转自：Karl Baum's Weblog
Karl Baum's Weblog
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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);
}
}
( Jul 08 2004, 09:39:55 AM EDT ) Permalink Comments [2]
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Comments:
A few things to keep in the back of your mind if you take this approach; 1. If any errors occur while attempting to lazy load relationships in the view (JSP) it would be hard to present a nice error to the user. 2. This would result in at least 2 hibernate sessions (db connections being open for any one request), so you might want to up the number of connections available. Cheers, Dan
Posted by Dan Washusen on July 08, 2004 at 09:02 PM EDT #
I am a little confused on why it would be difficult to show a nice error jsp. Couldn't we just use the provided servlet container error page mechanisms? In regards to the 2 hibernate sessions being opened. Are you saying that the OpenSessionInViewInterceptor would be run twice if an exception was thrown? Thanks for your feedback!
Posted by Karl Baum (63.170.158.133) on July 09, 2004 at 09:48 AM EDT #