3
Servlet Filters and Event Listeners

This chapter describes the following servlet features that were introduced in the servlet 2.3 specification:

Servlet Filters

Servlet filters are used for preprocessing Web application requests and post-processing responses. While this particular functionality was introduced in the servlet 2.3 specification, earlier Web servers have supported similar constructs.

The following sections cover servlet filters:

Overview of Servlet Filters

When the servlet container calls a method in a servlet on behalf of the client, the HTTP request that the client sent is, by default, passed directly to the servlet. The response that the servlet generates is, by default, passed directly back to the client, with its content unmodified by the container. In this scenario, the servlet must process the request and generate as much of the response as the application requires.

But there are many cases where some preprocessing of the request for servlets would be useful. In addition, it is sometimes useful to modify the response from a class of servlets. One example is encryption. A servlet, or a group of servlets in an application, might generate response data that is sensitive and should not go out over the network in clear-text form, especially when the connection has been made using a non-secure protocol such as HTTP. A filter can encrypt the responses. Of course, in this case the client must be able to decrypt the responses.

A common scenario for a filter is where you want to apply preprocessing or post-processing to requests or responses for a group of servlets, not just a single servlet. If you need to modify the request or response for just one servlet, there is no need to create a filter--just do what is required directly in the servlet itself.

Note that filters are not servlets. They do not implement and override HttpServlet methods such as doGet() or doPost(). Rather, a filter implements the methods of the javax.servlet.Filter interface. The methods are:

init()
destroy()
doFilter()

How the Servlet Container Invokes Filters

Figure 3-1 shows how the servlet container invokes filters. On the left is a scenario where no filters are configured for the servlet being called. On the right, several filters (1, 2, ..., N) have been configured in a chain to be invoked by the container before the servlet is called and after it has responded. The web.xml file specifies which servlets cause the container to invoke the filters.

Figure 3-1 Servlet Invocation with and without Filters

Text description of invfilt.gif follows.

Text description of the illustration invfilt.gif

The order in which filters are invoked depends on the order in which they are configured in the web.xml file. The first filter in web.xml is the first one invoked during the request, and the last filter in web.xml is the first one invoked during the response. Note the reverse order during the response.

Note:

Be careful in coordinating any use of multiple filters, in case of possible overlap in functionality or in what the filters are overwriting.

Filtering of Forward or Include Targets

In the OC4J 9.0.4 implementation, when a servlet is filtered, any servlets that are forwarded to or included from the filtered servlet are not filtered by default. You can change this behavior, however, through the following environment setting:

oracle.j2ee.filter.on.dispatch=true

This flag is set to false by default.

Note:

This flag is a temporary mechanism for the current release. Future releases will follow the next servlet specification (servlet 2.4), which also directs that servlets that are forwarded to or included from a filtered servlet are not filtered by default. But in accordance with the specification, this behavior will be configurable through the web.xml file.

See "Servlet Includes and Forwards" for general information about including and forwarding.

Filter Examples

This section lists and describes three servlet filter examples.

Filter Example 1

This section provides a simple filter example. Any filter must implement the three methods in the javax.servlet.Filter interface or must extend a class that implements them. So the first step is to write a class that implements these methods. This class, which we will call MyGenericFilter, can be extended by other filters.

Generic Filter

Here is the generic filter code. Assume this generic filter is part of a package com.example.filter and set up a corresponding directory structure.

This is an elementary example of an empty (or "pass-through") filter and could be used as a template.

package com.example.filter;
import javax.servlet.*;

public class MyGenericFilter implements javax.servlet.Filter {
  public FilterConfig filterConfig;                                 //1

  public void doFilter(final ServletRequest request,                //2
                       final ServletResponse response,
                       FilterChain chain)
      throws java.io.IOException, javax.servlet.ServletException { 
    chain.doFilter(request,response);                               //3
  } 

  public void init(final FilterConfig filterConfig) {               //4
    this.filterConfig = filterConfig;
  } 

  public void destroy() {                                           //5
  }
}

Save this code in a file called MyGenericFilter.java in the package directory. The numbered code notes refer to the following:

This declares a variable to save the filter configuration object.
The doFilter() method contains the code that implements the filter.
In the generic case, just call the filter chain.
The init() method saves the filter configuration in a variable.
The destroy() method can be overridden to accomplish any required finalization.

Filter Code: HelloWorldFilter.java

This filter overrides the doFilter() method of the MyGenericFilter class above. It prints a message on the console when it is called on entrance, then adds a new attribute to the servlet request, then calls the filter chain. In this example there is no other filter in the chain, so the container passes the request directly to the servlet. Enter the code that follows into a file called HelloWorldFilter.java.

package com.acme.filter;

import javax.servlet.*;

public class HelloWorldFilter extends MyGenericFilter {
  private FilterConfig filterConfig; 

  public void doFilter(final ServletRequest request,
                       final ServletResponse response,
                       FilterChain chain)
       throws java.io.IOException, javax.servlet.ServletException  { 
    System.out.println("Entering Filter");
    request.setAttribute("hello","Hello World!");
    chain.doFilter(request,response);
    System.out.println("Exiting HelloWorldFilter"); 
  } 
}

JSP Code: filter.jsp

To keep the example simple, the "servlet" to process the filter output is written as a JSP page. Here it is:

<HTML> 
<HEAD> 
<TITLE>Filter Example 1</TITLE> 
</HEAD>
<BODY> 
<HR>
<P><%=request.getAttribute("hello")%></P>
<P>Check your console output!</P>
<HR> 
</BODY> 
</HTML>

The JSP page gets the new request attribute, hello, that the filter added, and prints its value on the console. Put the filter.jsp page in the root directory of the OC4J standalone default Web application and make sure your console window is visible when you invoke filter.jsp from your browser.

Setting Up Example 1

To test the filter examples in this chapter, the OC4J standalone default Web application will be used. The filter should be configured in the web.xml file in the default Web application /WEB-INF directory (j2ee/home/default-web-app/WEB-INF by default).

You will need the following entries in the <web-app> element:

  <!-- Filter Example 1 -->
  <filter> 
    <filter-name>helloWorld</filter-name>
    <filter-class>com.acme.filter.HelloWorldFilter</filter-class> 
  </filter> 
  <filter-mapping> 
    <filter-name>helloWorld</filter-name>
    <url-pattern>/filter.jsp</url-pattern> 
  </filter-mapping> 
  <!-- end Filter Example 1 -->

The <filter> element defines the name of the filter and the Java class that implements the filter. The <filter-mapping> element defines the URL pattern that specifies to which targets the <filter-name> should apply. In this simple example, the filter applies to only one target: the JSP code in filter.jsp.

Note:

There is a <servlet-filter> element for the global-web-application.xml file or orion-web.xml file that has equivalent functionality. You can use either mechanism, but remember that web.xml settings override global-web-application.xml settings for the <web-app> element, and orion-web.xml settings override web.xml settings.

Running Example 1

Invoke filter.jsp from your Web browser. The console output should look something like this:

<hostname>% Entering Filter
Exiting HelloWorldFilter

The output to the Web browser is something like what is shown in Figure 3-2.

Figure 3-2 Example 1 Output

Text description of filters1.gif follows.

Text description of the illustration filters1.gif

Filter Example 2

A filter can be configured with initialization parameters in the web.xml file. This section provides a filter example that uses the following web.xml entry, which demonstrates a parameterized filter:

  <!-- Filter Example 2 -->
  <filter> 
    <filter-name>message</filter-name>
    <filter-class>com.acme.filter.MessageFilter</filter-class>
    <init-param> 
      <param-name>message</param-name>
      <param-value>A message for you!</param-value> 
    </init-param> 
  </filter> 
  <filter-mapping> 
    <filter-name>message</filter-name>
    <url-pattern>/filter2.jsp</url-pattern> 
  </filter-mapping> 
  <!-- end Filter Example 2 -->

Here, the filter named message has been configured with an initialization parameter, also called message. The value of the message parameter is "A message for you!"

Filter Code: MessageFilter.java

The code to implement the message filter example is shown below. Note that it uses the MyGenericFilter class from "Filter Example 1".

package com.acme.filter;
import javax.servlet.*;

public class MessageFilter extends MyGenericFilter { 
  public void doFilter(final ServletRequest request,
                       final ServletResponse response,
                       FilterChain chain)
       throws java.io.IOException, javax.servlet.ServletException { 
    System.out.println("Entering MessageFilter");
    String message = filterConfig.getInitParameter("message");
    request.setAttribute("message",message);
    chain.doFilter(request,response);
    System.out.println("Exiting MessageFilter"); 
  } 
}

This filter uses the filterConfig object that was saved in the generic filter. The filterConfig.getInitParameter() method returns the value of the initialization parameter.

JSP Code: filter2.jsp

As in the first example, this example uses a JSP page to implement the "servlet" that tests the filter. The filter named in the <url-pattern> tag above is filter2.jsp. Here is the code, which you can enter into a file filter2.jsp in the OC4J standalone default Web application root directory:

<HTML> 
<HEAD> 
<TITLE>Lesson 2</TITLE> 
</HEAD>
<BODY> 
<HR>
<P><%=request.getAttribute("message")%></P>
<P>Check your console output!</P>
<HR> 
</BODY> 
</HTML>

Running Example 2

Make sure that the filter configuration has been entered in the web.xml file, as shown above. Then access the JSP page with your browser. The console output should show something like the following:

Auto-deploying file:/private/tssmith/appserver/default-web-app/ (Assembly had 
been updated)...
Entering MessageFilter
Exiting MessageFilter

Note the message from the server showing that it redeployed the default Web application after the web.xml file was edited, and note the messages from the filter as it was entered and exited.

The Web browser screen should show something like what is shown in Figure 3-3.

Figure 3-3 Example 2 Output

Text description of filters2.gif follows.

Text description of the illustration filters2.gif

Filter Example 3

A particularly useful function for a filter is to manipulate the response to a request. To accomplish this, use the standard javax.servlet.http.HttpServletResponseWrapper class, a custom javax.servlet.ServletOutputStream object, and a filter. To test the filter, you also need a target to be processed by the filter. In this example, the target that is filtered is a JSP page.

There are three new classes to create to implement this example:

FilterServletOutputStream: This is a new implementation of ServletOutputStream for response wrappers.
GenericResponseWrapper: This is a basic implementation of the response wrapper interface.
PrePostFilter: This is the code that implements the filter.

This example uses the HttpServletResponseWrapper class to wrap the response before it is sent to the target. This class is an object that acts as a wrapper for the ServletResponse object (using a Decorator design pattern, as described in software design textbooks). It is used to wrap the real response so that it can be modified after the target of the request has delivered its response.

The HTTP servlet response wrapper developed in this example uses a custom servlet output stream that lets the wrapper manipulate the response data after the servlet (or JSP page, in this example) is finished writing it out. Normally, this cannot be done after the servlet output stream has been closed (essentially, after the servlet has committed it). That is the reason for implementing a filter-specific extension to the ServletOutputStream class in this example.

Output Stream: FilterServletOutputStream.java

The FilterServletOutputStream class is used to manipulate the response of another resource. This class overrides the three write() methods of the standard java.io.OutputStream class.

Here is the code for the new output stream:

package com.acme.filter;

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;

public class FilterServletOutputStream extends ServletOutputStream {

  private DataOutputStream stream; 

  public FilterServletOutputStream(OutputStream output) { 
    stream = new DataOutputStream(output); 
  }

  public void write(int b) throws IOException  { 
    stream.write(b); 
  }

  public void write(byte[] b) throws IOException  { 
    stream.write(b); 
  }

  public void write(byte[] b, int off, int len) throws IOException  { 
    stream.write(b,off,len); 
  } 

}

Save this code in the following directory under the default Web application root directory (j2ee/home/default-web-app by default), and compile it:

/WEB-INF/classes/com/acme/filter

Servlet Response Wrapper: GenericResponseWrapper.java

To use the custom ServletOutputStream class, implement a class that can act as a response object. This wrapper object is sent back to the client in place of the original response that was generated.

The wrapper must implement some utility methods, such as to retrieve the content type and content length of its content. The GenericResponseWrapper class accomplishes this:

package com.acme.filter;

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;

public class GenericResponseWrapper extends HttpServletResponseWrapper { 
  private ByteArrayOutputStream output;
  private int contentLength;
  private String contentType;

  public GenericResponseWrapper(HttpServletResponse response) { 
    super(response);
    output=new ByteArrayOutputStream();
  } 

  public byte[] getData() { 
    return output.toByteArray(); 
  } 

  public ServletOutputStream getOutputStream() { 
    return new FilterServletOutputStream(output); 
  } 
  
  public PrintWriter getWriter() { 
    return new PrintWriter(getOutputStream(),true); 
  } 

  public void setContentLength(int length) { 
    this.contentLength = length;
    super.setContentLength(length); 
  } 

  public int getContentLength() { 
    return contentLength; 
  } 

  public void setContentType(String type) { 
    this.contentType = type;
    super.setContentType(type); 
  } 


  public String getContentType() { 
    return contentType; 
  } 
}

Save this code in the following directory under the default Web application root directory (j2ee/home/default-web-app by default), and compile it:

/WEB-INF/classes/com/acme/filter

Writing the Filter

This filter adds content to the response after that target is invoked. This filter extends the filter from "Generic Filter".

Here is the filter code, PrePostFilter.java:

package com.acme.filter;
import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;

public class PrePostFilter extends MyGenericFilter { 

  public void doFilter(final ServletRequest request,
                       final ServletResponse response,
                       FilterChain chain)
       throws IOException, ServletException { 
  OutputStream out = response.getOutputStream();
  out.write("<HR>PRE<HR>".getBytes());
  GenericResponseWrapper wrapper = new 

         GenericResponseWrapper((HttpServletResponse) response); 
  chain.doFilter(request,wrapper);
  out.write(wrapper.getData());
  out.write("<HR>POST<HR>".getBytes());
  out.close(); 
  } 
}

Save this code in the following directory under the default Web application root directory (j2ee/home/default-web-app by default), and compile it:

/WEB-INF/classes/com/acme/filter

As in the previous examples, create a simple JSP page:

<HTML> 
<HEAD> 
<TITLE>Filter Example 3</TITLE> 
</HEAD>
<BODY> 
This is a testpage. You should see<br>
this text when you invoke filter3.jsp, <br>
as well as the additional material added<br>
by the PrePostFilter. 
<br>
</BODY> 
</HTML>

Save this JSP code in filter3.jsp in the root directory of the default Web application.

Configuring the Filter

The following <filter> element must be added to web.xml, after the configuration of the message filter:

  <!-- Filter Example 3 -->
  <filter> 
    <filter-name>prePost</filter-name>
    <display-name>prePost</display-name>
    <filter-class>com.acme.filter.PrePostFilter</filter-class> 
  </filter> 
  <filter-mapping> 
    <filter-name>prePost</filter-name>
    <url-pattern>/filter3.jsp</url-pattern> 
  </filter-mapping> 
  <!-- end Filter Example 3 -->

Running Example 3

Invoke the servlet in your Web browser.

You should see a page that looks something like what is shown in Figure 3-4.

Figure 3-4 Example 3 Output

Text description of filters3.gif follows.

Text description of the illustration filters3.gif

Event Listeners

The servlet specification includes the capability to track key events in your Web applications through event listeners. This functionality allows more efficient resource management and automated processing based on event status. The following sections describe servlet event listeners:

Event Categories and Listener Interfaces

There are two levels of servlet events:

Servlet context-level (application-level) event

This involves resources or state held at the level of the application servlet context object.
Session-level event

This involves resources or state associated with the series of requests from a single user session; that is, associated with the HTTP session object.

At each of these two levels, there are two event categories:

Lifecycle changes
Attribute changes

You can create one or more event listener classes for each of the four event categories. A single listener class can monitor multiple event categories.

Create an event listener class by implementing the appropriate interface or interfaces of the javax.servlet package or javax.servlet.http package. Table 3-1 summarizes the four categories and the associated interfaces.

Table 3-1 Event Listener Categories and Interfaces

Event Category	Event Descriptions	Interface
Servlet context lifecycle changes	Servlet context creation, at which point the first request can be serviced Imminent shutdown of the servlet context	javax.servlet. ServletContextListener
Servlet context attribute changes	Addition of servlet context attributes Removal of servlet context attributes Replacement of servlet context attributes	javax.servlet. ServletContextAttributeListener
Session lifecycle changes	Session creation Session invalidation Session timeout	javax.servlet.http. HttpSessionListener
Session attribute changes	Addition of session attributes Removal of session attributes Replacement of session attributes	javax.servlet.http. HttpSessionAttributeListener

Typical Event Listener Scenario

Consider a Web application consisting of servlets that access a database. A typical use of the event listener mechanism would be to create a servlet context lifecycle event listener to manage the database connection. This listener might function as follows:

The listener is notified of application startup.
The application logs in to the database and stores the connection object in the servlet context.
Servlets use the database connection to perform SQL operations.
The listener is notified of imminent application shutdown (shutdown of the Web server or removal of the application from the Web server).
Prior to application shutdown, the listener closes the database connection.

Event Listener Declaration and Invocation

Event listeners are declared in the application web.xml deployment descriptor through <listener> elements under the top-level <web-app> element. Each listener has its own <listener> element, with a <listener-class> subelement specifying the class name. Within each event category, event listeners should be specified in the order in which you would like them to be invoked when the application runs.

After the application starts up and before it services the first request, the servlet container creates and registers an instance of each listener class that you have declared. For each event category, listeners are registered in the order in which they are declared. Then, as the application runs, event listeners for each category are invoked in the order of their registration. All listeners will remain active until after the last request is serviced for the application.

Upon application shutdown, session event listeners are notified first, in reverse order of their declarations, then application event listeners are notified in reverse order of their declarations.

Here is an example of event listener declarations, from the Sun Microsystems Java Servlet Specification, Version 2.3:

<web-app>
   <display-name>MyListeningApplication</display-name>
   <listener>
      <listener-class>com.acme.MyConnectionManager</listenerclass>
   </listener>
   <listener>
      <listener-class>com.acme.MyLoggingModule</listener-class>
   </listener>
   <servlet>
      <display-name>RegistrationServlet</display-name>
      ...
   </servlet>
</web-app>

Assume that MyConnectionManager and MyLoggingModule both implement the ServletContextListener interface, and that MyLoggingModule also implements the HttpSessionListener interface.

When the application runs, both listeners will be notified of servlet context lifecycle events, and the MyLoggingModule listener will also be notified of session lifecycle events. For servlet context lifecycle events, the MyConnectionManager listener will be notified first, because of the declaration order.

Event Listener Coding and Deployment Guidelines

Be aware of the following rules and guidelines for event listener classes:

In a multithreaded application, attribute changes might occur simultaneously. There is no requirement for the servlet container to synchronize the resulting notifications; the listener classes themselves are responsible for maintaining data integrity in such a situation.
Each listener class must have a public zero-argument constructor.
Each listener class file must be packaged in the application WAR file, either under /WEB-INF/classes or in a JAR file in /WEB-INF/lib.

Note:

In a distributed environment, the scope of event listeners is one for each deployment descriptor declaration for each JVM. There is no requirement for distributed Web containers to propagate servlet context events or session events to additional JVMs. This is discussed in the Sun Microsystems Java Servlet Specification.

Event Listener Methods and Related Classes

This section lists event listener methods that are called by the servlet container when a servlet context event or session event occurs. These methods take different types of event objects as input, so these event classes and their methods are also discussed.

ServletContextListener Methods, ServletContextEvent Class

The ServletContextListener interface specifies the following methods:

void contextInitialized(ServletContextEvent sce)

The servlet container calls this method to notify the listener that the servlet context has been created and the application is ready to process requests.
void contextDestroyed(ServletContextEvent sce)

The servlet container calls this method to notify the listener that the application is about to be shut down.

The servlet container creates a javax.servlet.ServletContextEvent object that is input for calls to ServletContextListener methods. The ServletContextEvent class includes the following method, which your listener can call:

ServletContext getServletContext()

Use this to retrieve the servlet context object that was created or is about to be destroyed, from which you can obtain information as desired. See "Introduction to Servlet Contexts" for information about the javax.servlet.ServletContext interface.

ServletContextAttributeListener Methods, ServletContextAttributeEvent Class

The ServletContextAttributeListener interface specifies the following methods:

void attributeAdded(ServletContextAttributeEvent scae)

The servlet container calls this method to notify the listener that an attribute was added to the servlet context.
void attributeRemoved(ServletContextAttributeEvent scae)

The servlet container calls this method to notify the listener that an attribute was removed from the servlet context.
void attributeReplaced(ServletContextAttributeEvent scae)

The servlet container calls this method to notify the listener that an attribute was replaced in the servlet context.

The container creates a javax.servlet.ServletContextAttributeEvent object that is input for calls to ServletContextAttributeListener methods. The ServletContextAttributeEvent class includes the following methods, which your listener can call:

String getName()

Use this to get the name of the attribute that was added, removed, or replaced.
Object getValue()

Use this to get the value of the attribute that was added, removed, or replaced. In the case of an attribute that was replaced, this method returns the old value, not the new value.

HttpSessionListener Methods, HttpSessionEvent Class

The HttpSessionListener interface specifies the following methods:

void sessionCreated(HttpSessionEvent hse)

The servlet container calls this method to notify the listener that a session was created.
void sessionDestroyed(HttpSessionEvent hse)

The servlet container calls this method to notify the listener that a session was destroyed.

The container creates a javax.servlet.http.HttpSessionEvent object that is input for calls to HttpSessionListener methods. The HttpSessionEvent class includes the following method, which your listener can call:

HttpSession getSession()

Use this to retrieve the session object that was created or destroyed, from which you can obtain information as desired. See "Introduction to Servlet Sessions" for information about the javax.servlet.http.HttpSession interface.

HttpSessionAttributeListener Methods, HttpSessionBindingEvent Class

The HttpSessionAttributeListener interface specifies the following methods:

void attributeAdded(HttpSessionBindingEvent hsbe)

The servlet container calls this method to notify the listener that an attribute was added to the session.
void attributeRemoved(HttpSessionBindingEvent hsbe)

The servlet container calls this method to notify the listener that an attribute was removed from the session.
void attributeReplaced(HttpSessionBindingEvent hsbe)

The servlet container calls this method to notify the listener that an attribute was replaced in the session.

The container creates a javax.servlet.http.HttpSessionBindingEvent object that is input for calls to HttpSessionAttributeListener methods. The HttpSessionBindingEvent class includes the methods that follow, which your listener can call.

String getName()

Use this to get the name of the attribute that was added, removed, or replaced.
Object getValue()

Use this to get the value of the attribute that was added, removed, or replaced. In the case of an attribute that was replaced, this method returns the old value, not the new value.
HttpSession getSession()

Use this to retrieve the session object that had the attribute change.

Event Listener Sample

This section provides code for a sample that uses a servlet context lifecycle and session lifecycle event listener. This includes the following components:

SessionLifeCycleEventExample: This is the event listener class, implementing the ServletContextListener and HttpSessionListener interfaces.
SessionCreateServlet: This servlet creates an HTTP session.
SessionDestroyServlet: This servlet destroys an HTTP session.
index.jsp: This is the application welcome page (the user interface), from which you can invoke SessionCreateServlet or SessionDestroyServlet.
web.xml: This is the deployment descriptor, where the servlets and listener class are declared.

To download and run this application, go to the following link:

http://otn.oracle.com/tech/java/oc4j/htdocs/oc4j-how-to.html

If you do not already have an Oracle Technology Network membership, select the membership link at the following address:

http://otn.oracle.com/

Memberships are free of charge.

Welcome Page: index.jsp

Here is the welcome page, the user interface that enables you to invoke the session-creation servlet by clicking the Create New Session link, or to invoke the session-destruction servlet by clicking the Destroy Current Session link.

<%@page session="false" %>
<H2>OC4J - HttpSession Event Listeners </H2>
<P>
This example demonstrates the use of the HttpSession Event and Listener that is 
new with the Java Servlet 2.3 API.  
</P>
<P>
[<a href="servlet/SessionCreateServlet">Create New Session</A>] &nbsp;
[<a href="servlet/SessionDestroyServlet">Destroy Current Session</A>]
</P>
<P>
Click the Create link above to start a new HttpSession. An HttpSession
listener has been configured for this application. The servler container
will send an event to this listener when a new session is created or 
destroyed. The output from the event listener will be visible in the 
console window from where OC4J was started.
</P>

Note:

No new session object is created if you click the Create New Session link again after having already created a session from the same client, unless the session had reached a timeout limit or you had explicitly destroyed it in the meantime.

Deployment Descriptor: web.xml

The servlets and the event listener are declared in the web.xml file. This results in SessionLifeCycleEventExample being instantiated and registered upon application startup. Because of this, its methods are automatically called by the servlet container, as appropriate, upon the occurrence of servlet context or session lifecycle events. Here are the web.xml entries:

<web-app>
   <listener>
      <listener-class>SessionLifeCycleEventExample</listener-class> 
   </listener>
   <servlet>
      <servlet-name>sessioncreate</servlet-name> 
      <servlet-class>SessionCreateServlet</servlet-class> 
   </servlet>
   <servlet>
      <servlet-name>sessiondestroy</servlet-name> 
      <servlet-class>SessionDestroyServlet</servlet-class> 
   </servlet>
   <welcome-file-list>
      <welcome-file>index.jsp</welcome-file> 
   </welcome-file-list>
</web-app>

Listener Class: SessionLifeCycleEventExample

This section shows the listener class. Its sessionCreated() method is called by the servlet container when an HTTP session is created, which occurs when you click the Create New Session link in index.jsp. When sessionCreated() is called, it calls the log() method to print a "CREATE" message indicating the ID of the new session.

The sessionDestroyed() method is called when the HTTP session is destroyed, which occurs when you click the Destroy Current Session link. When sessionDestroyed() is called, it calls the log() method to print a "DESTROY" message indicating the ID and duration of the terminated session.

import javax.servlet.http.*;
import javax.servlet.*;

public class SessionLifeCycleEventExample 
   implements ServletContextListener, HttpSessionListener
{
   /* A listener class must have a zero-argument constructor: */
   public SessionLifeCycleEventExample()
   {
   }

   ServletContext servletContext;

   /* Methods from the ServletContextListener */
   public void contextInitialized(ServletContextEvent sce)
   {
      servletContext = sce.getServletContext();
   }

   public void contextDestroyed(ServletContextEvent sce)
   {
   }

   /* Methods for the HttpSessionListener */
   public void sessionCreated(HttpSessionEvent hse)
   {
      log("CREATE",hse);
   }
   public void sessionDestroyed(HttpSessionEvent hse)
   {

         HttpSession _session = hse.getSession();
         long _start = _session.getCreationTime();
         long _end = _session.getLastAccessedTime();
         String _counter = (String)_session.getAttribute("counter");
         log("DESTROY, Session Duration:" 
                   + (_end - _start) + "(ms) Counter:" + _counter, hse);
   }
   
   protected void log(String msg, HttpSessionEvent hse)
   {
      String _ID = hse.getSession().getId();
      log("SessionID:" + _ID + "    " + msg);
   }
   
   protected void log(String msg)
   {
      System.out.println("[" + getClass().getName() + "] " + msg);
   }
}

Session Creation Servlet: SessionCreateServlet.java

This servlet is invoked when you click the Create New Session link in index.jsp. Its invocation results in a request object and associated session object being created by the servlet container. Creation of the session object results in the servlet container calling the sessionCreated() method of the event listener class.

import java.io.*;
import java.util.Enumeration;
import java.util.Date;
import javax.servlet.*;
import javax.servlet.http.*;

public class SessionCreateServlet extends HttpServlet { 

    public void doGet (HttpServletRequest req, HttpServletResponse res)
      throws ServletException, IOException
      {
         //Get the session object
         HttpSession session = req.getSession(true);
        
        // set content type and other response header fields first
        res.setContentType("text/html");

        // then write the data of the response
        PrintWriter out = res.getWriter();
     
        String _sval = (String)session.getAttribute("counter");
        int _counter=1;
     
        if(_sval!=null)
        {
           _counter=Integer.parseInt(_sval);
           _counter++;
      }
   
       
      session.setAttribute("counter",String.valueOf(_counter));

      out.println("<HEAD><TITLE> " + "Session Created Successfully .. 
         Look at OC4J Console to see whether the HttpSessionEvent invoked " 
         + "</TITLE></HEAD><BODY>");
      out.println("<P>[<A HREF=\"SessionCreateServlet\">Reload</A>]&nbsp;");   
      out.println("[<A HREF=\"SessionDestroyServlet\">Destroy Session</A>]");   
      out.println("<h2>Session created Successfully</h2>");
      out.println("Look at the OC4J Console to see whether the HttpSessionEvent
                  was invoked");
      out.println("<h3>Session Data:</h3>");
      out.println("New Session: " + session.isNew());
      out.println("<br>Session ID: " + session.getId());
      out.println("<br>Creation Time: " + new Date(session.getCreationTime()));
      out.println("<br>Last Accessed Time: " + 
                   new Date(session.getLastAccessedTime()));
      out.println("<BR>Number of Accesses: " + session.getAttribute("counter"));
      
   }
}

Session Destruction Servlet: SessionDestroyServlet.java

This servlet is invoked when you click the Destroy Current Session link in index.jsp. Its invocation results in a call to the invalidate() method of the session object. This in turn results in the servlet container calling the sessionDestroyed() method of the event listener class.

import java.io.*;
import java.util.Enumeration;

import javax.servlet.*;
import javax.servlet.http.*;

public class SessionDestroyServlet extends HttpServlet { 

    public void doGet (HttpServletRequest req, HttpServletResponse res)
      throws ServletException, IOException
      {
         //Get the session object
         HttpSession session = req.getSession(true);
         // Invalidate Session
         session.invalidate(); 

         // set content type and other response header fields first
         res.setContentType("text/html");

         // then write the data of the response
         PrintWriter out = res.getWriter();

         out.println("<HEAD><TITLE> " + "Session Destroyed Successfully .. 
           Look at OC4J Console to see whether the HttpSessionEvent invoked " 
           + "</TITLE></HEAD><BODY>");
         out.println("<P>[<A HREF=\"../index.jsp\">Restart</A>]");
         out.println("<h2> Session Destroyed Successfully</h2>");
         out.println("Look at the OC4J Console to see whether the
                      HttpSessionEvent was invoked");
         out.close();
   }
}

3Servlet Filters and Event Listeners

Servlet Filters

Overview of Servlet Filters

How the Servlet Container Invokes Filters

Figure 3-1 Servlet Invocation with and without Filters

Filtering of Forward or Include Targets

Filter Examples

Filter Example 1

Generic Filter

Filter Code: HelloWorldFilter.java

JSP Code: filter.jsp

Setting Up Example 1

Running Example 1

Figure 3-2 Example 1 Output

Filter Example 2

Filter Code: MessageFilter.java

JSP Code: filter2.jsp

Running Example 2

Figure 3-3 Example 2 Output

Filter Example 3

Output Stream: FilterServletOutputStream.java

Servlet Response Wrapper: GenericResponseWrapper.java

Writing the Filter

Configuring the Filter

Running Example 3

Figure 3-4 Example 3 Output

Event Listeners

Event Categories and Listener Interfaces

Table 3-1 Event Listener Categories and Interfaces

Typical Event Listener Scenario

Event Listener Declaration and Invocation

Event Listener Coding and Deployment Guidelines

Event Listener Methods and Related Classes

ServletContextListener Methods, ServletContextEvent Class

ServletContextAttributeListener Methods, ServletContextAttributeEvent Class

HttpSessionListener Methods, HttpSessionEvent Class

HttpSessionAttributeListener Methods, HttpSessionBindingEvent Class

Event Listener Sample

Welcome Page: index.jsp

Deployment Descriptor: web.xml

Listener Class: SessionLifeCycleEventExample

Session Creation Servlet: SessionCreateServlet.java

Session Destruction Servlet: SessionDestroyServlet.java

3
Servlet Filters and Event Listeners