sunlabs.brazil.filter

Class FilterHandler

public class FilterHandler extends Object implements Handler

The FilterHandler captures the output of another Handler and allows the ouput to be modified. One or more {@link sunlabs.brazil.filter.Filter Filters} may be specified to change that output before it is returned to the client.

This handler provides one of the core services now associated with the Brazil Server: the ability to dynamically rewrite web content obtained from an arbitrary source.

For instance, the FilterHandler can be used as a proxy for a PDA. The wrapped Handler would go to the web to obtain the requested pages on behalf of the PDA. Then, a Filter would examine all "text/html" pages and rewrite the pages so they fit into the PDA's 200 pixel wide screen. Another Filter would examine all requested images and dynamically dither them to reduce the wireless bandwidth consumed by the PDA.

The following configuration parameters are used to initialize this Handler:

prefix, suffix, glob, match
Specify the URL that triggers this handler. (See {@link MatchString}).
handler
The name of the Handler whose output will be captured and then filtered. This is called the "wrapped handler".
filters
A list of Filter names. The filters are applied in the specified order to the output of the wrapped handler.
exitOnError
If set, the server's initFailure will set any of the filters fail to initialize. No handler prefix is required.
A sample set of configuration parameters illustrating how to use this handler follows:
 handler=filter
 port=8081

 filter.class=sunlabs.brazil.filter.FilterHandler
 filter.handler=proxy
 filter.filters=noimg

 proxy.class=sunlabs.brazil.proxy.ProxyHandler

 noimg.class=sunlabs.brazil.filter.TemplateFilter
 noimg.template=sunlabs.brazil.template.NoImageTemplate
 
These parameters set up a proxy server running on port 8081. As with a normal proxy, this proxy server forwards all HTTP requests to the target machine, but it then examines all HTML pages before they are returned to the client and strips out all <img> tags. By applying different filters, the developer could instead build a server See the description under {@link sunlabs.brazil.server.Handler#respond responnd} for a more detailed explaination.

Version: 2.3

Author: Stephen Uhler (stephen.uhler@sun.com) Colin Stevens (colin.stevens@sun.com)

Field Summary
Filter[]filters
Handlerhandler
Method Summary
booleaninit(Server server, String prefix)
Start the handler and filter classes.
booleanrespond(Request request)
Responds to an HTTP request by the forwarding the request to the wrapped Handler and filtering the output of that Handler before sending the output to the client.

Field Detail

filters

public Filter[] filters

handler

public Handler handler

Method Detail

init

public boolean init(Server server, String prefix)
Start the handler and filter classes.

respond

public boolean respond(Request request)
Responds to an HTTP request by the forwarding the request to the wrapped Handler and filtering the output of that Handler before sending the output to the client.

At several stages, the Filters are given a chance to short-circuit this process:

  1. Each Filter is given a chance to examine the request before it is sent to the Handler by invoking its respond() method. The Filter may decide to change the request's properties. A Filter may even return some content to the client now, by (calling request.sendResponse() and returning true), in which case, neither the Handler nor any further Filters are invoked at all.
  2. Assuming the respond() methods of all the filters returned false (which is normally the case), the handler's respond() method is called, and is expected to generate content. If no content is generated at this step, this handler returns false.
  3. After the Handler has generated the response headers, but before it has generated any content, each Filter is asked if it would be interested in filtering the content. If no Filter is, then the subsequent content from the Handler will be sent directly to the client.
  4. On the other hand, if any Filter is interested in filtering the content, then the output of the Handler will be sent to each of the interested Filters in order. The output of each interested Filter is sent to the next one; the output of the final Filter is sent to the client.
  5. At this point, any one of the invoked Filters can decide to reject the content completely, instead of rewriting it.
See {@link Filter} for a description of how to cause filters to implement the various behaviors defined above.

Parameters: request The HTTP request to be forwarded to one of the sub-servers.

Returns: true if the request was handled and content was generated, false otherwise.

Throws: IOException if there was an I/O error while sending the response to the client.