Reference : Servlet Spec 3.0
Basic :
A
servlet is a Java™ technology-based Web component, managed by a container,
that generates
dynamic content.
Multipurpose Internet Mail
Extensions (MIME)
In
functionality, servlets lie somewhere between Common Gateway Interface (CGI)
programs
and proprietary server extensions such as the Netscape Server API
(NSAPI) or Apache
Modules.
Servlet Interface
The
Servlet interface is the central abstraction of
the Java Servlet API. All servlets
implement
this interface either directly, or more commonly, by extending a class that
implements
the interface. The two classes in the Java Servlet API that implement the
Servlet interface are GenericServlet and HttpServlet. For
most purposes,
Developers will
extend HttpServlet to implement their servlets.
Request Handling:
The
basic Servlet interface defines a service
method for handling client requests.
This
method is called for each request that the servlet container routes to an
instance
of
a servlet.
The
handling of concurrent requests to a Web application generally requires that
the
Web
Developer design servlets that can deal with multiple threads executing within
the
service method at a particular time.
Generally
the Web container handles concurrent requests to the same servlet by
concurrent
execution of the service method on different threads.
HTTP SPECIIC
The
HttpServlet abstract subclass adds additional methods
beyond the basic
Servlet interface that are automatically called by the service
method in the
HttpServlet class to aid in processing HTTP-based
requests. These methods are:
doGet for handling HTTP GET requests
doPost for handling HTTP POST requests
doPut for handling HTTP PUT requests
doDelete for handling HTTP DELETE requests
doHead for handling HTTP HEAD requests
doOptions for handling HTTP OPTIONS requests
doTrace for handling HTTP TRACE requests
SingleThreadModel Interface is deprecated in this version of
the specification. V3
Life Cycle
This
life cycle is expressed in the API by the init, service, and
destroy
methods
of the javax.servlet.Servlet interface that
all servlets must implement
directly or
indirectly through the GenericServlet or HttpServlet abstract classes.
Loading and Instantiation
The
servlet container is responsible for loading and instantiating servlets. The
loading
and instantiation can occur when the container is started, or delayed until
the container
determines the servlet is needed to service a request
After loading the
Servlet class, the container instantiates it for use.
Initialization
After
the servlet object is instantiated, the container must initialize the servlet
before
it
can handle requests from clients. Initialization is provided so that a servlet
can
read
persistent configuration data, initialize costly resources (such as JDBC™ APIbased
connections),
and perform other one-time activities. The container initializes
the
servlet instance by calling the init method
of the Servlet interface with a
unique
(per servlet declaration) object implementing the ServletConfig
interface.
This
configuration object allows the servlet to access name-value initialization
parameters
from the Web application’s configuration information. The configuration
object
also gives the servlet access to an object (implementing the ServletContext
interface)
that describes the servlet’s runtime environment. See Chapter 4, “Servlet
Context” for more
information about the ServletContext interface.
A
typical sequence of
events
for asynchronous processing is:
1.
The request is received and passed via normal filters for authentication etc.
to the
servlet.
2.
The servlet processes the request parameters and/or content to determine the
nature
of the request.
3.
The servlet issues requests for resources or data, for example, sends a remote
web
service
request or joins a queue waiting for a JDBC connection.
4.
The servlet returns without generating a response.
5.
After some time, the requested resource becomes available, the thread handling
that
event continues processing either in the same thread or by dispatching to a
resource in the
container using the AsyncContext.
Request path elements
The
request path that leads to a servlet servicing a request is composed of many
important
sections. The following elements are obtained from the request URI path
and
exposed via the request object:
■ Context Path: The path prefix associated with the ServletContext
that this
servlet
is a part of. If this context is the “default” context rooted at the base of
the
Web
server’s URL name space, this path will be an empty string. Otherwise, if the
context
is not rooted at the root of the server’s name space, the path starts with a
/ character but does not end with a / character.
■ Servlet Path: The path section that directly corresponds to the mapping which
activated
this request. This path starts with a ’/’ character except in the case
where
the request is matched with the ‘/*’ or ““ pattern, in which case it is an
empty
string.
■ PathInfo: The part of the request path that is not part of the Context
Path or the
Servlet
Path. It is either null if there is no extra path, or is a string with a
leading
‘/’.
The
following methods exist in the HttpServletRequest interface to access this
information:
■ getContextPath
■ getServletPath
■ getPathInfo
It
is important to note that, except for URL encoding differences between the
request
URI
and the path parts, the following equation is always true:
requestURI = contextPath + servletPath +
pathInfo
To give a few
examples to clarify the above points, consider the following:
TABLE 3-1 Example Context
Set Up
Context Path /catalog
Servlet Mapping
Pattern: /lawn/*
Servlet: LawnServlet
Servlet Mapping
Pattern: /garden/*
Servlet: GardenServlet
Servlet Mapping
Pattern: *.jsp
Servlet: JSPServlet
TABLE 3-2 Observed Path
Element Behavior
Request Path Path Elements
/catalog/lawn/index.html
ContextPath: /catalog
ServletPath: /lawn
PathInfo: /index.html
/catalog/garden/implements/
ContextPath: /catalog
ServletPath: /garden
PathInfo: /implements/
/catalog/help/feedback.jsp
ContextPath: /catalog
ServletPath: /help/feedback.jsp
PathInfo: null
Path Translation
Methods
■ ServletContext.getRealPath
■ HttpServletRequest.getPathTranslated
Servlet Context
The
ServletContext interface defines a servlet’s view of the
Web application within
which the servlet
is running.
A
ServletContext is rooted at a known path within a Web
server. For example, a
servlet
context could be located at http://www.mycorp.com/catalog. All requests
that
begin with the /catalog request
path, known as the context path, are routed to
the Web
application associated with the ServletContext.
Scope :
There
is one instance object of the ServletContext interface associated with each
Web
application deployed into a container. In cases where the container is
distributed
over many virtual machines, a Web application will have an instance of
the ServletContext for
each JVM.
Servlets
in a container that were not deployed as part of a Web application are
implicitly
part of a “default” Web application and have a default ServletContext. In
a
distributed container, the default ServletContext is non-distributable and must
only exist in one
JVM.
Servlet
contexts can
not be shared
across virtual hosts.
Resources
The
ServletContext interface provides direct access only to
the hierarchy of static
content
documents that are part of the Web application, including HTML, GIF, and
JPEG
files, via the following methods of the ServletContext interface:
■ getResource
■ getResourceAsStream
Filtering
Filters
are Java components that allow on the fly transformations of payload and
header
information in both the request into a resource and the response from a
resource
