WebApplicationContextProvider


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56  
57  package org.objectstyle.cayenne.conf;
58  
59  import javax.servlet.ServletRequest  ;
60  import javax.servlet.ServletRequestEvent  ;
61  import javax.servlet.ServletRequestListener  ;
62  import javax.servlet.http.HttpServletRequest  ;
63  import javax.servlet.http.HttpSession  ;
64  
65  import org.objectstyle.cayenne.access.DataContext;
66  
67  /**
68   * WebApplicationContextProvider is a helper class integrating Cayenne with web applications.
69   * It performs two related operations - (1) whenever an HttpSession is started, it creates a 
70   * new instance of DataContext and sets it as a session attribute; (2) whenever a new request 
71   * comes in for the session, it retrieves previously stored session DataContext and binds it to
72   * the request worker thread for the duration of the request. This way any class can get access to 
73   * the correct instance of DataContext without knowing anything about the servlet environment. This 
74   * is useful for decoupling the controller layer (Servlets, Struts, etc.) from a DAO layer, 
75   * making public DAO API independent from DataContext.
76   * 
77   * <p><b>Installation in the web container:</b> WebApplicationContextProvider should be
78   * installed as a listener, similar to the WebApplicationListener). Per servlet specification,
79   * listener is configured in <code>web.xml</code> deployment descriptor:
80   * </p>
81   * 
82   * <pre>
83   * &lt;listener&gt;
84   *   &lt;listener-class&gt;org.objectstyle.cayenne.conf.WebApplicationContextProvider&lt;/listener-class&gt;
85   * &lt;/listener&gt;
86   * </pre>
87   * 
88   * <p><b>Accessing thread DataContext:</b> A DataContext bound to a worker thread can be retrieved 
89   * for the duration of the request processing by calling the static method
90   * {@link DataContext#getThreadDataContext()}.
91   * </p>
92   * 
93   * <p>
94   * <p><b>Upgrading from {@link WebApplicationListener}:</b> WebApplicationContextProvider supersedes
95   * WebApplicationListener. WebApplicationContextProvider relies on ServletRequestListener
96   * API that is a part of Servlet Specification since version 2.4. Therefore it requires a
97   * container that supports 2.4 spec, e.g. Tomcat 5.* </p>
98   * </p>
99   * 
100  * @author <a HREF="mailto:scott@smartblob.com">Scott McClure </a>
101  * @since 1.1
102  */
103 public class WebApplicationContextProvider extends WebApplicationListener implements
104         ServletRequestListener   {
105 
106     /**
107      * Retrieves the DataContext bound earlier to the HttpSession, and binds it to the
108      * current thread.
109      */
110     public void requestInitialized(ServletRequestEvent   sre) {
111 
112         ServletRequest   req = sre.getServletRequest();
113         if (req instanceof HttpServletRequest  ) {
114 
115             HttpSession   session = ((HttpServletRequest  ) req).getSession();
116             DataContext dataContext = ServletUtil.getSessionContext(session);
117 
118             // even if null, reset to null is a good thing here...
119             DataContext.bindThreadDataContext(dataContext);
120             if (dataContext == null) {
121                 throw new IllegalStateException  (
122                         "DataContext is null for the session, cannot bind to thread.");
123             }
124         }
125     }
126 
127     /**
128      * Removes the reference to the DataContext from the thread.
129      */
130     public void requestDestroyed(ServletRequestEvent   sre) {
131         // Prevent lingering object references
132         DataContext.bindThreadDataContext(null);
133     }
134 }
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