四件在我步入职业软件开发生涯那天起就该知道的事情

使用hibernate时，接口SessionFactory中获取session的方法挺多：openSession, getCurrentSession.如果使用spring，则有更多的用法。这些用法不免迷惑，他们分别是什么意思，哪些会自动提交呢？
下面是我结合实例与源码的分析。

没有使用spring管理的情况
主要区别openSession()和openCurrentSession()，openSession()即打开一个session，但程序中直接使用它，而是使用getCurrentSession()，典型用法如下（没有使用spring申明式管理管理事务）

private List listEvents() {
Session session = HibernateUtil.getSessionFactory().getCurrentSession();
session.beginTransaction();
List result = session.createQuery("from Event").list();
session.getTransaction().commit();
return result;
}

openCurrentSession()即获取当前上下文session

/**
* Obtains the current session.  The definition of what exactly "current"
* means controlled by the {@link org.hibernate.context.CurrentSessionContext} impl configured
* for use.
* <p/>
* Note that for backwards compatibility, if a {@link org.hibernate.context.CurrentSessionContext}
* is not configured but a JTA {@link org.hibernate.transaction.TransactionManagerLookup}
* is configured this will default to the {@link org.hibernate.context.JTASessionContext}
* impl.
*
* @return The current session.
*
* @throws HibernateException Indicates an issue locating a suitable current session.
*/
public org.hibernate.classic.Session getCurrentSession() throws HibernateException;

private CurrentSessionContext buildCurrentSessionContext() {
String impl = properties.getProperty( Environment.CURRENT_SESSION_CONTEXT_CLASS );
// for backward-compatability
if ( impl == null && transactionManager != null ) {
impl = "jta";
}

if ( impl == null ) {
return null;
}
else if ( "jta".equals( impl ) ) {
if ( settings.getTransactionFactory().areCallbacksLocalToHibernateTransactions() ) {
log.warn( "JTASessionContext being used with JDBCTransactionFactory; auto-flush will not operate correctly with getCurrentSession()" );
}
return new JTASessionContext( this );
}
else if ( "thread".equals( impl ) ) {
return new ThreadLocalSessionContext( this );
}
else if ( "managed".equals( impl ) ) {
return new ManagedSessionContext( this );
}
else {
try {
Class implClass = ReflectHelper.classForName( impl );
return ( CurrentSessionContext ) implClass
.getConstructor( new Class[] { SessionFactoryImplementor.class } )
.newInstance( new Object[] { this } );
}
catch( Throwable t ) {
log.error( "Unable to construct current session context [" + impl + "]", t );
return null;
}
}
}

从上面获取CurrentSessionContext的方法中看出，current session是由配置文件制定，默认是jta。如果使用spring管理，切勿配置该项，默认将为springContextProvider

current_session_context_class thread

使用spring管理
spring中的HibernateTemplate有对hibernate的封装，有很多便捷的方法，在DAO实现中

@Override
public Magic read(int id) {
return super.getHibernateTemplate().get(Magic.class, id);
}

但是如果需要自己使用复杂的组合查找，可以使用excute(new HiberanteCallback())

public <T> T get(final Class<T> entityClass, final Serializable id, final LockMode lockMode)
throws DataAccessException {

return executeWithNativeSession(new HibernateCallback<T>() {
@SuppressWarnings("unchecked")
public T doInHibernate(Session session) throws HibernateException {
if (lockMode != null) {
return (T) session.get(entityClass, id, lockMode);
}
else {
return (T) session.get(entityClass, id);
}
}
});
}

这样调用获取的session是通过下面这个方法

/**
* Return a Session for use by this template.
* <p>Returns a new Session in case of "alwaysUseNewSession" (using the same
* JDBC Connection as a transactional Session, if applicable), a pre-bound
* Session in case of "allowCreate" turned off, and a pre-bound or new Session
* otherwise (new only if no transactional or otherwise pre-bound Session exists).
* @return the Session to use (never <code>null</code>)
* @see SessionFactoryUtils#getSession
* @see SessionFactoryUtils#getNewSession
* @see #setAlwaysUseNewSession
* @see #setAllowCreate
*/
protected Session getSession() {
if (isAlwaysUseNewSession()) {
return SessionFactoryUtils.getNewSession(getSessionFactory(), getEntityInterceptor());
}
else if (isAllowCreate()) {
return SessionFactoryUtils.getSession(
getSessionFactory(), getEntityInterceptor(), getJdbcExceptionTranslator());
}
else if (SessionFactoryUtils.hasTransactionalSession(getSessionFactory())) {
return SessionFactoryUtils.getSession(getSessionFactory(), false);
}
else {
try {
return getSessionFactory().getCurrentSession();
}
catch (HibernateException ex) {
throw new DataAccessResourceFailureException("Could not obtain current Hibernate Session", ex);
}
}
}

SessionFactoryUtil中的逻辑如下

/**
* Get a Hibernate Session for the given SessionFactory. Is aware of and will
* return any existing corresponding Session bound to the current thread, for
* example when using {@link HibernateTransactionManager}. Will create a new
* Session otherwise, if "allowCreate" is <code>true</code>.
* <p>Same as {@link #getSession}, but throwing the original HibernateException.
* @param sessionFactory Hibernate SessionFactory to create the session with
* @param entityInterceptor Hibernate entity interceptor, or <code>null</code> if none
* @param jdbcExceptionTranslator SQLExcepionTranslator to use for flushing the
* Session on transaction synchronization (may be <code>null</code>)
* @param allowCreate whether a non-transactional Session should be created
* when no transactional Session can be found for the current thread
* @return the Hibernate Session
* @throws HibernateException if the Session couldn't be created
* @throws IllegalStateException if no thread-bound Session found and
* "allowCreate" is <code>false</code>
*/
private static Session doGetSession(
SessionFactory sessionFactory, Interceptor entityInterceptor,
SQLExceptionTranslator jdbcExceptionTranslator, boolean allowCreate)
throws HibernateException, IllegalStateException {

Assert.notNull(sessionFactory, "No SessionFactory specified");

SessionHolder sessionHolder = (SessionHolder) TransactionSynchronizationManager.getResource(sessionFactory);
if (sessionHolder != null && !sessionHolder.isEmpty()) {
// pre-bound Hibernate Session
Session session = null;
if (TransactionSynchronizationManager.isSynchronizationActive() &&
sessionHolder.doesNotHoldNonDefaultSession()) {
// Spring transaction management is active ->
// register pre-bound Session with it for transactional flushing.
session = sessionHolder.getValidatedSession();
if (session != null && !sessionHolder.isSynchronizedWithTransaction()) {
logger.debug("Registering Spring transaction synchronization for existing Hibernate Session");
TransactionSynchronizationManager.registerSynchronization(
new SpringSessionSynchronization(sessionHolder, sessionFactory, jdbcExceptionTranslator, false));
sessionHolder.setSynchronizedWithTransaction(true);
// Switch to FlushMode.AUTO, as we have to assume a thread-bound Session
// with FlushMode.MANUAL, which needs to allow flushing within the transaction.
FlushMode flushMode = session.getFlushMode();
if (flushMode.lessThan(FlushMode.COMMIT) &&
!TransactionSynchronizationManager.isCurrentTransactionReadOnly()) {
session.setFlushMode(FlushMode.AUTO);
sessionHolder.setPreviousFlushMode(flushMode);
}
}
}
else {
// No Spring transaction management active -> try JTA transaction synchronization.
session = getJtaSynchronizedSession(sessionHolder, sessionFactory, jdbcExceptionTranslator);
}
if (session != null) {
return session;
}
}

logger.debug("Opening Hibernate Session");
Session session = (entityInterceptor != null ?
sessionFactory.openSession(entityInterceptor) : sessionFactory.openSession());

// Use same Session for further Hibernate actions within the transaction.
// Thread object will get removed by synchronization at transaction completion.
if (TransactionSynchronizationManager.isSynchronizationActive()) {
// We're within a Spring-managed transaction, possibly from JtaTransactionManager.
logger.debug("Registering Spring transaction synchronization for new Hibernate Session");
SessionHolder holderToUse = sessionHolder;
if (holderToUse == null) {
holderToUse = new SessionHolder(session);
}
else {
holderToUse.addSession(session);
}
if (TransactionSynchronizationManager.isCurrentTransactionReadOnly()) {
session.setFlushMode(FlushMode.MANUAL);
}
TransactionSynchronizationManager.registerSynchronization(
new SpringSessionSynchronization(holderToUse, sessionFactory, jdbcExceptionTranslator, true));
holderToUse.setSynchronizedWithTransaction(true);
if (holderToUse != sessionHolder) {
TransactionSynchronizationManager.bindResource(sessionFactory, holderToUse);
}
}
else {
// No Spring transaction management active -> try JTA transaction synchronization.
registerJtaSynchronization(session, sessionFactory, jdbcExceptionTranslator, sessionHolder);
}

// Check whether we are allowed to return the Session.
if (!allowCreate && !isSessionTransactional(session, sessionFactory)) {
closeSession(session);
throw new IllegalStateException("No Hibernate Session bound to thread, " +
"and configuration does not allow creation of non-transactional one here");
}

return session;
}

可以看出，总体的获取session的顺序是先查看spring的transaction manager， 其次JTA， 再次调用getCurrentSession，在getCurrentSession也会有JTA的回退，可能有重复的地方。