No 1:
关于OpenSessionInViewFilter
转自：Potain 的BLOG
OpenSessionInView
Created by potian. Last edited by admin 61 days ago. Viewed 181 times.
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Hibernate的Lazy初始化1:n关系时，你必须保证是在同一个Session内部使用这个关系集合，不然Hiernate将抛出例外。
另外，你不愿意你的DAO测试代码每次都打开关系Session，因此，我们一般会采用OpenSessionInView模式。
OpenSessionInViewFilter解决Web应用程序的问题
如果程序是在正常的Web程序中运行，那么Spring的OpenSessionInViewFilter能够解决问题，它：
protected void doFilterInternal(HttpServletRequest request,
HttpServletResponse response,
FilterChain filterChain) throws ServletException, IOException {
SessionFactory sessionFactory = lookupSessionFactory();
logger.debug("Opening Hibernate Session in OpenSessionInViewFilter");
Session session = getSession(sessionFactory);
TransactionSynchronizationManager.bindResource(sessionFactory,
new SessionHolder(session));
try {
filterChain.doFilter(request, response);
}
finally {
TransactionSynchronizationManager.unbindResource(sessionFactory);
logger.debug("Closing Hibernate Session in OpenSessionInViewFilter");
closeSession(session, sessionFactory);
}
}
可以看到，这个Filter在request开始之前，把sessionFactory绑定到TransactionSynchronizationManager，和这个SessionHolder相关。这个意味着所有request执行过程中将使用这个session。而在请求结束后，将和这个sessionFactory对应的session解绑，并且关闭Session。
为什么绑定以后，就可以防止每次不会新开一个Session呢？看看HibernateDaoSupport的情况：
public final void setSessionFactory(SessionFactory sessionFactory) {
this.hibernateTemplate = new HibernateTemplate(sessionFactory);
}
protected final HibernateTemplate getHibernateTemplate() {
return hibernateTemplate;
}
我们的DAO将使用这个template进行操作：
public abstract class BaseHibernateObjectDao
extends HibernateDaoSupport
implements BaseObjectDao {
protected BaseEntityObject getByClassId(final long id) {
BaseEntityObject obj =
(BaseEntityObject) getHibernateTemplate()
.execute(new HibernateCallback() {
public Object doInHibernate(Session session)
throws HibernateException {
return session.get(getPersistentClass(),
new Long(id));
}
});
return obj;
}
public void save(BaseEntityObject entity) {
getHibernateTemplate().saveOrUpdate(entity);
}
public void remove(BaseEntityObject entity) {
try {
getHibernateTemplate().delete(entity);
} catch (Exception e) {
throw new FlexEnterpriseDataAccessException(e);
}
}
public void refresh(final BaseEntityObject entity) {
getHibernateTemplate().execute(new HibernateCallback() {
public Object doInHibernate(Session session)
throws HibernateException {
session.refresh(entity);
return null;
}
});
}
public void replicate(final Object entity) {
getHibernateTemplate().execute(new HibernateCallback() {
public Object doInHibernate(Session session)
throws HibernateException {
session.replicate(entity,
ReplicationMode.OVERWRITE);
return null;
}
});
}
而HibernateTemplate试图每次在execute之前去获得Session，执行完就力争关闭Session
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());
}
}
而这个SessionFactoryUtils能否得到当前的session以及closeSessionIfNecessary是否真正关闭session，端取决于这个session是否用sessionHolder和这个sessionFactory在我们最开始提到的TransactionSynchronizationManager绑定。
public static void closeSessionIfNecessary(Session session,
SessionFactory sessionFactory)
throws CleanupFailureDataAccessException {
if (session == null ||
TransactionSynchronizationManager.hasResource(sessionFactory)) {
return;
}
logger.debug("Closing Hibernate session");
try {
session.close();
}
catch (JDBCException ex) {
// SQLException underneath
throw new CleanupFailureDataAccessException(
"Cannot close Hibernate session", ex.getSQLException());
}
catch (HibernateException ex) {
throw new CleanupFailureDataAccessException(
"Cannot close Hibernate session", ex);
}
}
HibernateInterceptor和OpenSessionInViewInterceptor的问题
使用同样的方法，这两个Interceptor可以用来解决问题。但是关键的不同之处在于，它们的力度只能定义在DAO或业务方法上，而不是在我们的Test方法上，除非我们把它们应用到TestCase的方法上，但你不大可能为TestCase去定义一个接口，然后把Interceptor应用到这个接口的某些方法上。直接使用HibernateTransactionManager也是一样的。因此，如果我们有这样的测试：
Category parentCategory = new Category ();
parentCategory.setName("parent");
dao.save(parentCategory);
Category childCategory = new Category();
childCategory.setName("child");
parentCategory.addChild(childCategory);
dao.save(childCategory);
Category savedParent = dao.getCategory("parent");
Category savedChild = (Category ) savedParent.getChildren().get(0);
assertEquals(savedChild, childCategory);
将意味着两件事情：
• 每次DAO执行都会启动一个session和关闭一个session
• 如果我们定义了一个lazy的关系，那么最后的Category savedChild = (Category ) savedParent.getChildren().get(0);将会让hibernate报错。
解决方案
一种方法是对TestCase应用Interceptor或者TransactionManager，但这个恐怕会造成很多麻烦。除非是使用增强方式的AOP.我前期采用这种方法(Aspectwerkz)，在Eclipse里面也跑得含好。
另一种方法是在TestCase的setup和teardown里面实现和Filter完全一样的处理，其他的TestCase都从这个TestCase继承，这种方法是我目前所使用的。
NO 2:
转自：Karl Baum's Weblog
Karl Baum's Weblog
All | General | Java
Thursday July 08, 2004
Lazy Initialization and the DAO pattern with Hibernate and Spring
Hibernate and Lazy Initialization
Hibernate object relational mapping offers both lazy and non-lazy modes of object initialization. Non-lazy initialization retrieves an object and all of its related objects at load time. This can result in hundreds if not thousands of select statements when retrieving one entity. The problem is compounded when bi-directional relationships are used, often causing entire databases to be loaded during the initial request. Of course one could tediously examine each object relationship and manually remove those most costly, but in the end, we may be losing the ease of use benefit sought in using the ORM tool.
The obvious solution is to employ the lazy loading mechanism provided by hibernate. This initialization strategy only loads an object's one-to-many and many-to-many relationships when these fields are accessed. The scenario is practically transparent to the developer and a minimum amount of database requests are made, resulting in major performance gains. One drawback to this technique is that lazy loading requires the Hibernate session to remain open while the data object is in use. This causes a major problem when trying to abstract the persistence layer via the Data Access Object pattern. In order to fully abstract the persistence mechanism, all database logic, including opening and closing sessions, must not be performed in the application layer. Most often, this logic is concealed behind the DAO implementation classes which implement interface stubs. The quick and dirty solution is to forget the DAO pattern and include database connection logic in the application layer. This works for small applications but in large systems this can prove to be a major design flaw, hindering application extensibility.
Being Lazy in the Web Layer
Fortunately for us, the Spring Framework has developed an out of box web solution for using the DAO pattern in combination with Hibernate lazy loading. For anyone not familiar with using the Spring Framework in combination with Hibernate, I will not go into the details here, but I encourage you to read Hibernate Data Access with the Spring Framework. In the case of a web application, Spring comes with both the OpenSessionInViewFilter and the OpenSessionInViewInterceptor. One can use either one interchangeably as both serve the same function. The only difference between the two is the interceptor runs within the Spring container and is configured within the web application context while the Filter runs in front of Spring and is configured within the web.xml. Regardless of which one is used, they both open the hibernate session during the request binding this session to the current thread. Once bound to the thread, the open hibernate session can transparently be used within the DAO implementation classes. The session will remain open for the view allowing lazy access the database value objects. Once the view logic is complete, the hibernate session is closed either in the Filter doFilter method or the Interceptor postHandle method. Below is an example of the configuration of each component:
Interceptor Configuration
<beans>
<bean id="urlMapping"
class="org.springframework.web.servlet.handler.SimpleUrlHandlerMapping">
<property name="interceptors">
<list>
<ref bean="openSessionInViewInterceptor"/>
</list>
</property>
<property name="mappings">
...
</bean>
...
<bean name="openSessionInViewInterceptor"
class="org.springframework.orm.hibernate.support.OpenSessionInViewInterceptor">
<property name="sessionFactory"><ref bean="sessionFactory"/></property>
</bean>
</beans>
Filter Configuration
<web-app>
...
<filter>
<filter-name>hibernateFilter</filter-name>
<filter-class>
org.springframework.orm.hibernate.support.OpenSessionInViewFilter
</filter-class>
</filter>
...
<filter-mapping>
<filter-name>hibernateFilter</filter-name>
<url-pattern>*.spring</url-pattern>
</filter-mapping>
...
</web-app>
Implementing the Hibernate DAO's to use the open session is simple. In fact, if you are already using the Spring Framework to implement your Hibernate DAO's, most likely you will not have to change a thing. The DAO's must access Hibernate through the convenient HibernateTemplate utility, which makes database access a piece of cake. Below is an example DAO.
Example DAO
public class HibernateProductDAO extends HibernateDaoSupport implements ProductDAO {
public Product getProduct(Integer productId) {
return (Product)getHibernateTemplate().load(Product.class, productId);
}
public Integer saveProduct(Product product) {
return (Integer) getHibernateTemplate().save(product);
}
public void updateProduct(Product product) {
getHibernateTemplate().update(product);
}
}
Being Lazy in the Business Layer
Even outside the view, the Spring Framework makes it easy to use lazy load initialization, through the AOP interceptor HibernateInterceptor. The hibernate interceptor transparently intercepts calls to any business object configured in the Spring application context, opening a hibernate session before the call, and closing the session afterward. Let's run through a quick example. Suppose we have an interface BusinessObject:
public interface BusinessObject {
public void doSomethingThatInvolvesDaos();
}
The class BusinessObjectImpl implements BusinessObject:
public class BusinessObjectImpl implements BusinessObject {
public void doSomethingThatInvolvesDaos() {
// lots of logic that calls
// DAO classes Which access
// data objects lazily
}
}
Through some configurations in the Spring application context, we can instruct the HibernateInterceptor to intercept calls to the BusinessObjectImpl allowing it's methods to lazily access data objects. Take a look at the fragment below:
<beans>
<bean id="hibernateInterceptor" class="org.springframework.orm.hibernate.HibernateInterceptor">
<property name="sessionFactory">
<ref bean="sessionFactory"/>
</property>
</bean>
<bean id="businessObjectTarget" class="com.acompany.BusinessObjectImpl">
<property name="someDAO"><ref bean="someDAO"/></property>
</bean>
<bean id="businessObject" class="org.springframework.aop.framework.ProxyFactoryBean">
<property name="target"><ref bean="businessObjectTarget"/></property>
<property name="proxyInterfaces">
<value>com.acompany.BusinessObject</value>
</property>
<property name="interceptorNames">
<list>
<value>hibernateInterceptor</value>
</list>
</property>
</bean>
</beans>
When the businessObject bean is referenced, the HibernateInterceptor opens a hibernate session and passes the call onto the BusinessObjectImpl. When the BusinessObjectImpl has finished executing, the HibernateInterceptor transparently closes the session. The application code has no knowledge of any persistence logic, yet it is still able to lazily access data objects.
Being Lazy in your Unit Tests
Last but not least, we'll need the ability to test our lazy application from J-Unit. This is easily done by overriding the setUp and tearDown methods of the TestCase class. I prefer to keep this code in a convenient abstract TestCase class for all of my tests to extend.
public abstract class MyLazyTestCase extends TestCase {
private SessionFactory sessionFactory;
private Session session;
public void setUp() throws Exception {
super.setUp();
SessionFactory sessionFactory = (SessionFactory) getBean("sessionFactory");
session = SessionFactoryUtils.getSession(sessionFactory, true);
Session s = sessionFactory.openSession();
TransactionSynchronizationManager.bindResource(sessionFactory, new SessionHolder(s));
}
protected Object getBean(String beanName) {
//Code to get objects from Spring application context
}
public void tearDown() throws Exception {
super.tearDown();
SessionHolder holder = (SessionHolder) TransactionSynchronizationManager.getResource(sessionFactory);
Session s = holder.getSession();
s.flush();
TransactionSynchronizationManager.unbindResource(sessionFactory);
SessionFactoryUtils.closeSessionIfNecessary(s, sessionFactory);
}
}