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1.5. The EventManager Web Application
1.5.1. About the EventManager
A Hibernate web application uses
Session
and Transaction
almost like a standalone application. However, some common patterns are useful. You can now write an EventManagerServlet
. This servlet can list all events stored in the database, and it provides an HTML form to enter new events.
1.5.2. Writing the Basic Servlet
First we need create our basic processing servlet. Since our servlet only handles HTTP
GET
requests, we will only implement the doGet()
method:
package org.hibernate.tutorial.web; // Imports public class EventManagerServlet extends HttpServlet { protected void doGet( HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { SimpleDateFormat dateFormatter = new SimpleDateFormat( "dd.MM.yyyy" ); try { // Begin unit of work HibernateUtil.getSessionFactory().getCurrentSession().beginTransaction(); // Process request and render page... // End unit of work HibernateUtil.getSessionFactory().getCurrentSession().getTransaction().commit(); } catch (Exception ex) { HibernateUtil.getSessionFactory().getCurrentSession().getTransaction().rollback(); if ( ServletException.class.isInstance( ex ) ) { throw ( ServletException ) ex; } else { throw new ServletException( ex ); } } } }
Save this servlet as
src/main/java/org/hibernate/tutorial/web/EventManagerServlet.java
The pattern applied here is called session-per-request. When a request hits the servlet, a new Hibernate
Session
is opened through the first call to getCurrentSession()
on the SessionFactory
. A database transaction is then started. All data access occurs inside a transaction irrespective of whether the data is read or written. Do not use the auto-commit mode in applications.
Do not use a new Hibernate
Session
for every database operation. Use one Hibernate Session
that is scoped to the whole request. Use getCurrentSession()
, so that it is automatically bound to the current Java thread.
Next, the possible actions of the request are processed and the response HTML is rendered. We will get to that part soon.
Finally, the unit of work ends when processing and rendering are complete. If any problems occurred during processing or rendering, an exception will be thrown and the database transaction rolled back. This completes the
session-per-request
pattern. Instead of the transaction demarcation code in every servlet, you could also write a servlet filter. See the Hibernate website and Wiki for more information about this pattern called Open Session in View. You will need it as soon as you consider rendering your view in JSP, not in a servlet.
1.5.3. Processing and Rendering
Now you can implement the processing of the request and the rendering of the page.
// Write HTML header PrintWriter out = response.getWriter(); out.println("<html><head><title>Event Manager</title></head><body>"); // Handle actions if ( "store".equals(request.getParameter("action")) ) { String eventTitle = request.getParameter("eventTitle"); String eventDate = request.getParameter("eventDate"); if ( "".equals(eventTitle) || "".equals(eventDate) ) { out.println("<b><i>Please enter event title and date.</i></b>"); } else { createAndStoreEvent(eventTitle, dateFormatter.parse(eventDate)); out.println("<b><i>Added event.</i></b>"); } } // Print page printEventForm(out); listEvents(out, dateFormatter); // Write HTML footer out.println("</body></html>"); out.flush(); out.close();
This coding style, with a mix of Java and HTML, would not scale in a more complex application—keep in mind that we are only illustrating basic Hibernate concepts in this tutorial. The code prints an HTML header and a footer. Inside this page, an HTML form for event entry and a list of all events in the database are printed. The first method is trivial and only outputs HTML:
private void printEventForm(PrintWriter out) { out.println("<h2>Add new event:</h2>"); out.println("<form>"); out.println("Title: <input name='eventTitle' length='50'/><br/>"); out.println("Date (e.g. 24.12.2009): <input name='eventDate' length='10'/><br/>"); out.println("<input type='submit' name='action' value='store'/>"); out.println("</form>"); }
The
listEvents()
method uses the Hibernate Session
bound to the current thread to execute a query:
private void listEvents(PrintWriter out, SimpleDateFormat dateFormatter) { List result = HibernateUtil.getSessionFactory() .getCurrentSession().createCriteria(Event.class).list(); if (result.size() > 0) { out.println("<h2>Events in database:</h2>"); out.println("<table border='1'>"); out.println("<tr>"); out.println("<th>Event title</th>"); out.println("<th>Event date</th>"); out.println("</tr>"); Iterator it = result.iterator(); while (it.hasNext()) { Event event = (Event) it.next(); out.println("<tr>"); out.println("<td>" + event.getTitle() + "</td>"); out.println("<td>" + dateFormatter.format(event.getDate()) + "</td>"); out.println("</tr>"); } out.println("</table>"); } }
Finally, the
store
action is dispatched to the createAndStoreEvent()
method, which also uses the Session
of the current thread:
protected void createAndStoreEvent(String title, Date theDate) { Event theEvent = new Event(); theEvent.setTitle(title); theEvent.setDate(theDate); HibernateUtil.getSessionFactory() .getCurrentSession().save(theEvent); }
The servlet is now complete. A request to the servlet will be processed in a single
Session
and Transaction
. As earlier in the standalone application, Hibernate can automatically bind these objects to the current thread of execution. This gives you the freedom to layer your code and access the SessionFactory
in any way you like. Usually you would use a more sophisticated design and move the data access code into data access objects (the DAO pattern).
1.5.4. Deploying and Testing
To deploy this application for testing we must create a Web ARchive (WAR). First we must define the WAR descriptor as
src/main/webapp/WEB-INF/web.xml
<?xml version="1.0" encoding="UTF-8"?> <web-app version="2.4" xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd"> <servlet> <servlet-name>Event Manager</servlet-name> <servlet-class>org.hibernate.tutorial.web.EventManagerServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>Event Manager</servlet-name> <url-pattern>/eventmanager</url-pattern> </servlet-mapping> </web-app>
To build and deploy call
mvn package
in your project directory and copy the hibernate-tutorial.war
file into your $JBOSS_HOME/server/$CONFIG/deploy
directory.
Once deployed and JBoss is running, access the application at
http://localhost:8080/hibernate-tutorial/eventmanager
. Watch the server log (in $JBOSS_HOME/server/$CONFIG/log/server.log
) to see Hibernate initialize when the first request hits your servlet (the static initializer in HibernateUtil
is called) and to get the detailed output if any exceptions occurs.
1.5.5. Summary
This tutorial covered the basics of writing a simple standalone Hibernate application and a small web application. More tutorials are available from the Hibernate website.