Custom Servlet Implementation

package com.example.web.component;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class CustomHttpServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("CustomServlet Invoked");
        resp.getWriter().write("Response from Custom Servlet: " + req.getContextPath());
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

Custom Filter Implementation

package com.example.web.filter;

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

public class LoggingFilter implements Filter {
    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) 
            throws IOException, ServletException {
        System.out.println("LoggingFilter: Request intercepted");
        chain.doFilter(request, response);
    }
}

Static Resource Filter

package com.example.web.filter;

import javax.servlet.*;
import javax.servlet.http.HttpServletRequest;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.OutputStream;

public class AssetDeliveryFilter implements Filter {
    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) 
            throws IOException, ServletException {
        HttpServletRequest httpReq = (HttpServletRequest) request;
        String uri = httpReq.getServletPath();
        String diskPath = httpReq.getServletContext().getRealPath(uri);
        
        try (OutputStream out = response.getOutputStream(); 
             FileInputStream fis = new FileInputStream(diskPath)) {
            byte[] buffer = new byte[1024];
            int bytesRead;
            while ((bytesRead = fis.read(buffer)) != -1) {
                out.write(buffer, 0, bytesRead);
            }
            out.flush();
        }
    }
}

Custom Listener Implementation

package com.example.web.listener;

import javax.servlet.ServletRequestEvent;
import javax.servlet.ServletRequestListener;

public class RequestAuditListener implements ServletRequestListener {
    @Override
    public void requestInitialized(ServletRequestEvent sre) {
        System.out.println("Request Initialized");
    }

    @Override
    public void requestDestroyed(ServletRequestEvent sre) {
        System.out.println("Request Destroyed");
    }
}

Method 1: Using WebApplicationInitializer (Servlet 3.0+)

To register these components programmatically, implement the WebApplicationInitializer interface. Spring detects this via the Servlet 3.0 ServletContainerInitializer mechanism.

package com.example.web.setup;

import com.example.web.component.CustomHttpServlet;
import com.example.web.filter.AssetDeliveryFilter;
import com.example.web.filter.LoggingFilter;
import com.example.web.listener.RequestAuditListener;
import org.springframework.web.WebApplicationInitializer;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;

public class AppInitializer implements WebApplicationInitializer {
    @Override
    public void onStartup(ServletContext context) throws ServletException {
        // Register Servlet
        context.addServlet("customServlet", CustomHttpServlet.class)
               .addMapping("/custom");

        // Register Filters
        context.addFilter("loggingFilter", LoggingFilter.class)
               .addMappingForUrlPatterns(null, false, "/*");
               
        context.addFilter("assetFilter", AssetDeliveryFilter.class)
               .addMappingForUrlPatterns(null, false, "/assets/*");

        // Register Listeners
        context.addListener(RequestAuditListener.class);
    }
}

Registration API Overloads

The ServletContext provides several overloads for adding components:

• Servlets: addServlet(String name, String className | Servlet instance | Class<?>)
• Filters: addFilter(String name, String className | Filter instance | Class<?>)
• Listeners: addListener(String className | Listener instance | Class<?>)

Filter Execution Order

When using addMappingForUrlPatterns(EnumSet<DispatcherType>, boolean isMatchAfter, String...), the execution order is determined by the registration sequence in the onStartup method. Filters registered with isMatchAfter = false take precedence over those set to true.

Mechanism Behind the Magic

In a Servlet 3.0+ container, the server scans for implementations of ServletContainerInitializer. Spring provides SpringServletContainerInitializer, which specifically looks for classes annotated with @HandlesTypes(WebApplicationInitializer.class). It then instantiates and runs the onStartup method of all discovered implementations.

// Simplified logic from SpringServletContainerInitializer
public void onStartup(Set<Class<?>> webAppInitializerClasses, ServletContext servletContext) {
    List<WebApplicationInitializer> initializers = new ArrayList<>();
    
    for (Class<?> clazz : webAppInitializerClasses) {
        // Ensure it's a concrete class implementing the interface
        if (!clazz.isInterface() && WebApplicationInitializer.class.isAssignableFrom(clazz)) {
            initializers.add((WebApplicationInitializer) clazz.newInstance());
        }
    }

    // Sort and execute
    AnnotationAwareOrderComparator.sort(initializers);
    for (WebApplicationInitializer initializer : initializers) {
        initializer.onStartup(servletContext); // Your custom logic runs here
    }
}