-
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.
SCJP 5.0 Simulator 642+ Questions With Explanations 