Java Reference
In-Depth Information
A
post
request includes form data as a header sent separately from the URL. This is
generally preferred, and it's required when confidential information is being collected on
the form. Also, some web servers and browsers do not support URLs longer than 255
characters, which limits the amount of information that can be sent in a
get
request.
Java servlets handle both of these requests through methods inherited from the
HttpServlet
class:
doGet(
HttpServletRequest
,
HttpServletResponse
)
and
doPost(
HttpServletRequest
,
HttpServletResponse
)
. These methods throw two kinds
of exceptions:
ServletException
, which is part of the
javax.servlet
package, and
IOException
, an exception in the standard
java.io
package that involves input and out-
put streams.
The
doGet()
and
doPost()
methods have two arguments: an
HttpServletRequest
object and an
HttpServletResponse
object. One is called when a
get
request is used to
execute the servlet, and the other is called with
post
. A common technique in Java
servlet programming is to use one method to call the other, as in the following example:
public void doGet(HttpServletRequest request,
HttpServletResponse response) throws ServletException, IOException {
doPost(request, response);
}
The request and response objects belong to classes in the
javax.servlet.http
package.
A servlet receives information about how it was run by calling methods of the
HttpServletRequest
class. For example, when a web form is submitted to a servlet,
each field on the form is stored as a string by the
HttpServletRequest
class.
You can retrieve these fields in a servlet by calling the
getParameter(
String
)
method
with the name of the field as an argument. This method returns
null
if no field of that
name exists.
A servlet communicates with the user by sending back an HTML document, a graphics
file, or another type of information supported by a web browser. It sends this information
by calling the methods of the
HttpServletResponse
class.
The first thing you must do when preparing a response is to define the kind of content
the servlet is sending to a browser. Call the
setContentType(
String
)
method with the
content type as an argument.
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The most common form for a response is HTML, which is set by calling
setContentType(“text/html”)
. You also can send a response as text (“text/plain”),
graphics files (“image/gif”, “image/jpeg”), and application-specific formats such as
“application/msword”.
