Everything about The Java Servlet totally explained
The
Java Servlet API allows a
software developer to add dynamic content to a
Web server using the
Java platform. The generated content is commonly
HTML, but may be other data such as
XML. Servlets are the Java counterpart to non-Java dynamic Web content technologies such as
PHP,
CGI and
ASP.NET. Servlets can maintain state across many server transactions by using
HTTP cookies,
session variables or
URL rewriting.
The Servlet
API, contained in the
Java package hierarchy, defines the expected interactions of a
Web container and a servlet. A Web container is essentially the component of a Web server that interacts with the servlets. The Web container is responsible for managing the lifecycle of servlets, mapping a URL to a particular servlet and ensuring that the URL requester has the correct access rights.
A is an
object that receives a request and generates a response based on that request. The basic servlet package defines Java objects to represent servlet requests and responses, as well as objects to reflect the servlet's configuration parameters and execution environment. The package defines
HTTP-specific subclasses of the generic servlet elements, including session management objects that track multiple requests and responses between the Web server and a client. Servlets may be packaged in a
WAR file as a
Web application.
Servlets can be generated automatically by
JavaServer Pages (JSP), or alternately by template engines such as
WebMacro. Often servlets are used in conjunction with JSPs in a pattern called "
Model 2", which is a flavor of the
model-view-controller pattern.
History
The complete servlet specification was created by
Sun Microsystems, with version 1.0 finalized in June 1997. Starting with version 2.3, the servlet specification was developed under the
Java Community Process. JSR 53 defined both the Servlet 2.3 and JavaServer Page 1.2 specifications. JSR 154 specifies the Servlet 2.4 and 2.5 specifications. As of
May 10,
2006, the current version of the servlet specification is 2.5.
In his blog on java.net, Sun veteran and
GlassFish lead Jim Driscoll details the history of servlet technology.
(External Link
) James Gosling first thought of servlets in the early days of
Java, but the concept didn't become a product until Sun shipped the Java Web Server product. This was before what is now the
Java Platform, Enterprise Edition was made into a specification.
| Servlet API version |
Released |
Platform |
Important Changes |
| Servlet 2.5 |
September 2005 |
JavaEE 5, J2SE 5.0 |
Requires J2SE 5.0, supports annotations |
| Servlet 2.4 |
November 2003 |
J2EE 1.4, J2SE 1.3 |
web.xml uses XML Schema |
| Servlet 2.3 |
August 2001 |
J2EE 1.3, J2SE 1.2 |
Addition of Filters |
| Servlet 2.2 |
August 1999 |
J2EE 1.2, J2SE 1.2 |
Becomes part of J2EE, introduced independent web applications in .war files |
| Servlet 2.1 |
November 1998 |
Unspecified |
First official specification, added RequestDispatcher, ServletContext |
| Servlet 2.0 |
|
JDK 1.1 |
Part of Java Servlet Development Kit 2.0 |
| Servlet 1.0 |
June 1997 |
|
|
Lifecycle of a Servlet
The Servlet lifecycle consists of the following steps:
- The Servlet class is loaded by the container during start-up.
- The container calls the
init method. This method initializes the servlet and must be called before the servlet can service any requests. In the entire life of a servlet, the init method is called only once.
- After initialization, the servlet can service client-requests. Each request is serviced in its own separate thread. The container calls the
service method of the servlet for every request. The service method determines the kind of request being made and dispatches it to an appropriate method to handle the request. The developer of the servlet must provide an implementation for these methods. If a request for a method that isn't implemented by the servlet is made, the method of the parent class is called, typically resulting in an error being returned to the requester.
- Finally, the container calls the
destroy method which takes the servlet out of service. The destroy method like init is called only once in the lifecycle of a Servlet.
ServletConfig and ServletContext
There is only one
ServletContext in every application. This object can be used by all the servlets to obtain application level information or container details. Every servlet, on the other hand, gets its own ServletConfig object. This object provides initialization parameters for a servlet. A developer can obtain the reference to ServletContext using either the ServletConfig object or ServletRequest object.
Servlet containers
A
Servlet container is a specialized web server that supports Servlet execution. It combines the basic functionality of a web server with certain Java/Servlet specific optimizations and extensions – such as an integrated Java runtime environment, and the ability to automatically translate specific URLs into Servlet requests. Individual Servlets are registered with a Servlet container, providing the container with information about what functionality they provide, and what URL or other resource locator that'll use to identify themselves. The Servlet container is then able to initialize the Servlet as necessary and deliver requests to the Servlet as they arrive. Many containers have the ability to dynamically add and remove Servlets from the system, allowing new Servlets to quickly be deployed or removed without affecting other Servlets running from the same container. Servlet containers are also referred to as
web containers or
web engines.
Like the other Java APIs, different vendors provide their own implementation of the Servlet container standard. For a list of some of the free and commercial web containers, see
the list of Servlet containers. (Note that 'free' means that non-commercial use is free. Some of the commercial containers, for example Resin and Orion, are free to use in a server environment for non-profit organizations).
Further Information
Get more info on 'Java Servlet'.
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