Java.Servlets.What is deployment descriptor ?

📄 What is it exactly?

➤ File Name:

WEB-INF/web.xml

➤ It’s read by the servlet container (like Tomcat) when deploying your app.

🧠 What It Does:

• Maps URLs to servlets
• Declares filters, listeners
• Configures session settings
• Sets up security (authentication, roles)
• Controls app initialization order
• Configures welcome files (like index.html)

🧪 Minimal web.xml Example

<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee" version="3.1">

    <servlet>
        <servlet-name>MyServlet</servlet-name>
        <servlet-class>com.myapp.MyServlet</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>MyServlet</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>

</web-app>

This tells the servlet container:

• Load class com.myapp.MyServlet
• Map it to URL: /hello

So when a user hits: http://localhost:8080/yourapp/hello, the container calls that servlet.

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🛡️ Other Things You Can Configure

1. Welcome Page

<welcome-file-list>
    <welcome-file>index.html</welcome-file>
</welcome-file-list>

2. Session Timeout

<session-config>
    <session-timeout>30</session-timeout> <!-- in minutes -->
</session-config>

3. Security Constraints

<security-constraint>
    <web-resource-collection>
        <web-resource-name>Admin Area</web-resource-name>
        <url-pattern>/admin/*</url-pattern>
    </web-resource-collection>
    <auth-constraint>
        <role-name>admin</role-name>
    </auth-constraint>
</security-constraint>

💡 Modern Usage Note

Starting with Servlet 3.0, web.xml is optional.

You can now use annotations instead:

@WebServlet("/hello")
public class MyServlet extends HttpServlet { ... }

But web.xml is still useful for:

• Large/legacy apps
• Externalizing config
• Fine-grained control

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✅ TL;DR

Feature	Details
File Location	WEB-INF/web.xml
Role	Deployment configuration
Declares	Servlets, filters, listeners, security
Still Needed?	Optional in modern apps (Servlet 3.0+)