Servlet Filters

• Idea is that you want to filter calls to a servlet first through a filter (or chain of multiple filters) to perform some kind of "checking" task before the servlet is invoked.
• Examples - authentication, login (Authentication ---e.g. Facebook application authentication to get access to social data/channels!!!!)
• Create a filter by extending javax.servlet.Filter class and setting up @WebFilter annotationle to have a filter class(es) associated with it that the server will invoke first.
• Look here (old version here )for an example associated with our facebook app in java.

Servlet 3.* + standard for Filter Declaration

• NOW use annotation @WebFilter

  package example;    
  import java.io.*;  
  import javax.servlet.*;    
  
  
  @WebFilter (value="/hello",
              initParams=({  @WebInitParam(name="message", value="Servlet says: ") }))
    public TestFilter implements Filter {
      private FilterConfig _filterConfig;
  
      public void init(FilterConfig filterConfig)
        throws ServletException    {
        _filterConfig = filterConfig;    }
  
      public void doFilter(ServletRequest req,ServletResponse res, FilterChain chain)
        throws ServletException, IOException    { 
        PrintWriter out = res.getWriter();
        out.print(_filterConfig.getInitParameter("message"));
      }
  
      public void destroy() {
        // destroy    }
   } 
  
  

   

   

  Metadata specified in WebFilter are available via FilterConfig object, so the output of http://localhost:8080/${app-name}/hello should render text

  Servlet says: Hello World!
              

   

Another @WebFilter example

@WebFilter(filterName = "TimeOfDayFilter",
           urlPatterns = {"/*"},
           initParams = { @WebInitParam(name = "mood", value = "awake")}
          )

The @WebFilter annotation has the following attributes   

• boolean	asyncSupported Declares whether the filter supports asynchronous operation mode.	optional
  java.lang.String	description The description of the filter	optional
  DispatcherType[]	dispatcherTypes The dispatcher types to which the filter applies	optional
  java.lang.String	displayName The display name of the filter	optional
  java.lang.String	filterName The name of the filter	optional
  WebInitParam[]	initParams The init parameters of the filter	optional
  java.lang.String	largeIcon The large-icon of the filter	loptional
  java.lang.String[]	servletNames The names of the servlets to which the filter applies. (if not given will match directly to URL patterns only)	optional (possibly)
  java.lang.String	smallIcon The small-icon of the filter	optional
  java.lang.String[]	urlPatterns The URL patterns to which the filter applies	REQUIRED WHEN NO value set **( Use the value attribute when the only attribute on the annotation is the URL pattern; use the urlPatterns attribute when other attributes are also used.)
  java.lang.String[]	value The URL patterns to which the filter applies	REQUIRED**( Use the value attribute when the only attribute on the annotation is the URL pattern; use the urlPatterns attribute when other attributes are also used.)

Specifying Filter Parameters     

Depending on IDE you use, you can setup Filter's class parameters in a GUI as shown below for Eclipse

Make a new fileter New->Filter

Click on Add to target a specific Servlet

Here are the results

package grewe.filter;

import java.io.IOException;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebFilter;

/**
* Servlet Filter implementation class MyFilter2
*/
@WebFilter(urlPatterns = { "/MyFilter2" }, servletNames = { "DummyServlet" })
public class MyFilter2 implements Filter {

/**
* Default constructor.
*/
public MyFilter2() {

Specifying Filter Mappings     

• The filters are invoked in the order in which filter mappings appear in the filter mapping list of a WAR.
  You specify a filter mapping list for a WAR in its deployment descriptor

• If you want to log every request to a web application, you map the hit counter filter to the URL pattern /*.

• You can map a filter to one or more web resources, and you can map more than one filter to a web resource.

filter F1 is mapped to servlets S1, S2, and S3;
filter F2 is mapped to servlet S2;
filter F3 is mapped to servlets S1 and S2.

Specifying Filter Chain order of Filters     

• Recall that a filter chain is one of the objects passed to the doFilter method of a filter.

• This chain is formed indirectly by means of filter mappings.

Option 1 -- you may be able to set in container specific deployment file (see specs for your container if possible)    

• The order of the filters in the chain is the same as the order in which filter mappings appear in the web application deployment descriptor.

• EXAMPLE: When a filter is mapped to servlet S1, the web container invokes the doFilter method of F1. The doFilter method of each filter in S1’s filter chain is invoked by the preceding filter in the chain by means of the chain.doFilter method. Because S1’s filter chain contains filters F1 and F3, F1’s call to chain.doFilter invokes the doFilter method of filter F3. When F3’s doFilter method completes, control returns to F1’s doFilter method.

• can not find any evidence for glassfish

Option 2 -- you have to go to Old style use of web.xml file for deployment (see below)

Option 3 -- IF YOU DONT CARE the order then simply define them with @WebFilter and don't worry         

OLD Servlet standard for declaring Filter classes used web.xml file

Example a Filter being defined to be the class net.thlievweb.facebook.FacebbokAuthFilter used for Facebook authentication

<web-app>
<display-name>
  Facebook Application
</display-name>
<filter>       

<filter-name>
 FaceBookAuthFilter
</filter-name>       

<filter-class>
 net.theliveweb.facebook.FaceBookAuthFilter
</filter-class>       

<init-param>
  <param-name>api_key</param-name>
  <param-value>[api-key]</param-value>
</init-param>
<init-param>
  <param-name>secret_key</param-name>
  <param-value>[secret-key]</param-value>
</init-param>
</filter>


<filter-mapping>
  <filter-name>FaceBookAuthFilter</filter-name>
  <url-pattern>*.do</url-pattern>
</filter-mapping>       

</web-app>