Ok, I've alredy found the cause.
I'm using Eclipse to build my application, and the Eclipse is pointing to J2EE version 1.4 (that's in my computer) for the compilation of the Servlets and TagHandlers.
But, I'm using Tomcat 4.1.29, or JBoss 3.2.3 to test the application, and these version aren't compatible with version 1.4 of J2EE, they are compatible with version 1.3
In J2EE 1.3 there's no class called JspContext just in the J2EE 1.4
That's all confusion.
That's was my fault, sorry.