what is the use filter mapping in web.xml

Filters wrap around servlets. They obviate the need to replicate code that needs to be executed on requests to any servlet.
As they are pluggable, they're far easier replaced than some abstract base class (a prime example of composition over inheritance).

Think of things like validating requests, adding specific fields to responses, analysing headers, logging requests and responses, things like that.
Or add some functionality to an application when you can't change the actual servlet code itself.

Filters are an under-appreciated feature of the Java servlet platform, ideal for writing components that can be added transparently to any web application. A filter is like a lightweight servlet that doesn't generate its own content, instead it plugs into the request handling process and executes in addition to the normal page processing.

Filters might record information about requests, convert content to a different format, or even redirect access to a different page. Filters can be applied to any resources served by a servlet engine, whether it's flat HTML, graphics, a JSP page, servlet , or whatever. They can be added to an existing web application without either the filter or the application being aware of one another. Filters are essentially a server plug-in that works with any servlet container compliant with version 2.3 or later of the servlet specification.

A filter implements the interface javax.servlet.Filter, and configured in the web application web.xml file, where the URL's it will process are defined. For each request, the servlet container decides which filters to apply, and adds those filters to a chain in the same order they appear in web.xml. Each filter has its Filter.doFilter() method called, and triggers the invocation of the next filter in the chain or the loading of the final resource (HTML page, servlet, or whatever).

Writing a simple filter
To write a filter, we create a class implementing the Filter interface, which requires three methods: init(), doFilter(), and destroy(). init() and destroy() are called when the filter is first initialized and when it is destroyed, respectively, to allow configuration and cleanup. For the moment we'll ignore these and focus on doFilter(), which is the meat of the filter.



sample web.xml looks like

 <filter>
        <filter-name>Timer</filter-name>
        <filter-class>com.kief.FilterDemo.TimerFilter</filter-class>
        <description>
            This filter times the execution of the request after
            the filter itself, and prints the execution time to
            the standard output.
        </description>
    </filter>