2.4 Basic Servlet Design

When writing Servlets, you will usually extend a class named javax.servlet.http.HttpServlet.

This is a base class that provides support for HTTP requests.

	The HttpServlet class, in turn, extends javax.servlet.GenericServlet, which provides some basic Servlet
	functionality.

Finally, GenericServlet implements the primary Servlet API interface, javax.servlet.Servlet.

	It also implements an interface called ServletConfig, which allows it to provide easy access to Servlet
	configuration information (Mukhar, et al. 2006).

Notice that Servlet defines only a small number of methods.

You can probably guess that init() and destroy() don't handle any requests.

We will look at these methods later when we discuss the Servlet lifecycle in more detail.

The getServletConfig() and getServletInfo() methods don't handle requests either.

That leaves only service() to handle requests (Mukhar, et al. 2006).

2.4 Basic Servlet Design

The service() Method

The service() method is the main method to perform the actual task.

	The servlet container calls the service() method to handle requests coming from the client and to write the
	formatted response back to the client.

Each time the server receives a request for a servlet; the server spawns a new thread and calls service.

	The service() method checks the HTTP request type (GET, POST, PUT, DELETE, etc.) and calls doGet,
	doPost, doPut, doDelete, etc. methods as appropriate (Solutions 2012).

Here is the signature of this method:

You should NOT override the service() method.

Your job is to override the doGet() and/or doPost() methods.

	Although this approach takes a couple of extra lines of code, it has several advantages over directly overriding
	service.

2.4 Basic Servlet Design

	First, you can later add support for other HTTP request methods by adding doPut, doTrace, etc., perhaps in a
	subclass (Hall and Brown 2004).

Overriding service directly precludes this possibility.

Second, you can add support for modification dates by adding a getLastModified method.

Since getLastModified is invoked by the default service method, overriding service eliminates this option.

Finally, service gives you automatic support for HEAD, OPTION, and TRACE requests (Hall and Brown 2004).

The doGet, doPost, and doXxx Methods

These methods contain the real meat of your servlet.

	Ninety-nine percent of the time, you only care about GET or POST requests, so you override doGet and/or
	doPost.

	However, you can also override doDelete for DELETE requests, doPut for PUT, doOptions for OPTIONS, and
	doTrace for TRACE.

Recall, however, that you have automatic support for OPTIONS and TRACE (Solutions 2012).

2.4 Basic Servlet Design

The difference between GET and POST

GET is meant to be used for getting things.

	Sure, you might use the parameters to help figure out what to send back, but the point is-you're not making any
	changes on the server! POST is meant to be used for sending data to be processed.

	This could be as simple as query parameters used to figure out what to send back, just as with a GET, but when
	you think of POST, think: update.

Think: use the data from the POST body to change something on the server (Bates, Sierra and Basham 2008).

POST has a body.

That's the key.

	Both GET and POST can send parameters, but with GET, the parameter data is limited to what you can stuff
	into the Request line.

2.4 Basic Servlet Design

	But there is another issue other than size, when you use GET, the parameter data shows up in the browser's
	input bar, right after actual URL (and separated with a "?"), therefore GET is not secured in some situations.

Also still another issue is whether you need or want end-users to be able to bookmark the request page.

GET requests can be bookmarked; POST requests cannot.

	Another important difference is that GET is idempotent while POST is not; by idempotent we mean that a
	method has ability to handle requests that are sent multiple times without altering system state / data / resource
	(e.g. when the client clicks on submit button twice after providing his credit card information) in case of POST, if
	it is not handled, the client will pay twice!

	But what determines whether the browser sends a GET or POST request? A simple hyperlink always means a
	GET.

	While in HTML from you can choose to whether you want to send a GET or a POST request by setting the
	method property of form tag.

If you left form tag without explicitly specify a method property then, GET is the default.