Java - Servlet Java class

About

A Servlet is an java object that receives a HTTP request and generates a HTTP response based on that request.

A Java servlet is:

  • a J2ee web component that responds to HTTP requests.
  • a HTTP-specific servlet class in Java EE that conforms to the Java Servlet technology API, a protocol by which a Java class may respond to requests.

Servlets are the Java counterpart to non-Java dynamic Web content technologies such as:

  • Php,
  • ASP,
  • and CGI

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”.

Servlets are best suited for:

  • service-oriented applications (web service endpoints can be implemented as servlets)
  • and the control functions of a presentation-oriented application, such as dispatching requests and handling nontextual data.

Use

Servlets can maintain state in session variables across many server transactions by using HTTP cookies, or URL rewriting.

Servlets are most often used to

  • process or store data that was submitted from an HTML form
  • provide dynamic content such as the results of a database query
  • manage state information that does not exist in the stateless HTTP protocol, such as filling the articles into the shopping cart of the appropriate customer.

Example

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.

Simple

Here is a simple servlet that just generates HTML.

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;

//or use
//import java.io.*;
//import javax.servlet.*;
//import javax.servlet.http.*;

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 World</title></head>\n" +
                "<body>\n" +
                "<h1>Hello, world!</h1>\n" +
                "</body></html>");
  }
}

With a Bean

With a enterprise bean:

The myServlet class uses dependency injection to obtain a reference to the Bean MyBean. The javax.ejb.EJB (@EJB) annotation is added to the declaration of the private member variable myBean, which is of type MyBean.

In this case, MyBean exposes a local, no-interface view, so the enterprise bean implementation class is the variable type.

@WebServlet
public class myServlet extends HttpServlet {
@EJB
MyBean myBean;
...
}

Note the annotation @webservlet (javax.servlet.annotation.WebServlet)

Documentation / Reference


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