Java Servlet

Life of a JSP file.

Servlets are Java programming language objects that dynamically process requests and construct responses. 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, and as such some find it easier to think of them as 'Java scripts'. 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 javax.servlet, 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 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 by JavaServer Pages (JSP) compiler, or alternately use template engines such as WebMacro or Apache Velocity to generate HTML. 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. 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.

Lifecycle of a servlet

The servlet lifecycle consists of the following steps:

1. The servlet class is loaded by the container during start-up.
2. 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.
3. 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 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.
4. Finally, the container calls the destroy() method that takes the servlet out of service. The destroy() method like init() is called only once in the lifecycle of a servlet.

Here is a simple servlet that just generates HTML. Note that HttpServlet is a subclass of GenericServlet, an implementation of the Servlet interface. The service() method dispatches requests to methods doGet(), doPost(), doPut(), doDelete(), etc., according to the HTTP request.

import java.io.IOException;
import java.io.PrintWriter;
 
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
 
public class HelloWorld extends HttpServlet {
  public void doGet(HttpServletRequest request, HttpServletResponse response)
      throws ServletException, IOException {
    PrintWriter out = response.getWriter();
    out.println("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.0 " +
                                        "Transitional//EN\">\n" +
                "<html>\n" +
                "<head><title>Hello WWW</title></head>\n" +
                "<body>\n" +
                "<h1>Hello WWW</h1>\n" +
                "</body></html>");
  }
}

External links

• Sun's servlet tutorial
• Beginning and intermediate servlet and JSP tutorials With source code
• Sun's servlet product description
• JSR
  • JSR 315 - Servlet 3.0
  • JSR 154 - Servlet 2.4 and 2.5
  • JSR 53 - Servlet 2.3
• New features added to Servlet 2.5 at JavaWorld
• Java Documentation of the Servlet 2.5 API

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