doGet for handling HTTP GET requests.
protected void doGet(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException
Called by the server (via the service method) to allow a
servlet to handle a GET request.
Overriding this method to support a GET request also
automatically supports an HTTP HEAD request. A
HEAD request is a GET request that
returns NO BODY in the response, only the request header fields. When overriding
this method, read the request data, write the response headers, get the
response's writer or output stream object, and finally, write the response data.
It's best to include content type and encoding. When using a
PrintWriter object to return the response, set the content
type before accessing the PrintWriter object.
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class DisplayServlet extends HttpServlet {
public void doGet(HttpServletRequest req, HttpServletResponse resp)
throws IOException, ServletException {
resp.setContentType("text/html");
PrintWriter out = resp.getWriter();
out.println("<html><head><title>Display Information");
out.println("</title></head><body>");
out.println("Hello, World");
out.println("</body></html>");
}
}
The servlet container must write the headers before committing the response,
because in HTTP the headers must be sent before the response body.
The GET method should be safe, that is, without any
side effects for which users are held responsible. For example, most form
queries have no side effects. If a client request is intended to change stored
data, the request should use some other HTTP method (for example,
POST method).
The GET method should also be idempotent, meaning that it
can be safely repeated. Sometimes making a method safe also makes it
idempotent. For example, repeating queries is both safe and idempotent, but
buying a product online or modifying data is neither safe nor idempotent.
GET method purpose.
The GET method means retrieve whatever
information (in the form of an entity) is identified by the
Request-URI. If the Request-URI
refers to a data-producing process, it is the produced data which shall be
returned as the entity in the response and not the source text of the
process, unless that text happens to be the output of the process.
In short, this method should be used for getting (retrieving) data only.
It should not be used for storing data in DB.
GET method technical characteristics.
Query string or form data during this method is simply appended
to the URL as name-value pairs separated with '&'.
name1=value1&name2=value2&name3=value3
Query length is limited (it depends on servlet container's plaform, but usually
should not exceed 1024 bytes). Users can see data in the browser's address bar.
http://some-server.com/some-script?name1=value1&name2=value2&name3=value3
Only ASCII (text) data can be sent to server with
GET method.
Easy to bookmark.
GET method triggers.
The web browser sends an HTTP GET request when:
The user types a URL in the browser's address bar.
The user clicks a link.
Retrieve a resource which was defined in
src (image) or href
(cascade style sheet) attributes.
<img src="image.gif">
<link href="style.css" rel="stylesheet" type="text/css">
The user (or JavaScript) submits a form that specifies attribute
method="GET":
<form action="/servlet/display" method="GET">
First Name: <input type="text" name="firstName"><p>
Last Name: <input type="text" name="lastName"><p>
<input type="submit" value="Display">
</form>
The user (or JavaScript) submits a form that specifies
NO method attribute (forms use
method GET BY DEFAULT):
<form action="/servlet/display">
First Name: <input type="text" name="firstName"><p>
Last Name: <input type="text" name="lastName"><p>
<input type="submit" value="Display">
</form>
doPost for handling HTTP POST requests.
protected void doPost(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException
Called by the server (via the service method) to allow
a servlet to handle a POST request. The HTTP
POST method allows the client to send data of UNLIMITED
length to the Web server a single time and is useful when posting information
such as credit card numbers.
When overriding this method, read the request data, write the response headers,
get the response's writer or output stream object, and finally, write the
response data. It's best to include content type and encoding. When using a
PrintWriter object to return the response, set the
content type before accessing the PrintWriter object.
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class DisplayServlet extends HttpServlet {
public void doPost(HttpServletRequest req, HttpServletResponse resp)
throws IOException, ServletException {
resp.setContentType("text/html");
PrintWriter out = resp.getWriter();
out.println("<html><head><title>Display Information");
out.println("</title></head><body>");
out.println("Hello, World");
out.println("</body></html>");
}
}
The servlet container must write the headers before committing the response,
because in HTTP the headers must be sent before the response body.
This method does not need to be either safe or idempotent. Operations
requested through POST can have side effects for
which the user can be held
accountable, for example, updating stored data or buying items online.
POST method purpose.
The POST method is used to request that the origin
server accept the entity enclosed in the request as a new subordinate
of the resource identified by the Request-URI in
the Request-Line. POST is
designed to allow a uniform method to cover the following functions:
Annotation of existing resources;
Posting a message to a bulletin board, newsgroup, mailing list,
or similar group of articles;
Providing a block of data, such as the result of submitting a
form, to a data-handling process;
Extending a database through an append operation.
In short, this method should be used for posting newgroups messages,
submitting long data fields to a database (such as a SQL insert of lengthy
string), or sending binary files to server.
POST method technical characteristics.
Sends information to the server such as form fields, large text
bodies, and key-value pairs.
Hides form data from users because it is not passed as a query string, but
in the message body.
Sends UNLIMITED length data as part of its HTTP request body.
For sending ASCII (text) or binary data.
Disallows bookmarks.
POST method triggers.
The web browser sends an HTTP POST request when:
The user (or JavaScript) submits a form that specifies attribute
method="POST":
<form action="/servlet/display" method="POST">
First Name: <input type="text" name="firstName"><p>
Last Name: <input type="text" name="lastName"><p>
<input type="submit" value="Display">
</form>
doPut for handling HTTP PUT requests.
protected void doPut(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException
Called by the server (via the service method) to
allow a servlet to handle a PUT request.
The PUT operation allows a client to place a file
on the server and is similar to sending a file by FTP.
When overriding this method, leave intact any content headers sent with the
request (including Content-Length,
Content-Type, Content-Transfer-Encoding,
Content-Encoding, Content-Base,
Content-Language, Content-Location,
Content-MD5, and Content-Range).
If your method cannot handle a content header, it must issue an error message
and discard the request.
This method does not need to be either safe or idempotent. Operations that
doPut performs can have side effects for which the
user can be held accountable. When using this method, it may be useful
to save a copy of the affected URL in temporary storage.
The fundamental difference between the POST and
PUT requests is reflected in the different meaning of
the Request-URI. The URI in a POST
request identifies the resource that will handle the enclosed
entity. That resource might be a data-accepting process, a gateway to
some other protocol, or a separate entity that accepts annotations.
In contrast, the URI in a PUT request identifies the
entity enclosed with the request -- the user agent knows what URI is
intended and the server MUST NOT attempt to apply the request to some
other resource.
<form enctype="multipart/form-data" action="some_url" method="PUT">
<input type="file" name="fileToUpload" value="Select File">
<input type="submit" value="Upload">
<input type="reset" value="Reset">
</form>
doDelete for handling HTTP DELETE
requests.
protected void doDelete(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException
Called by the server (via the service method) to
allow a servlet to handle a DELETE request.
The DELETE operation allows a client to remove a
document or Web page from the server.
This method does not need to be either safe or idempotent. Operations
requested through DELETE can have side effects for
which users can be held accountable. When using this method, it may
be useful to save a copy of the affected URL in temporary storage.
doHead for handling HTTP HEAD requests.
protected void doHead(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException
Receives an HTTP HEAD request from the protected
service method and handles the request. The client
sends a HEAD request when it wants to see
ONLY the HEADERS of a response, such as Content-Type or
Content-Length. The HTTP HEAD method
counts the output bytes in the response to set the
Content-Length header accurately.
The doHead method in HttpServlet
is a specialized form of the doGet method that returns
only the headers produced by the doGet method.
doOptions for handling HTTP OPTIONS
requests.
protected void doOptions(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException
Called by the server (via the service method) to allow a
servlet to handle a OPTIONS request.
The OPTIONS request determines which HTTP methods
the server supports and returns an appropriate header. For example, if a servlet
overrides doGet, this method returns the following header:
Allow: GET, HEAD, TRACE, OPTIONS
doTrace for handling HTTP TRACE requests.
protected void doTrace(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException
Called by the server (via the service method) to
allow a servlet to handle a TRACE request.
The doTrace method generates a response containing
all instances of the headers sent in the TRACE request
to the client, so that they can be used in debugging. There's no need
to override this method.
