✅ Short answer:
The connection pool (like HikariDataSource, Tomcat DataSource, etc.) should be destroyed when the web application is shutting down.

That means:

• When Tomcat (or another servlet container) undeploys your app,
• Or stops,
• Or reloads the application.

Otherwise:

• Database connections stay open → resource leak on database side.
• Eventually database runs out of connections → errors like “Too many connections”.

✅ How to destroy it properly?

You must close the connection pool inside a ServletContextListener in the contextDestroyed() method.

Here’s the right lifecycle:

Event	What to do
App startup (contextInitialized)	Create connection pool and store it in ServletContext
App shutdown (contextDestroyed)	Close the connection pool safely

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✅ Example again (proper destruction):

import jakarta.servlet.ServletContextEvent;
import jakarta.servlet.ServletContextListener;
import jakarta.servlet.annotation.WebListener;
import com.zaxxer.hikari.HikariDataSource;

@WebListener
public class AppContextListener implements ServletContextListener {
    private HikariDataSource dataSource;

    @Override
    public void contextInitialized(ServletContextEvent sce) {
        dataSource = new HikariDataSource();
        dataSource.setJdbcUrl("jdbc:mysql://localhost:3306/mydb");
        dataSource.setUsername("user");
        dataSource.setPassword("pass");
        
        // Save it in the ServletContext so servlets can use it
        sce.getServletContext().setAttribute("DataSource", dataSource);
    }

    @Override
    public void contextDestroyed(ServletContextEvent sce) {
        // Properly close the pool when the application shuts down
        HikariDataSource ds = (HikariDataSource) sce.getServletContext().getAttribute("DataSource");
        if (ds != null) {
            ds.close(); // 🔥 Important: release all DB connections
        }
    }
}

✅ What happens when you call ds.close()?

• All idle connections are closed.
• Active connections are closed after use.
• No new connections are accepted.
• The pool is safely shutdown.

In short: the application cleans up after itself like a good citizen.

✅ If you forget to close:

• Memory leak inside Tomcat or your server.
• Database keeps connections busy unnecessarily.
• Can cause deployment errors if you redeploy the app many times.

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✅ Real-world checklist:

Thing	Should happen
During contextInitialized	Create + configure DataSource
Save it to	ServletContext attributes
During contextDestroyed	Fetch DataSource and call .close()
After closing	No memory leaks, no DB leaks

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🌟 Visual Lifecycle:

App Startup
    ↓
ServletContextListener.contextInitialized()
    ↓
Create Connection Pool (DataSource)
    ↓
Servlets use connections from pool
    ↓
App Shutdown
    ↓
ServletContextListener.contextDestroyed()
    ↓
Close Connection Pool properly

✨ Final Reminder:

• Connection objects are borrowed and returned automatically (with try-with-resources).
• But the pool itself must be manually closed once when the app ends.