Java Reference
In-Depth Information
Before we look at the output from this Web page, we need to consider just what
our servlet will look like…
8.5
Servlet Structure
Servlets must import the following two packages:
javax.servlet
javax.servlet.http
As of Tomcat 7, it is also necessary to import the following annotation type:
javax.servlet.annotation.WebServlet
In addition, since servlet output uses a
PrintWriter
stream, package java.io is
required. Servlets that use the HTTP protocol (which means
all
servlets, at the
present time) must extend class
HttpServlet
from package
java.servlet.http
. The two
most common HTTP requests (as specifi ed in the HTML pages that make use of
servlets) are
GET
and
POST
. At the servlet end, method
service
will despatch either
method
doGet
or method
doPost
in response to these requests. The programmer
should override (at least) one of these two methods.
You should use the
POST
method for multiple data items and/or items that need
to be transmitted securely. If you are transmitting just single items that have no
security implications, then use
GET
. All three methods (
doGet
,
doPost
and
service
)
have a
void
return type and take the following two arguments:
an
HttpServletRequest
object;
an
HttpServletResponse
object.
The former encapsulates the HTTP request from the browser and has several
methods, but none will be required by our fi rst servlet. The second argument holds
the servlet's response to the client's request. There are just two methods of this
HttpServletResponse
object that are of interest to us at present and these are
shown below.
void setContentType(String <type>)
This specifi es the data type of the response. Normally, this will be “
text/HTML”
.
PrintWriter getWriter()
Returns the output stream object to which the servlet can write character data to
the client (using method
println
).
There are four basic steps in a servlet…
1. Execute the
setContentType
method with an argument of “
text/HTML
”.
2. Execute the
getWriter
method to generate a
PrintWriter
object.
3. Retrieve any parameter(s) from the initial Web page.
(Not required in our fi rst servlet.)
4. Use the
println
method of the above
PrintWriter
object to create elements of the
Web page to be 'served up' by our Web server.
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