ServletContext represents the application context for a web application. The server creates a single instance per web application, which is shared by all servlets within that application. This object is globally unique and serves as a shared resource container.

Key Functions of ServletContext

• Converting relative paths to absolute paths.
• Retrieving server container information.
• Reading global configuration parameters.
• Acting as a global attribute store.

Using ServletContext

Obtaining the Application Deployment Path

String appPath = context.getContextPath();

Converting Relative Paths to Absolute Paths (Useful for file upload/download operations)

String absolutePath = context.getRealPath("/uploads");

Retrieving Server Information

String serverDetails = servletContext.getServerInfo(); // Returns container name and version
int primaryVersion = servletContext.getMajorVersion(); // Main supported Servlet version
int secondaryVersion = servletContext.getMinorVersion(); // Minor supported Servlet version

Reading Global Configuration from web.xml Define parameters in web.xml:

<context-param>
    <param-name>databaseUrl</param-name>
    <param-value>jdbc:mysql://localhost:3306/appdb</param-value>
</context-param>

Access them in code:

String dbUrl = servletContext.getInitParameter("databaseUrl");
Enumeration<String> paramNames = servletContext.getInitParameterNames();
while(paramNames.hasMoreElements()) {
    String name = paramNames.nextElement();
    System.out.println(name + ": " + servletContext.getInitParameter(name));
}

Using ServletContext as a Global Attribute Store

// Storing data
servletContext.setAttribute("userSessionData", sessionObject);
// Retrieving data
Object data = servletContext.getAttribute("userSessionData");
// Removing data
servletContext.removeAttribute("userSessionData");

Lifecycle of ServletContext The ServletContext object is created when the web application starts and remains available until the application is shut down. Due to its long lifespan, it's not recommended to store transient business data in this context.

ServletConfig Object ServletConfig contains initialization parameters specific to an individual servlet, corresponding to configuration within the <servlet> tag in web.xml. When the container initializes a servlet, it packages the servlet's configuration into a ServletConfig object.

Servlet-Specifci Configuration in web.xml

<servlet>
    <servlet-name>ProductServlet</servlet-name>
    <servlet-class>com.example.ProductServlet</servlet-class>
    <init-param>
        <param-name>maxItems</param-name>
        <param-value>50</param-value>
    </init-param>
    <init-param>
        <param-name>cacheTimeout</param-name>
        <param-value>300</param-value>
    </init-param>
</servlet>

Accessing ServletConfig Parameters

ServletConfig config = this.getServletConfig();
String maxItems = config.getInitParameter("maxItems");
String timeout = config.getInitParameter("cacheTimeout");
Enumeration<String> paramNames = config.getInitParameterNames();

Example Implementation

MainServlet.java

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.IOException;
import java.util.*;

public class MainServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // Different ways to obtain ServletContext
        ServletContext ctx1 = request.getServletContext();
        ServletContext ctx2 = this.getServletContext();
        System.out.println("Same context? " + (ctx1 == ctx2));

        // Application path
        String appPath = ctx1.getContextPath();
        System.out.println("Application Path: " + appPath);

        // Path conversion
        String uploadPath = ctx1.getRealPath("fileStorage");
        System.out.println("Upload Directory: " + uploadPath);

        // Server info
        System.out.println("Server: " + ctx1.getServerInfo());
        System.out.println("Servlet Version: " + ctx1.getMajorVersion() + "." + ctx1.getMinorVersion());

        // Global parameters
        String dbUser = ctx1.getInitParameter("dbUsername");
        String dbPass = ctx1.getInitParameter("dbPassword");
        System.out.println("Database Credentials: " + dbUser + "/" + dbPass);

        // Storing global attributes
        List<String> users = Arrays.asList("Alice", "Bob", "Charlie");
        ctx1.setAttribute("activeUsers", users);
        ctx1.setAttribute("appTheme", "dark");
    }
}

SecondaryServlet.java

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.IOException;
import java.util.*;

public class SecondaryServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ServletContext context = this.getServletContext();

        // Reading all global parameters
        Enumeration<String> globalParams = context.getInitParameterNames();
        while(globalParams.hasMoreElements()) {
            String param = globalParams.nextElement();
            System.out.println(param + ": " + context.getInitParameter(param));
        }

        // Retrieving stored attributes
        List<String> users = (List<String>) context.getAttribute("activeUsers");
        String theme = (String) context.getAttribute("appTheme");
        System.out.println("Users: " + users);
        System.out.println("Theme: " + theme);
    }
}

ConfigServlet.java

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.IOException;

public class ConfigServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ServletConfig config = this.getServletConfig();
        System.out.println("Manufacturer: " + config.getInitParameter("manufacturer"));
        System.out.println("Display: " + config.getInitParameter("displayType"));
    }
}

web.xml Configuration

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee 
         http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0">

    <context-param>
        <param-name>dbUsername</param-name>
        <param-value>admin</param-value>
    </context-param>
    <context-param>
        <param-name>dbPassword</param-name>
        <param-value>secure123</param-value>
    </context-param>

    <servlet>
        <servlet-name>MainServlet</servlet-name>
        <servlet-class>MainServlet</servlet-class>
    </servlet>
    <servlet>
        <servlet-name>SecondaryServlet</servlet-name>
        <servlet-class>SecondaryServlet</servlet-class>
    </servlet>
    <servlet>
        <servlet-name>ConfigServlet</servlet-name>
        <servlet-class>ConfigServlet</servlet-class>
        <init-param>
            <param-name>manufacturer</param-name>
            <param-value>Dell</param-value>
        </init-param>
        <init-param>
            <param-name>displayType</param-name>
            <param-value>IPS</param-value>
        </init-param>
    </servlet>

    <servlet-mapping>
        <servlet-name>MainServlet</servlet-name>
        <url-pattern>/main</url-pattern>
    </servlet-mapping>
    <servlet-mapping>
        <servlet-name>SecondaryServlet</servlet-name>
        <url-pattern>/secondary</url-pattern>
    </servlet-mapping>
    <servlet-mapping>
        <servlet-name>ConfigServlet</servlet-name>
        <url-pattern>/config</url-pattern>
    </servlet-mapping>
</web-app>