Test to illustrate starvation on a webclient call

WebClientStarvationTest.java

import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.mockito.junit.jupiter.MockitoExtension;
import org.springframework.http.client.reactive.ReactorClientHttpConnector;
import org.springframework.web.reactive.function.client.WebClient;
import reactor.netty.http.client.HttpClient;
import reactor.netty.resources.ConnectionProvider;

import java.time.Duration;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;

import static org.junit.jupiter.api.Assertions.assertThrows;

@ExtendWith(MockitoExtension.class)
public class WebClientStarvationTest {

    private WebClient createWebClient(int maxConnections) {
        ConnectionProvider connectionProvider = ConnectionProvider.builder("test_pool")
                .maxConnections(maxConnections)  // Low limit to simulate starvation
                .pendingAcquireTimeout(Duration.ofSeconds(5)) // Timeout for acquiring a connection
                .maxIdleTime(Duration.ofSeconds(10))
                .build();

        HttpClient httpClient = HttpClient.create(connectionProvider);

        return WebClient.builder()
                .clientConnector(new ReactorClientHttpConnector(httpClient))
                .build();
    }

    @Test
    public void testConnectionStarvation() throws InterruptedException {
        WebClient webClient = createWebClient(2); // Only 2 connections allowed

        CountDownLatch latch = new CountDownLatch(2);

        // Simulate long-running requests to the first service
        Runnable longRunningRequest = () -> {
            webClient.get()
                    .uri("https://httpbin.org/delay/10") // Simulates a 10-second delay
                    .retrieve()
                    .bodyToMono(String.class)
                    .doFinally(signal -> latch.countDown()) // Release when done
                    .subscribe();
        };

        new Thread(longRunningRequest).start();
        new Thread(longRunningRequest).start();

        // Wait a short time to let those requests consume all connections
        Thread.sleep(500);

        // Third request should get blocked and eventually timeout
        assertThrows(Exception.class, () -> {
            try {
                webClient.get()
                        .uri("https://httpbin.org/get") // Different host
                        .retrieve()
                        .bodyToMono(String.class)
                        .block(Duration.ofSeconds(10)); // Should timeout
            } catch (Exception e) {
                System.out.println(e);
                throw e;
            }
        });

        // Wait for long requests to complete
        latch.await(15, TimeUnit.SECONDS);
    }
}