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Filter questions

stefania ferrarelli

Joined: Jul 26, 2009
Posts: 29
if I want to modify request parameters inside a filter is it right to set request attributes after modifying? Is it the only strategy or is there a better one?
If I have chained more than one filter for the same servlet or url the order of execution is that of filter tag regardless of the order of filter mapping?I tried to chain two filters but it is not exactly clear to me the order of execution(request, response, target servlet) - for instance when destroy() is called for each filter.It makes no difference using <servlet-name> instead of <url-pattern>?Apart from Compression Filter, could you suggest me another example of Wrapper?<dispatcher> INCLUDE, FORWARD, ERROR(how can I call an ERROR filter?) let other components of the application-instead of the client- to interact with the filter but if I have a RequestDispatcher.forward() to the first filter of a chain, also the following filters of the chain will have a FORWARD <dispatcher>?Can change the <dispatcher>inside a chain?Every kind of component can use a filter before it reaches a target servlet??The target must be a servlet, that is filters apply only to servlets?
Not very clear ideas about filters!!
stefania ferrarelli

Joined: Jul 26, 2009
Posts: 29
I tried the code of CompressionFilter of the HF book on page 722.On the Compression Filter method doFilter I get a NullPointerException on line

it can be ServletContext ctx or FilterConfig cfg, could you give me a suggestion?
Paul Anilprem
Enthuware Software Support
Ranch Hand

Joined: Sep 23, 2000
Posts: 3412
Filters can be applied to ...url patterns and/or servlet names. If multiple filters are specified for a url patter or a servlet name, they are invoked in the order they are specified in the DD.

Destroy is called when the filter is taken out of service (for example, when the web app is shutting down). It has nothing to do with request processing, just like init.

The target of a filter can be anything because a url pattern can point to image, html file, javascript file, jsp, and of course a servlet.

You might want to read the Filters chapter from Servlet Specification. It will clear all you doubts.

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Frits Walraven
Creator of Enthuware JWS+ V6

Joined: Apr 07, 2010
Posts: 1927

Hi Stefania,

Have a look at this old thread Compression-Filter-Head-First

You might get the answer for your NullPointerException there.

subject: Filter questions