为什么Spring的HibernateTemplate一般情况下不支持数据的惰性加载的源码分析

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HibernateTemplate template = (MyHibernateTemplate) context.getBean("hibernateTemplate");

Emp emp = (Emp) template.load(Emp.class, new Long(7369));

long no = emp.getEmpno();

log.info("empno = " + no);

String name = emp.getEname();

log.info("Empname = " + name);

上面的代码在一般情况执行到String name=emp.getEname()；的时候就会抛出org.hibernate.LazyInitializationException的异常，这个是容易理解的，因为Emp emp = (Emp) template.load(Emp.class, new Long(7369));默认是延迟数据的加载的，HibernateTemplate在执行完Emp emp = (Emp) template.load(Emp.class, new Long(7369));这句话后打开的session就关闭了。因而你在执行 String name = emp.getEname();这条语句时，这个时候才会向数据查询，但是这个时候session已经关闭，因而会出现上面的异常。为什么session会关闭呢？看一下Hibernate的源码就知道了，其核心代码是下面这段：

public Object execute(HibernateCallback action, boolean exposeNativeSession) throws DataAccessException {
        Assert.notNull(action, "Callback object must not be null");

        Session session = getSession();
        boolean existingTransaction = SessionFactoryUtils.isSessionTransactional(session, getSessionFactory());
        if (existingTransaction) {
            logger.debug("Found thread-bound Session for HibernateTemplate");
        }

        FlushMode previousFlushMode = null;
        try {
            previousFlushMode = applyFlushMode(session, existingTransaction);
            enableFilters(session);
            Session sessionToExpose = (exposeNativeSession ? session : createSessionProxy(session));
            Object result = action.doInHibernate(sessionToExpose);
            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 {
            if (existingTransaction) {
                logger.debug("Not closing pre-bound Hibernate Session after HibernateTemplate");
                disableFilters(session);
                if (previousFlushMode != null) {
                    session.setFlushMode(previousFlushMode);
                }
            }
            else {
                // Never use deferred close for an explicitly new Session.
                if (isAlwaysUseNewSession()) {
                    SessionFactoryUtils.closeSession(session);
                }
                else {
                    SessionFactoryUtils.closeSessionOrRegisterDeferredClose(session, getSessionFactory());
                }
            }
        }
    }

因为HibernateTemplate是通过方法的回调执行你所操作的方法的。对应的话你调用的方法对应的就是

Object result = action.doInHibernate(sessionToExpose);这一句当这些代码执行完后，最后看finally语句，应该可以看到，如果existingTransaction这个变量为false，也就是说，你所进行的操作没有进行事务管理的，最总就关闭session了。大家可以代码跟踪一下就知道existingTransaction一般情况下的值就是为false的。那么我们就可以考虑让existingTransaction为true，也就是让自己的操作进入事务管理，这个时候session就不会关闭了，除非你自己关闭。比如我们进行一个junit测试如下：

package jimmee.cn.edu.zju;

import java.util.Iterator;

import java.util.Set;

import junit.framework.TestCase;

import org.apache.log4j.Logger;

import org.hibernate.Session;

import org.hibernate.SessionFactory;

import org.springframework.context.ApplicationContext;

import org.springframework.context.support.ClassPathXmlApplicationContext;

import org.springframework.orm.hibernate3.MyHibernateTemplate;

import org.springframework.orm.hibernate3.MySessionFactoryUtils;

import org.springframework.orm.hibernate3.SessionFactoryUtils;

import org.springframework.orm.hibernate3.SessionHolder;

import org.springframework.transaction.support.TransactionSynchronizationManager;

public class TestSession extends TestCase

{

Logger log = Logger.getLogger(TestSession.class);

SessionFactory sessionFactory = null;

Session session = null;

ApplicationContext bf = null;

public void testSession()

{

MyHibernateTemplate template = (MyHibernateTemplate) bf

.getBean("hibernateTemplate");

Emp emp = (Emp) template.load(Emp.class, new Long(7369));

long no = emp.getEmpno();

log.info("empno = " + no);

String name = emp.getEname();

log.info("Empname = " + name);

Dept dept = (Dept) template.load(Dept.class, new Long(30));

log.info("dept no =" + dept.getDeptno());

log.info("dept name=" + dept.getDname());

Set set = dept.getEmps();

for (Iterator iterator = set.iterator(); iterator.hasNext();)

{

Emp emp3 = (Emp) iterator.next();

log.info("empname = " + emp3.getEname());

}

@Override

protected void setUp() throws Exception

{

super.setUp();

bf = new ClassPathXmlApplicationContext("applicationContext.xml");

sessionFactory = (SessionFactory) bf.getBean("sessionFactory");

session = MySessionFactoryUtils.getSession(sessionFactory, true);

TransactionSynchronizationManager.bindResource(sessionFactory,

new SessionHolder(session));

}

@Override

protected void tearDown() throws Exception

{

super.tearDown();

SessionHolder holder = (SessionHolder) TransactionSynchronizationManager

.getResource(sessionFactory);

Session s = holder.getSession();

s.flush();

TransactionSynchronizationManager.unbindResource(sessionFactory);

MySessionFactoryUtils.closeSession(s);

sessionFactory.close();

}

为了明显的看到一些信息，我继承了HibernateTemplate了，在MyHibernateTemplate里重写了 public Object execute(HibernateCallback action, boolean exposeNativeSession) throws DataAccessException方法，自己添加了一些打印信息，最终输出结果如下：

2008-03-10 14:08:46,921 INFO [jimmee.cn.edu.zju.TestSession] - empno = 7369
Hibernate: select emp0_.EMPNO as EMPNO0_0_, emp0_.DEPTNO as DEPTNO0_0_, emp0_.ENAME as ENAME0_0_, emp0_.JOB as JOB0_0_, emp0_.MGR as MGR0_0_, emp0_.HIREDATE as HIREDATE0_0_, emp0_.SAL as SAL0_0_, emp0_.COMM as COMM0_0_ from SCOTT.EMP emp0_ where emp0_.EMPNO=?
2008-03-10 14:08:47,015 INFO [jimmee.cn.edu.zju.TestSession] - Empname = SMITH
2008-03-10 14:08:47,015 DEBUG [org.springframework.transaction.support.TransactionSynchronizationManager] - Retrieved value [org.springframework.orm.hibernate3.SessionHolder@2bccb2] for key [org.hibernate.impl.SessionFactoryImpl@18bbf55] bound to thread [main]
2008-03-10 14:08:47,015 INFO [org.springframework.orm.hibernate3.MyHibernateTemplate] - session id=26578114
2008-03-10 14:08:47,015 INFO [org.springframework.orm.hibernate3.MyHibernateTemplate] - factory id=org.hibernate.impl.SessionFactoryImpl@18bbf55
2008-03-10 14:08:47,015 DEBUG [org.springframework.transaction.support.TransactionSynchronizationManager] - Retrieved value [org.springframework.orm.hibernate3.SessionHolder@2bccb2] for key [org.hibernate.impl.SessionFactoryImpl@18bbf55] bound to thread [main]
2008-03-10 14:08:47,015 INFO [org.springframework.orm.hibernate3.MyHibernateTemplate] - existingTransaction = true
2008-03-10 14:08:47,015 DEBUG [org.springframework.orm.hibernate3.MyHibernateTemplate] - Found thread-bound Session for HibernateTemplate
2008-03-10 14:08:47,015 DEBUG [org.springframework.orm.hibernate3.MyHibernateTemplate] - Not closing pre-bound Hibernate Session after HibernateTemplate
2008-03-10 14:08:47,015 INFO [jimmee.cn.edu.zju.TestSession] - dept no =30
Hibernate: select dept0_.DEPTNO as DEPTNO1_0_, dept0_.DNAME as DNAME1_0_, dept0_.LOC as LOC1_0_ from SCOTT.DEPT dept0_ where dept0_.DEPTNO=?
2008-03-10 14:08:47,031 INFO [jimmee.cn.edu.zju.TestSession] - dept name=SALES
Hibernate: select emps0_.DEPTNO as DEPTNO1_, emps0_.EMPNO as EMPNO1_, emps0_.EMPNO as EMPNO0_0_, emps0_.DEPTNO as DEPTNO0_0_, emps0_.ENAME as ENAME0_0_, emps0_.JOB as JOB0_0_, emps0_.MGR as MGR0_0_, emps0_.HIREDATE as HIREDATE0_0_, emps0_.SAL as SAL0_0_, emps0_.COMM as COMM0_0_ from SCOTT.EMP emps0_ where emps0_.DEPTNO=?
2008-03-10 14:08:47,046 INFO [jimmee.cn.edu.zju.TestSession] - empname = BLAKE
2008-03-10 14:08:47,046 INFO [jimmee.cn.edu.zju.TestSession] - empname = WARD
2008-03-10 14:08:47,046 INFO [jimmee.cn.edu.zju.TestSession] - empname = TURNER
2008-03-10 14:08:47,046 INFO [jimmee.cn.edu.zju.TestSession] - empname = ALLEN
2008-03-10 14:08:47,046 INFO [jimmee.cn.edu.zju.TestSession] - empname = MARTIN
2008-03-10 14:08:47,046 INFO [jimmee.cn.edu.zju.TestSession] - empname = JAMES
2008-03-10 14:08:47,046 DEBUG [org.springframework.transaction.support.TransactionSynchronizationManager] - Retrieved value [org.springframework.orm.hibernate3.SessionHolder@2bccb2] for key [org.hibernate.impl.SessionFactoryImpl@18bbf55] bound to thread [main]
2008-03-10 14:08:47,062 DEBUG [org.springframework.transaction.support.TransactionSynchronizationManager] - Removed value [org.springframework.orm.hibernate3.SessionHolder@2bccb2] for key [org.hibernate.impl.SessionFactoryImpl@18bbf55] from thread [main]
2008-03-10 14:08:47,062 INFO [org.springframework.orm.hibernate3.SessionFactoryUtils] - Closing Hibernate Session
2008-03-10 14:08:47,062 DEBUG [org.springframework.orm.hibernate3.SessionFactoryUtils] - Closing Hibernate Session
2008-03-10 14:08:47,062 INFO [org.springframework.orm.hibernate3.SessionFactoryUtils] - session id=26578114
2008-03-10 14:08:47,078 INFO [org.springframework.orm.hibernate3.SessionFactoryUtils] - Closed Hibernate Session
2008-03-10 14:08:47,078 INFO [org.hibernate.impl.SessionFactoryImpl] - closing