Programmer's Guide
Filters

Overview of filters

Filters process request objects before they get to the server or process response objects after they leave the server and before they are returned to the client. Filters can also be invoked in a chain so that one filter can pass the request or the response to another in the chain, and so on. You can use filters to do the following tasks:

• Take flow-control logic out of web application components.
• Examine and modify the request object before the server receives it from the client.
• Examine and modify the response object before the client receives it from the server.
• Change the content of the response.

The order and usage of filters are defined in the web application's configuration files and are not compiled into the web application itself. As a result, you can rearrange, add and remove filters without recompiling the application. This loose coupling of filters with web applications makes filters useful for many tasks by many members of a development team.

Specifically, filters are often implemented as:

• Request dispatchers
• User authenticators and authorizers
• Request and response loggers and auditors
• Form validators
• Image converters
• Data compressors and decompressors
• Data encryptors and decryptors
• Response output tokenizers
• Triggers for resource access
• XML transformers
• Content localizers

When compiling filter examples in this chapter, you must include the jrun_root/lib/jrun.jar file in your classpath. For example:

>javac -classpath .;c:/jrun4/lib/jrun.jar *.java -d c:/jrun4/servers/default/deploy/default-app/web-inf/classes/

Most examples in this chapter extend the GenericFilter class to avoid having to override all methods of the Filter interface. For the GenericFilter code, see "Creating a generic filter class".

The default JRun server includes a filter that displays the total page execution time for every request. For more information, see "Viewing execution times".

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