Test using WebTestClient

Let's assume we have a simple controller that handles HTTP requests for a RESTful API:

import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;
import reactor.core.publisher.Mono;

@RestController
public class HelloController {

    @GetMapping("/hello")
    public Mono<ResponseEntity<String>> sayHello() {
        return Mono.just(ResponseEntity.ok("Hello, World!"));
    }
}

Now, let's create JUnit test cases for this controller using WebTestClient:

import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.web.reactive.AutoConfigureWebTestClient;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.http.MediaType;
import org.springframework.test.web.reactive.server.WebTestClient;
import reactor.core.publisher.Mono;

@SpringBootTest
@AutoConfigureWebTestClient
public class HelloControllerTest {

    @Autowired
    private WebTestClient webTestClient;

    @Test
    public void testSayHello() {
        // Send GET request to /hello endpoint
        webTestClient.get().uri("/hello")
                .accept(MediaType.APPLICATION_JSON)
                .exchange()
                // Verify status code and response body
                .expectStatus().isOk()
                .expectBody(String.class).isEqualTo("Hello, World!");
    }

    // Additional test cases can be added for error scenarios, etc.
}

In this example:

• We annotate the test class with @SpringBootTest to enable Spring Boot test support.
• We use @AutoConfigureWebTestClient to configure WebTestClient automatically.
• We inject an instance of WebTestClient using @Autowired.
• In the testSayHello() method, we send a GET request to the /hello endpoint and verify the response using WebTestClient's fluent API. We expect the status code to be OK (200) and the response body to be "Hello, World!".
• Additional test cases can be added for handling error scenarios, testing different request parameters, or verifying behavior under specific conditions.

These test cases ensure that the controller behaves correctly and responds appropriately to HTTP requests. They provide confidence in the behavior of the non-blocking API and help catch any regressions or issues introduced during development.