There is nothing called servlet tunneling but certainly there is a concept called HTTP tunneling.
So let us try to understand what does HTTP tunneling means.
Most of the intranets work within the confines of a firewall that guards the intranet from direct influence of the external network. The firewall acts as a security wall for the intranet that protects it from malicious influences of internet.A proxy server,for example,acts as a firewall that allows the intranet users to surf through the web but does not allow them to connect to the network applications.
A typical firewall restricts all external communications from the intranet except the HTTP communications.So if a client behind a firewall(say an applet) wants to connect to an HTTP Server(where the servlet resides),then opening a simple socket on the client for initiating the communicaion with the server wont help since the firewall will block this request for being non HTTP.
In order to get through the firewall,u will have to wrap the client request in an HTTP message so that it will pierce throgh the firewall and reach the server.
This mechanism of encapsulating the client request in a HTTP message so that it may penetrate the firewall is called HTTP FIREWALL TUNNELING.
U will have to use the java.net.URL and java.net.URLConnection
classes for doing this.
This is a brief explanation. Any doubts,plz ask