的方法执行时,Spring的TransactionManager会自动Open Sesion,自动开启事务,并且将此Sesion绑定到SpringSessionContext(实际上是TransactionSynchronizationManager的ThreadLocal的Map)中..

2:SessionFactory.getCurrentSession()方法执行时,调用SpringSessionContext.currentSession()从TransactionSynchronizationManager的上下文中查找当前的Session

3:找到后返回当前的Session,找不到,则返回HibernateException("NoSessionfound for current thread")

上述第一点是未验证的,现在我们来分析一下源代码:

Public UserService{   @Transactional   public void addUser(User user) throwsException   {      Session session =sessionFactory.getCurrentSession();          session.save(user);   }}

//当然,声明式事务管理实质也是用AOP实现的,此时取得的一个Proxy..UserService userService = (UserService) beanFactory.getBean("userService");             User user = new User();user.setName("Mark");             userService.addUser(user);  //在执行这一句时,应该是调用Proxy来开启一个事务,并绑定到某个上下文中,我们跟进去看看

先进入到JdkDynamicAopProxyimplements.invoke()方法

然后到ReflectiveMethodInvocation. proceed(),这里调用TransactionInterceptor.invoke(this)方法:

典型的拦截器模式:

1:按需开启事务

2:递归执行下一个拦截器或执行代理目标方法

3:提交事务

public class TransactionInterceptor extends TransactionAspectSupport implements MethodInterceptor,Serializable {     public Object invoke(final MethodInvocationinvocation) throws Throwable {                                                                                                                                                  .......              //就在这一句开启事务              TransactionInfotxInfo = createTransactionIfNecessary(tm, txAttr,joinpointIdentification);              ......              retVal= invocation.proceed();   //执行下一个拦截器 或 执行代理目标的方法              ......                                                                                                                              //提交事务                                     commitTransactionAfterReturning(txInfo);                                                                                   return retVal;              .......    } }

进入createTransactionIfNecessary(tm,txAttr, joinpointIdentification), 其中的tm.getTransaction(txAttr); //这一句应该是取得事务的,跟进去

……………………………………….

AbstractPlatformTransactionManager.getTransaction()方法:

如果当前有事务,取出并根据事务传播行为的配置去处理,如果当前没有事务,调用doBegin开启一个新事务

public abstract classAbstractPlatformTransactionManager implementsPlatformTransactionManager, Serializable { ......   public final TransactionStatus getTransaction(TransactionDefinitiondefinition) throws TransactionException {              //这里是取得当前已绑定到TransactionSynchronizationManager的上下文的事务,主要为事务传播行为而设              Object transaction =doGetTransaction();              ........              //如果事务已存在,根据事务传播行为来处理,              if(isExistingTransaction(transaction)) {                     // Existingtransaction found -> check propagation behavior to find out how to behave.                     return handleExistingTransaction(definition, transaction, debugEnabled);              }              ......              //这里最关键,开启一个新的事务,并放进TransactionSynchronizationManager的上下文中,并绑定到当前线程)              doBegin(transaction,definition);              return status;              .......       }......}

HibernateTransactionManager.doGetTransaction()方法:

尝试获取当前已绑定到TransactionSynchronizationManager的上下文的事务,主要为事务传播行为而设

public class HibernateTransactionManager extends AbstractPlatformTransactionManagerimplements ResourceTransactionManager, InitializingBean {protected Object doGetTransaction() {HibernateTransactionObject txObject = new HibernateTransactionObject();......//在TransactionSynchronizationManager的上下文中查找当前的Session(实质上也是与线程绑定的)SessionHolder sessionHolder =(SessionHolder) TransactionSynchronizationManager.getResource(getSessionFactory());......return txObject;}}

HibernateTransactionManager.doBegin()方法:

开启一个新的事务,并放进TransactionSynchronizationManager的上下文(ThreadLocal的Map)中,绑定到当前线程)

public class HibernateTransactionManager extends AbstractPlatformTransactionManagerimplements ResourceTransactionManager, InitializingBean {......     protected void doBegin(Object transaction, TransactionDefinition definition) {                HibernateTransactionObject txObject = (HibernateTransactionObject) transaction;.........try {if (txObject.getSessionHolder() == null || txObject.getSessionHolder().isSynchronizedWithTransaction()) {//这里才真正的打开SessionSession newSession = SessionFactoryUtils.openSession(getSessionFactory());.........}session = txObject.getSessionHolder().getSession();                        .........Transaction hibTx;// Register transaction timeout..........if (timeout != TransactionDefinition.TIMEOUT_DEFAULT) {.........}else {//这里才真正地开启事务hibTx = session.beginTransaction();}.........                        //如果这新开启的Session,则将SessionHolder(Session和Transaction)放到TransactionSynchronizationManager的上下文中(绑定到当前线程)// Bind the session holder to the thread.if (txObject.isNewSessionHolder()) {                                                                                                                                        //以键值对<SessionFactory,SessionHolder>形式绑定到TransactionSynchronizationManager的上下文中TransactionSynchronizationManager.bindResource(getSessionFactory(), txObject.getSessionHolder());}.......}.........     }......}

Spring @Transactional 如何开启事务总结:

@Transactional声明的方法执行时会调用AbstractPlatformTransactionManager.getTransaction()取得当前事务

而getTransaction()的执行流程如下:

1:尝试获取当前已绑定到TransactionSynchronizationManager的上下文的事务

(调用HibernateTransactionManager.doGetTransaction()方法)

2:如果取得已存在的事务,则根据事务传播属性的设置来处理

(调用AbstractPlatformTransactionManager.handleExistingTransaction()方法)

3:如果没有事务,则打开新的Session,开启新的事务,并将该Session和事务绑定到TransactionSynchronizationManager的上下文中

(调用HibernateTransactionManager.doBegin()方法)

-------------------------------------------------

核心还是理解TransactionSynchronizationManager,要懂得Spring的TransactionManager开启事务后是以键值对<SessionFactory,SessionHolder>形式存放到 TransactionSynchronizationManager的上下文(ThreadLocal的Map)中(以ThreadLocal的方式与当前线程绑定).

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