1.4. 第三部分 - EventManager web应用程序

Let's turn the following discussion into a small web application...

Hibernate web应用程序使用SessionTransaction的方式几乎和独立应用程序是一样的。但是,有一些常见的模式(pattern)非常有用。现在我们编写一个EventManagerServlet。这个servlet可以列出数据库中保存的所有的events,还提供一个HTML表单来增加新的events。

1.4.1. 编写基本的servlet

在你的源代码目录的events包中创建一个新的类:

package events;

// Imports

public class EventManagerServlet extends HttpServlet {

    // Servlet code
}

我们后面会用到dateFormatter 的工具, 它把Date对象转换为字符串。只要一个formatter作为servlet的成员就可以了。

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();
        throw new ServletException(ex);
    }

}

The pattern we are applying 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. Then a database transaction is started-all data access as to occur inside a transaction, no matter if data is read or written (we don't use the auto-commit mode in applications).

我们称这里应用的模式为每次请求一个session(session-per-request)。当有请求到达这个servlet的时候,通过对SessionFactory的第一次调用,打开一个新的Hibernate Session。然后启动一个数据库事务-所有的数据访问都是在事务中进行,不管是读还是写(我们在应用程序中不使用auto-commit模式)。

不要为每次数据库操作都使用一个新的Hibernate Session。将Hibernate Session的范围设置为整个请求。要用getCurrentSession(),这样它自动会绑定到当前Java线程。

Finally, the unit of work ends when processing and rendering is complete. If any problem 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'll need it as soon as you consider rendering your view in JSP, not in a servlet.

1.4.2. 最后,当处理与渲染都结束的时候,这个工作单元就结束了。假若在处理或渲染的时候有任何错误发生,会抛出一个异常,回滚数据库事务。这样,session-per-request模式就完成了。为了避免在每个servlet中都编写事务边界界定的代码,可以考虑写一个servlet 过滤器(filter)来更好地解决。关于这一模式的更多信息,请参阅Hibernate网站和Wiki,这一模式叫做Open Session in View-只要你考虑用JSP来渲染你的视图(view),而不是在servlet中,你就会很快用到它。

处理与渲染

// 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();

Granted, 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>");
}

listEvents()方法使用绑定到当前线程的Hibernate Session来执行查询:

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>");
        for (Iterator it = result.iterator(); 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>");
    }
}

最后,store动作会被导向到createAndStoreEvent()方法,它也使用当前线程的Session:

protected void createAndStoreEvent(String title, Date theDate) {
    Event theEvent = new Event();
    theEvent.setTitle(title);
    theEvent.setDate(theDate);

    HibernateUtil.getSessionFactory()
                    .getCurrentSession().save(theEvent);
}

That's it, the servlet is 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'd use a more sophisticated design and move the data access code into data access objects (the DAO pattern). See the Hibernate Wiki for more examples.

1.4.3. 部署与测试

要发布这个程序,你得把它打成web发布包:WAR文件。把下面的脚本加入到你的build.xml中:

<target name="war" depends="compile">
    <war destfile="hibernate-tutorial.war" webxml="web.xml">
        <lib dir="${librarydir}">
          <exclude name="jsdk*.jar"/>
        </lib>

        <classes dir="${targetdir}"/>
    </war>
</target>

这段代码在你的开发目录中创建一个hibernate-tutorial.war的文件。它把所有的类库和web.xml描述文件都打包进去,web.xml 文件应该位于你的开发根目录中:

<?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>events.EventManagerServlet</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>Event Manager</servlet-name>
        <url-pattern>/eventmanager</url-pattern>
    </servlet-mapping>
</web-app>

Before you compile and deploy the web application, note that an additional library is required: jsdk.jar. This is the Java servlet development kit, if you don't have this library already, get it from the Sun website and copy it to your library directory. However, it will be only used for compilation and excluded from the WAR package.

在你的开发目录中,调用ant war来构建、打包,然后把hibernate-tutorial.war文件拷贝到你的tomcat的webapps目录下。假若你还没安装Tomcat,就去下载一个,按照指南来安装。对此应用的发布,你不需要修改任何Tomcat的配置。

在部署完,启动Tomcat之后,通过http://localhost:8080/hibernate-tutorial/eventmanager进行访问你的应用,在第一次servlet 请求发生时,请在Tomcat log中确认你看到Hibernate被初始化了(HibernateUtil的静态初始化器被调用),假若有任何异常抛出,也可以看到详细的输出。