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Servlet filters intercept requests and responses, allowing you to manipulate or transform the contained information. While we re not going to delve into filters in great detail, the following example should give you
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Decoupling Hibernate from the web layer
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an initial idea of what filters can do. The filter class shown in listing 8.7 has two responsibilities. First, it places a Session instance into the servlet request scope. The controller servlet retrieves the Session and uses it. Second, the filter has to close the Session instance. Listing 8.7 Session management filter
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package com.manning.hq.ch08; import import import import import import javax.servlet.Filter; javax.servlet.FilterChain; javax.servlet.FilterConfig; javax.servlet.ServletException; javax.servlet.ServletRequest; javax.servlet.ServletResponse;
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import org.hibernate.SessionFactory; public class SessionFilter implements Filter { private SessionFactory factory; Initializes the SessionFactory
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public void init(FilterConfig cfg) throws ServletException { factory = new Configuration().configure().buildSessionFactory(); } public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain) throws ServletException { Opens a new Session Session session = factory.openSession(); req.setAttribute("hibernate.session", session); Places the Session into the request scope try { chain.doFilter(req, res); Calls the next item in } the filter chain, which is your servlet finally { if ((session != null) && (session.isOpen())) { session.close(); After the servlet returns, closes the Session } } } public void destroy() {} }
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Web frameworks: WebWork, Struts, and Tapestry
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Once the filter is created, you need to edit the web.xml file to configure it. Configuring the filter consists of telling the application server which requests should pass through the filter. Listing 8.8 shows the filter configuration. Listing 8.8 Filter configuration
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<filter> Declares the filter <filter-name>smFilter</filter-name> <filter-class>com.manning.hq.ch08.SessionFilter</filter-class> </filter> Maps the filter to the desired servlet <filter-mapping> <filter-name>Session Management Filter Mapping</filter-name> <servlet-name>myControllerServlet</servlet-name> </filter-mapping> <servlet> Declares the servlet <servlet-name>myControllerServlet</servlet-name> <servlet-class> com.manning.hq.ch08.ControllerServlet </servlet-class> </servlet> Maps the servlet to a URL pattern <servlet-mapping> <servlet-name>myControllerServlet</servlet-name> <url-pattern>/controller</url-pattern> </servlet-mapping>
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Using the configuration in listing 8.8, your filter will always be called before myControllerServlet is executed. The filter guarantees that you will always have a valid Hibernate Session before the request reaches the servlet and that the Session will be properly cleaned up when the request has finished with it. Of course, if you have multiple controller servlets in the application, you need to intercept each of them. If there is a common naming scheme for all servlets, you can simply use a URL pattern instead of a servlet name, for example:
<filter-mapping> <filter-name>Session Management Filter Mapping</filter-name>
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<url-pattern>*Servlet</url-pattern> </filter-mapping>
Making the Hibernate Session available to the Controller is fairly straightforward using either of the two methods detailed above. If you re using a web framework, such as Struts, you have other options for making Sessions available. We ll look at the options for each framework in the next section.
8.3.4 Accessing the Session from the Service layer
Since the Service layer is encapsulated from the web application, you can t easily pass the Session instance to the relevant classes from the web layer. Instead, one component of the Service layer, the DAO, needs to obtain Sessions. A preferred pattern for this is the Thread Local Session. The Thread Local Session pattern makes use of the java.lang.ThreadLocal class to create a Session that is accessible from a single application thread. This is particularly convenient in multithreaded applications, such as web applications. The core of the pattern is defined in one class, shown in listing 8.9. Listing 8.9 Thread Local Session provider
package com.manning.hq.ch08; import import import import org.hibernate.HibernateException; org.hibernate.Session; org.hibernate.SessionFactory; org.hibernate.cfg.Configuration;
public class ServiceLocator { private static final ThreadLocal t = new ThreadLocal(); private static SessionFactory factory; static { try { Builds the SessionFactory factory = new Configuration().configure().buildSessionFactory(); } catch (HibernateException e) {
Web frameworks: WebWork, Struts, and Tapestry
e.printStackTrace(); } } Retrieves the public static Session getSession() Session from the ThreadLocal throws HibernateException { instance Session s = (Session) t.get(); if (s == null) { If the returned Session is s = factory.openSession(); null, opens a new Session t.set(s); Sets it as the } current Session for the ThreadLocal return s; instance } public void closeSession() { Session s = (Session) t.get(); if (s != null) { try { s.close(); } catch (HibernateException e) { e.printStackTrace(); } Sets the contained } Session to null t.set(null); } }
If there is any magic to the ServiceLocator class, it s in how the ThreadLocal class behaves. By storing the initialized Session instance in the ThreadLocal variable, you are ensured that only one Session is created per request thread. When the Session is retrieved the first time, a new Session is opened by the SessionFactory and set in the ThreadLocal instance. Subsequent calls to getSession() return the same Session instance as long as the Session hasn t been closed with a call to closeSession().
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