A servlet is a Java programming language class used to extend the capabilities of servers that host applications accessed via a request-response programming model. Although servlets can respond to any type of request, they are commonly used to extend the applications hosted by Web servers. Thus, it can be thought of as a Java Applet that runs on a server instead of a browser.[1]
A Servlet is a Java class in Java EE that conforms to the Java Servlet API, a protocol by which a Java class may respond to requests. They are not tied to a specific client-server protocol, but are most often used with the HTTP protocol. Therefore, the word "Servlet" is often used in the meaning of "HTTP Servlet".[2] Thus, a software developer may use a servlet 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 CGI and ASP.NET. Servlets can maintain state in session variables across many server transactions by using HTTP cookies, or URL rewriting.
To deploy and run a Servlet, a Web container must be used. 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.
The servlet API, contained in the Java package hierarchy javax.servlet, defines the expected interactions of the Web container and a servlet.[2]
A Servlet 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 javax.servlet.http 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 from JavaServer Pages (JSP) by the JavaServer Pages compiler. The difference between Servlets and JSP is that Servlets typically embed HTML inside Java code, while JSPs embed Java code in HTML. While the direct usage of Servlets to generate HTML (as shown in the example below) has become rare, the higher level MVC web framework in Java EE (JSF) still explicitly uses the Servlet technology for the low level request/response handling via the FacesServlet. A somewhat older usage is to use servlets in conjunction with JSPs in a pattern called "Model 2", which is a flavor of the model-view-controller pattern.
A servlet is a Java component that can be plugged into a Java-enabled web server to provide custom services. These services can include:
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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 March 26, 2010, the current version of the servlet specification is 3.0.
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 | December 2009 | JavaEE 6, JavaSE 6 | Pluggability, Ease of development, Async Servlet, Security, File Uploading |
| Servlet 2.5 | September 2005 | JavaEE 5, JavaSE 5 | Requires JavaSE 5, supports annotation |
| Servlet 2.4 | November 2003 | J2EE 1.4, J2SE 1.3 | web.xml uses XML Schema by pankaj kuamr biet jhansi |
| Servlet 2.3 | August 2001 | J2EE 1.3, J2SE 1.2 | Addition of Filter |
| 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 |
The advantages of using servlets are their fast performance and ease of use combined with more power over traditional CGI (Common Gateway Interface). Traditional CGI scripts written in Java have a number of disadvantages when it comes to performance:
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 that takes the servlet out of service. The destroy() method, like init(), is called only once in the lifecycle of a servlet.Three methods are central to the life cycle of a servlet. These are init( ), service( ), and destroy( ). They are implemented by every servlet and are invoked at specific times by the server. Let us consider a typical user scenario to understand when these methods are called.
init() method of the servlet.
service() method of the servlet.
service() method is called for each HTTP request.destroy() method to relinquish any resources such as file handles that are allocated for the servlet; important data may be saved to a persistent store.The following example servlet prints a "Hello world" HTML page.
Note that HttpServlet is a subclass of GenericServlet, an implementation of the Servlet interface.
The service() method dispatches requests to the methods doGet(), doPost(), doPut(), doDelete(), and so on; according to the HTTP request.
import java.io.IOException; import javax.servlet.ServletConfig; import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; public class ServletLifeCycleExample extends HttpServlet { private int count; @Override public void init(ServletConfig config) throws ServletException { super.init(config); getServletContext().log("init() called"); count=0; } @Override protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { getServletContext().log("service() called"); count++; response.getWriter().write("Incrementing the count: Count = "+count); } @Override public void destroy() { getServletContext().log("destroy() called"); } }
Servlets are most often used to
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