How to Return 404 with Spring WebFlux?

The Spring Reactive allows developers to build Asynchronous, Non-Blocking, and Event-Driven Web Applications. When compared with the Spring MVC framework the Spring Reactive Web framework provides more functionality. And we can easily manage the web applications through this Spring Reactive Framework. If we want to use this framework, we need to integrate Spring Web flux in our dependencies File.

In this article, we will see how to Return a 404 error status code with Spring WebFlux.

The 404 came while no data was found on the server, The HTTP protocol returns this error code as output. The 404 error is returned by the HTTP protocol when a server cannot find the requested resource.

Prerequisites:

To understand how a flux works in reactive streams, below we have listed some topics:

• Spring WebFlux
• Functionality of Publisher, Subscriber
• Reactive Streams in Spring
• Spring Annotations
• APIs working flow
• Know Information about HTTP error codes.

Implementation Steps to Return 404 Error Status Code with Spring WebFlux

Below is the implementation to get a 404 error status code with Spring WebFlux.

Step 1: Add Dependencies

To access the Spring Reactive, we need add below Dependencies in our Spring Boot project.

dependencies {
    // Spring WebFlux
    implementation 'org.springframework.boot:spring-boot-starter-webflux'
    // Reactor Core
    implementation 'io.projectreactor:reactor-core'
    // Spring Boot Starter Test (if needed)
    testImplementation 'org.springframework.boot:spring-boot-starter-test'
}

Step 2: Java Class Creation

After creation of Spring stater project with above mentioned dependencies, we will create one Java class in project main package. This class is worked as RestController for handling API end points. For Creating RestController we need use @RestController annotation in Spring WebFlux.

Java

package com.reactive.app;

import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;

import reactor.core.publisher.Mono;

@RestController
public class RestControllerClass {

    @GetMapping("/api/resource")
    public Mono<ResponseEntity<String>> getResource(@RequestParam(required = false) String id) {
        // Check if resource exists
        if (id == null || id.isEmpty() || resourceNotFound(id)) {
            // Return 404 response
            return Mono.just(ResponseEntity.notFound().build());
        }

        // If resource exists, return it
        return Mono.just(ResponseEntity.ok("Resource with ID " + id));
    }

    // Dummy method to check if resource exists
    private boolean resourceNotFound(String id) {
        return true;
    }
}

Explanation of the Code:

• In the above Java class, we have created one API end point by using @GetMapping for handling GET method with HTTP protocol.
• After that we have created one method that is getResource(). This Method take request parameter as input. This Parameter request take Id value in the API URL as input.
• If given Id is not found, then HTTP protocol returns 404 error message. Here we intentionally give wrong Id to show Not Found error means 404 status code.
• After that by using If statement we check the weather the id null or resourceNotFound(id). If it is true, it returns 404 status code by using ResponseEntity.
• Here resourceNotFound is a method for handling Id value from the user given in the API URL.

Step 3: getResource API

Here, we have created one API with HTTP GET method by using @GetMapping annotation. Then we have used Mono Publisher. This publisher takes ResponseEntity as return type and it returns a publisher in the form of ResponseEntity with HTTP status code. This ResponseEntity take class as String to handle User ID.

Java

@GetMapping("/api/resource")
    public Mono<ResponseEntity<String>> getResource(@RequestParam(required = false) String id) {
        // Check if resource exists
        if (id == null || id.isEmpty() || resourceNotFound(id)) {
            // Return 404 response
            return Mono.just(ResponseEntity.notFound().build());
        }

        // If resource exists, return it
        return Mono.just(ResponseEntity.ok("Resource with ID " + id));
    }

Step 4: resourceNotFound method

Here, we have created one boolean method to handle user id. This method takes user id as argument through method signature. Then we return the true value in any case. This method is called in API endpoint, we can observe this in the API code.

private boolean resourceNotFound(String id) {
        return true;
}

Output:

After running this project as Spring Boot API, we have tested this API end point using PostMan tool. After successfully tested API endpoint, we got expected output that is Return 404 error.

Below is the output image for your reference.