compiling servlet

Hi,

I was reading as below (http://www.javatpoint.com/steps-to-create-a-servlet-using-tomcat-server)

3)Compile the servlet

For compiling the Servlet, jar file is required to be loaded. Different Servers provide different jar files:
Jar file      Server
1) servlet-api.jar      Apache Tomacat
2) weblogic.jar      Weblogic
3) javaee.jar      Glassfish
4) javaee.jar      JBoss

I wonder why different servers provides different jars for compiling servlet rather than supplying single jar.


Please advise
Any links resources ideas highly appreciated. Thanks in advance