Servlet filter & mapping

[Kevin P Smith]

Quick question, just been looking at Servlet 3.0 and was wondering....

What is the difference plus/minus between mapping a Servlet and Filter a Servlet?

I've always used mappings, and by the looks of the examples I can find Filters are pretty much the same.

Cheers

KS

[Bear Bibeault]

They're not even remotely alike. A mapping specifies what URL patterns are to be applied to a Servlet, a filter is a type of listener class thats invoked before/after another resource is executed.

Are you asking what the difference is between servlet mappings and filter mappings?

[Kevin P Smith]

Yes it was the mappings I was talking about sorry, what's the difference between

<servlet-mapping>
and
<filter-mapping>

What confussed me as well is the way Servlets 3.0 uses Annotations to call these, I was reading that a Filter and Servlet seemed to be the same thing. If it helps this is the tutorial I was looking at:

http://today.java.net/pub/a/today/2008/10/14/introduction-to-servlet-3.html

@ServletFilter
@FilterMapping("/foo")
public class MyFilter {
  public void doFilter(HttpServletRequest req, 
                       HttpServletResponse res) {
        .....
  }
}
 

Then a Servlet class is:

@Servlet(urlMappings={"/MyApp"}, initParams ={@InitParam(name="lang", value="english")})
public class MyServlet {
    public void doGet(HttpServletRequest req, 
                          HttpServletResponse res) {
                ....
    }
}
  

[Bear Bibeault]

The servlet mappings define what URLs will trigger the servlet, while the filter mappings server the same purpose for what requests the filter will be invoked upon.

This is not a new concept with Servlets 3 -- just the annotations (in lieu of web.xml entries) are.

[Kevin P Smith]

So just to dumb it down...

The filter-mapping simply maps a URI pattern (/foo) to the filter like the servlet-mapping would map it (/foo) to the Servlet?

So if I was to do a Filter:

@WebFilter(urlPatterns = {"/foo"})
public class MyFilter {  
  public void doFilter(HttpServletRequest req,   
                       HttpServletResponse res) {  

        <stuff happens here>

  }  
}  

and a Servlet:

@Servlet(urlMappings={"/foo"}, 
	initParams ={@InitParam	(name="lang", value="english")}
)  
public class MyServlet {  
    public void doGet(HttpServletRequest req,   
                          HttpServletResponse res) {  

	<more stuff happens here>

    }  
}

and executed the URI '/foo' it would call the ServletFilter which would in turn do some 'stuff' under the doFilter method, then call the Servlet then return to the Filter before going off to wherever I have set?

example:
JSP --> ServletFilter --> Servlet --> <wherever>


(If would appear that a quick browse would show that the website I was using is already slightly out of date in the annotations!)