使用Spring的HibernateDAOSupport可能遇到的Lazy initialization問題

在專案中我們使用DAO這個pattern來封裝資料庫存取層，Hibernate Quickly (Manning)中建議我們可以繼承Spring所提供的HibernateDAOSupport來簡化DAO程式碼，這是很常見的作法。

原本method重複性很高的部份：

public void create(Event event) throws DataAccessLayerException {
try {
    startOperation();
    session.save(event);
    tx.commit();
} catch (HibernateException e) {
    handleException(e);
} finally {
    HibernateFactory.close(session);
}
}

可以被簡化為：

public abstract class AbstractSpringDao  extends HibernateDaoSupport{

public AbstractSpringDao() { }

protected void saveOrUpdate(Object obj) {
 getHibernateTemplate().saveOrUpdate(obj);
}

protected void delete(Object obj) {
 getHibernateTemplate().delete(obj);
}
...
}

都透過Sprgin提供的HibernateTemplate幫我們作掉那些重複的開、關session, transaction動作。

以下擷取自Spring原始碼

package org.springframework.orm.hibernate3;
...
public class HibernateTemplate extends HibernateAccessor implements HibernateOperations {
...
public Object execute(HibernateCallback action) throws DataAccessException {
   Session session = (!this.allowCreate ?
       SessionFactoryUtils.getSession(getSessionFactory(),false) :
       SessionFactoryUtils.getSession(getSessionFactory(), getEntityInterceptor(), getJdbcExceptionTranslator()));
   boolean existingTransaction =
     TransactionSynchronizationManager.hasResource(getSessionFactory());
   if (!existingTransaction && getFlushMode() == FLUSH_NEVER) {
       session.setFlushMode(FlushMode.NEVER);
   }
   try {
       Object result = action.doInHibernate(session);
       flushIfNecessary(session, existingTransaction);
       return result;
   }
   catch (HibernateException ex) {
       throw convertHibernateAccessException(ex);
   }
   catch (SQLException ex) {
       throw convertJdbcAccessException(ex);
   }
   catch (RuntimeException ex) {
       // callback code threw application exception
       throw ex;
   }
   finally {
       SessionFactoryUtils.closeSessionIfNecessary(session, getSessionFactory());
   }
}
...
}

在execute()最後的finally中，session會被關掉。因此從DAO層的method執行結束後，session就被關掉，因此如果在JSP頁面中存取到某些one-to-many （或其他lazy=true的關聯物件時）就會產生exception：
failed to lazily initialize a collection of ...
因為無法再透過session去抓資料。

通常此時會採用Spring提供的OpenSessionInViewFilter來解決，這個filter會在發出request的時候開啟session直到request結束，因此JSP頁面上仍可以透過這個session來取得因為lazy initialization未取得的資料。

但設定上去後，如果你採用Hibernate 3則會產生另一個問題。
當你呼叫DAO的SaveOrUpdate()資料並沒有存進資料庫，只會顯示一堆select 敘述：

Hibernate: select employee_.EMPLOYEE_ID, employee_.DEPARTMENT_NAME as DEPARTMENT2_0_, employee_.DEPARTMENT_NO as DEPARTMENT3_0_, employee_.INAUGURATION_DATE as INAUGURA4_0_, employee_.DISCHARGE_DATE as DISCHARGE5_0_, employee_.PROFESSIONAL_TITLE as PROFESSI6_0_, employee_.JOB_LEVEL as JOB7_0_, employee_.DIRECT_SUPERVISOR as DIRECT8_0_, employee_.PERSONAL_PHOTO as PERSONAL9_0_, employee_.CHINESE_NAME as CHINESE10_0_, ... from EMPLOYEES employee_ where employee_.EMPLOYEE_ID=?

並且產生以下Exception：
org.springframework.dao.InvalidDataAccessApiUsageException: Write operations are not allowed in read-only mode (FlushMode.NEVER/MANUAL): Turn your Session into FlushMode.COMMIT/AUTO or remove 'readOnly' marker from transaction definition.

/**
 * Get a Session for the SessionFactory that this filter uses.
 * Note that this just applies in single session mode!
 * The default implementation delegates to the
 * SessionFactoryUtils.getSession method and
 * sets the Session's flush mode to "NEVER".
 * 

Can be overridden in subclasses for creating a Session with a
 * custom entity interceptor or JDBC exception translator.
 * @param sessionFactory the SessionFactory that this filter uses
 * @return the Session to use
 * @throws DataAccessResourceFailureException if the Session could not be created
 * @see org.springframework.orm.hibernate3.SessionFactoryUtils#getSession(SessionFactory, boolean)
 * @see org.hibernate.FlushMode#NEVER
 */
protected Session getSession(SessionFactory sessionFactory) throws DataAccessResourceFailureException {
 Session session = SessionFactoryUtils.getSession(sessionFactory, true);
 FlushMode flushMode = getFlushMode();
 if (flushMode != null) {
  session.setFlushMode(flushMode);
 }
 return session;
}

reference:
1.使用語法高亮度http://sjcywg.blogspot.com/2008/02/syntax-highlighting.html
2.相關問題簡述Lazy Initialization and the DAO pattern with Hibernate and Spring