Problem Scenario: -

I have a spring boot application that acts as the backend for a frontned application that many users use. it has many rest controllers and hence many end points.

One of the endpoints is an excel generation endpoint. But, since I use a library that consumes lot of RAM to generate and build the excel, I cannot allow more than 3 excel generation concurrent requests to be processed by my spring boot application, else the app will crash with out of memory exception. Write a program to achieve this.

Answer:-

We can achieve this in Spring Boot by implementing a custom HandlerInterceptor along with a Semaphore to limit concurrent access to the Excel generation endpoint.

import org.springframework.stereotype.Component;
import org.springframework.web.servlet.HandlerInterceptor;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.util.concurrent.Semaphore;

@Component
public class ExcelGenerationInterceptor implements HandlerInterceptor {

    private final Semaphore semaphore;

    public ExcelGenerationInterceptor() {
        this.semaphore = new Semaphore(3); // Limiting to 3 concurrent requests
    }

    @Override
    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
        if ("/generateExcel".equals(request.getRequestURI())) {
            try {
                semaphore.acquire(); // Acquire a permit
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
                return false;
            }
        }
        return true;
    }

    @Override
    public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception {
        if ("/generateExcel".equals(request.getRequestURI())) {
            semaphore.release(); // Release the permit after request completion
        }
    }
}

This interceptor limits access to the /generateExcel endpoint by allowing only 3 concurrent requests to proceed at a time. Requests to other endpoints are not affected.

You need to register this interceptor in your Spring Boot application. You can do this by adding it to your WebMvcConfigurer configuration:

import org.springframework.context.annotation.Configuration;
import org.springframework.web.servlet.config.annotation.InterceptorRegistry;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;

@Configuration
public class WebConfig implements WebMvcConfigurer {

    @Override
    public void addInterceptors(InterceptorRegistry registry) {
        registry.addInterceptor(new ExcelGenerationInterceptor()).addPathPatterns("/**");
    }
}

With this setup, your Spring Boot application will only allow a maximum of 3 concurrent requests to the Excel generation endpoint while allowing other endpoints to function normally.