How do JSP tag files and libraries work?

I've been learning about JSPs and came across tag files and libraries. I know that they are custom actions and useful for pointing out errors instead of using JavaBeans for example, but I still don't unders开发者_如何学Gotand how they work. Lets say for example you do:

<jsp:directive.attribute name = "amount" required = "true" />

And later, assuming that calc is defined using jsp:useBean, amount can be used by:

<c:set target="${calc}" property = "amount" value ="${amount}" />

But what happens behind the scenes?

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${calc} represents value of variable calc which may be there in pageCOntext,request,session,Servletcontext

By the statement

<c:set target="${calc}" property = "amount" value ="${amount}" />

it will store value in calc var's amount represented by value

Behind the scene it would be

calc.setAmount(amount);

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Tag files are custom tag handlers which are written in JSP. These jsp are converted to java tag handlers by the JSP compiler.