Page Filter

How It Works

Architecture

The plugin maintains a database of approved web sites/pages. New sites/pages can be added to the database by using a web form. Each time the browser makes a request, the database is consulted as to whether the requested page is approved. To be approved, the requested URL must match at least one of the entries in the database. If the request is approved, then the page is retrieved and is displayed to the user. If the request is not approved, then the browser is diverted to a web page that contains a list approved links.

When an approved page is retrieved from the web, each link is checked to determine whether it points to an approved page. If it points to an approved page, then the link is not edited. If not, then the link is removed (i.e., the anchor tag is removed but the anchor text remains).

MEG Model

The PageFilter plugin consists of two generators and two editors. The following diagram illustrates the way a request gets processed by the PageFilter plugin. In the diagram, "RE" refers to the one request editor, "G" refers to either of the two generators, and "DE" refers to the one document editor.

• Step 1 (RE): When the browser makes a request, the request editor checks to see whether the requested web page is in the database of approved pages. If the requested page is in the database, then the request editor does not change the request. If the page is not in the database, then the request editor changes the URL in the request.
• Step 2 (G): The PageFilter plugin uses two generators, one for each of the cases in Step 1 (request approved vs. request not approved). Requests for approved pages are handled by WBI's default generator. This generator acts as a transparent proxy, retrieving the requested page from its server. Requests for unapproved pages are handled by a special pagefilter generator. This generator creates a web page containing links to approved sites.
• Step 3 (DE): Once a page has been received from a generator, a document editor checks whether the page contains any links to unapproved sites. If there are any such links, the document editor replaces these links with normal text. Once the document editor is done, the page is ready to be displayed in the browser. Note that the document editor does not actually need to be run on pages that come from the pagefilter generator, as the links on these pages always point to approved sites.

Implementation Details

• Filtering out unapproved pages: The PageRequestEditor (a request editor) checks whether the requested page is approved or not. If it is approved, then the PageRequestEditor throws a RequestRejectedException. If not, a new request is created (with a different URL) which will make the browser display a web page of links to approved pages/sites.

• Editing links: The PageFilterEditor (a document editor) changes approved pages by editing out links to unapproved web pages/sites. A LinkAnnotationEditor is used to remove links to unapproved sites. More precisely, PageFilterEditor extends LinkAnnotationEditor. This means that each link (i.e., each anchor tag and its anchor text) is passed to the editLinkmethod. If the href of the tag matches a pattern in the database (the link points to an approved site), then editLink does nothing. Otherwise, editLink sets the link tag to null, indicating that the link is to be removed. Note that only the anchor tag is removed in this case, and not the anchor text.

• Retrieving approved pages: WBI's DefaultHttpGenerator retrieves approved web pages.

• Creating page of approved links: The PageFilterGenerator handles requests which were created by the PageRequestEditor. The generator creates a String of HTML code containing links to approved sites. Then, it uses a StaticHtmlGenerator to display the HTML code in a web page.

• Storing approved pages/sites: Each approved web page or web site is represented by a Page object. A Page contains a pattern to match URLs against and a sample URL and title. The sample is used to generate the page of links to approved sites. The plugin maintains a small database of these Page objects.

• Changing the "approved" database: The PageFilterFormGenerator deals with the web form to add Page objects to the "approved" database. The generator can create the form and process any data that was entered into the form. The database is actually an instance of Section.

• Some key WBI classes that were used:

  com.ibm.wbi.protocol.http.beans
  StaticHtmlGenerator	used by PageFilterGenerator and PageFilterFormGenerator
  FixContentTypeEditor	make sure that content-type in http header matches actual content type of page
  FormHelper	helps extract query data from web forms
  LinkAnnotationEditor	extended by PageFilterEditor (edit out unapproved links)
  NewUrlRequestEditor	used by PageRequestEditor
  com.ibm.wbi.protocol.http
  HttpGenerator	extended by PageFilterGenerator and PageFilterFormGenerator
  HttpRequestEditor	extended by PageRequestEditor
  com.ibm.wbi.util
  Section	used by the PageFilter plugin itself (to keep track of Pages)

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  Source Files

  Page.java

  Contains the class definition for Page (data structure for approved page).

  pagefilter.ini

  Initial database of approved sites/pages.

  PageFilter.java

  Contains the class definitions for PageFilter (the plugin itself), PageFilterGenerator (page of links to approved pages), PageFilterFormGenerator (form to change list of approved sites), PageFilterEditor (edit out links to unapproved sites), and PageRequestEditor (change request URL if destined for an unapproved site).

  pagefilter.reg

  Contains the code necessary to register the plugin.

  Pattern.java

  Contains the class definitions for Pattern and PatternPart (data structures used to match requested URLs against entries in approved database).