Do I need servlets for server-side Java programming?

Question

I need to program a small server to handle requests from an Android application. (The user sends a number to the server, the server does some math on the numbers received from each user - the average, let's say - and returns it).

I've just read this simple introduction from Oracle (http://docs.oracle.com/javase/tutorial/networking/sockets/clientServer.html).

It explains how to leave a server listening on a specific door. Normal java, no special application needed (such as Tomcat).

I know people generally use 'servlets' (with Tomcat) to accomplish this kind of thing. Why is that? What are the advantages? Can't i just program a sample server like the one described in that simple tutorial, and leave it always running on a computer?

Answer 1

Using a socket is very simple. At first.

You create your server class, you have it bind to a port and away you go.

The first hurdle you will hit, covered in the comments, is multi-threading. But a simple producer/consumer pattern will solve that for you in no time.

The next problem you will hit is protocol.

• Who talks first?

• How do you respond?

• How do you deal with an invalid request?

• What happens if the stream collapses during a request?

• Do you open a new socket for each request or does a client hold onto a socket and write multiple requests?

• Maybe you want some sort of non-blocking IO?

This is where HTTP comes in, it is a protocol for communicating over TCP/IP (actually over anything, you could use bits of paper and a bike). It defines answers to all the above questions (and many more).

So, you run a webserver (tomcat, glassfish) and it deals with the raw sockets and sending the right information.

A servlet is an abstraction, when Tomcat has a connection and it has negotiated compression, encryption etc it will pass a request onto the servlet.

The servlet doesn't have to worry about the raw socket, it reads the request and writes a response.

It's worth pointing out that HTTP isn't the only protocol, it's just the one happens to be used for web-browsing. And so the one used by web-servers.

Answer 2

You do not need servlets.

Servlets are helpful because they manage the socket handling for you but the drawback is you need a container (such as Tomcat) to run your servlets on. You may want to look at Netty which is really built to do the kind of work you are talking about.