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 -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) compiler, 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.
In his blog on java.net, Sun veteran and GlassFish lead Jim Driscoll details the history of servlet technology. James Gosling first thought of servlets in the early days of Java, but the concept did not 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 3.0||Not yet released||JavaEE 6 , J2SE 6.0||Pluggability, Ease of development, Async Servlet, Security|
|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 |
|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 |
|Servlet 2.0||JDK 1.1||Part of Java Servlet Development Kit 2.0|
|Servlet 1.0||June 1997|
The Servlet lifecycle consists of the following steps:
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.
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 is not implemented by the servlet is made, the method of the parent class is called, typically resulting in an error being returned to the requester.
destroy()method which takes the servlet out of service. The
init()is called only once in the lifecycle of a Servlet.
Here is a simple servlet that just generates HTML
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.
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, e.g. Resin and Orion, are free to use in a server environment for non-profit organizations).