JAVA EE: Servlets : ServletConfig

Tuesday, 25 March 2014

Servlets : ServletConfig

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What Is ServletConfig in Java Servlets?

In Java web applications, servlets often need configuration parameters initialized at startup—such as database connection strings, email configurations, or specific file paths. Hardcoding these values directly into the Java class creates rigid code that requires recompilation whenever a parameter changes. The ServletConfig object acts as a bridge between the servlet container and an individual servlet, providing access to servlet-specific initialization parameters defined in the deployment descriptor (web.xml) or annotations.

Key Concepts & Object Lifecycle

One per Servlet: Unlike ServletContext (which is shared across the entire web application), a unique ServletConfig object is created for each individual servlet in the container.
Creation During Initialization: The servlet container instantiates ServletConfig during the servlet initialization phase and passes it to the servlet via the init(ServletConfig config) method.
Accessing the ServletContext: The ServletConfig object provides a direct reference to the application-wide ServletContext via getServletContext().

Essential ServletConfig Methods

The javax.servlet.ServletConfig interface defines four fundamental methods:

getInitParameter(String name): Returns a String containing the value of the named initialization parameter, or null if it does not exist.
getInitParameterNames(): Returns an Enumeration<String> of all initialization parameter names defined for the servlet.
getServletName(): Returns the name of the servlet instance as defined in the deployment descriptor or @WebServlet annotation.
getServletContext(): Returns the reference to the surrounding ServletContext object.

Why Use ServletConfig?

Configurability Without Recompilation: Change servlet startup values in web.xml without touching compiled Java source code.
Encapsulation & Scope Isolation: Keeps configuration parameters scoped exclusively to the servlet that requires them, preventing accidental access or collisions across other servlets.
Convenient Access: Because GenericServlet implements ServletConfig, methods like getInitParameter() and getServletContext() can be called directly within any servlet subclass.

Recommended Next Videos

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ServletConfig

ServletConfig Project Dir Structure

ServletConfig

ServletConfigServlet.java

import java.io.IOException;
import java.io.PrintWriter;
import java.util.Enumeration;

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 ServletConfigServlet extends HttpServlet
{

  private static final long serialVersionUID = 1L;

  ServletConfig             servletConfig           = null;

  public void init( ServletConfig servletConfig )
  {
    this.servletConfig = servletConfig;
    System.out.println("\n-----------------------------------------------------");
    System.out.println("init method has been called and servlet is initialized");
    System.out.println("-----------------------------------------------------");

    System.out.println("Using 'getInitParameter(String name)' method get the value");
    String driverName = servletConfig.getInitParameter("driverName");
    System.out.println("Driver Name : " + driverName);

    System.out.println("\nUsing 'getInitParameterNames()'  method get the value");
    Enumeration<String> paramNames = servletConfig.getInitParameterNames();

    while( paramNames.hasMoreElements() )
    {
      String paramName = paramNames.nextElement();

      String value = servletConfig.getInitParameter(paramName);

      System.out.println("paramName : " + paramName + ", Value: " + value);

    }

  }

  public void doGet( HttpServletRequest request, HttpServletResponse response ) throws ServletException,
                  IOException
  {

    System.out.println("\n-----------------------------------------------------");
    System.out.println("doGet method has been called");
    System.out.println("-----------------------------------------------------");

    ServletConfig servletConfig = getServletConfig();
    System.out.println("Got servletconfig object using getServletConfig() method : " + servletConfig);
    System.out.println("Servlet Name : " + servletConfig.getServletName());
    System.out.println("ServletContext Object : " + servletConfig.getServletContext());

    response.setContentType("text/html");
    PrintWriter out = response.getWriter();
    out.print("<html><body>");
    out.print("<b>hello HTTP servlet</b>");
    out.print("</body></html>");
  }

  public void destroy()
  {
    System.out.println("-----------------------------------------------------");
    System.out.println("destroy method has been called and servlet is destroyed");
    System.out.println("-----------------------------------------------------");
  }

  public ServletConfig getServletConfig()
  {
    return servletConfig;
  }
}

web.xml

<?xml version="1.0" encoding="ISO-8859-1"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" metadata-complete="true" version="3.0">
  <display-name>ServletConfigServlet</display-name>
  <description>
        This is a simple web application with a source code organization
        based on the recommendations of the Application Developer's Guide.
    </description>
  <servlet>
    <servlet-name>ServletConfigServlet</servlet-name>
    <servlet-class>ServletConfigServlet</servlet-class>
    
     <init-param>  
      <param-name>driverName</param-name>  
      <param-value>oracle.jdbc.driver.OracleDriver</param-value>  
    </init-param>
    
     <init-param>  
      <param-name>username</param-name>  
      <param-value>ram</param-value>  
    </init-param>
    
    <init-param>  
      <param-name>password</param-name>  
      <param-value>ram</param-value>  
    </init-param>   
    
  </servlet>
  
  <servlet-mapping>
    <servlet-name>ServletConfigServlet</servlet-name>
    <url-pattern>/servletConfig</url-pattern>
  </servlet-mapping>
</web-app>

index.html

<!DOCTYPE HTML><html lang="en"><head>
<meta charset="UTF-8">
<title>ServletConfigServlet</title>
</head>
<body>
<p>
<h3>ServletConfigServlet</H3>
<p></p>
<ul>

<li><a href="servletConfig">ServletConfigServlet</a></li>

</ul>
</body></html>

Environment Used:

JDK version :1.6.0_30
Tomcat version : 7.0.50

To Download ServletConfigApp Project Click the below link

https://sites.google.com/site/javaee4321/servlets/ServletConfigApp.zip?attredirects=0&d=1

See also:

Servlets Tutorial

All Design Patterns Links