Role of filter-mapping

I'm trying out Spring and I've met the 开发者_运维百科filter-mapping tag. What is its role when compared to the servlet-mapping tag? Is it executed in the background when the urls are called?

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<filter-mapping> specifies when a javax.servlet.Filter is invoked.

Filters are invoked before the servlet they are defined for (you can map a filter to either an URL pattern or to a specific servlet)