Throwing an Exception vs Mono.error() in Spring WebFlux

In Spring Webflux, reactive programming is embraced, introducing concepts like Mono and Flux for asynchronous data handling. In this article, we will learn the differences between throwing exceptions and using Mono.error() in Spring Webflux.

Exceptions and Error Handling in Spring Webflux

• Exceptions: Exceptions are commonly used in programs to indicate unexpected errors that disrupt the normal flow of execution.
• Reactive Streams: Spring Webflux defines that errors within a reactive stream should be signaled through error notifications rather than exceptions.

Mono.error()

• Signaling Errors in Reactive Streams: In Spring WebFlux, we primarily utilize Mono.error() to signal errors within a Mono stream. This function accepts an exception object as an argument and returns a new Mono that emits an error notification downstream. Subsequent subscribers to this Mono will receive the error notification instead of receiving any data.
• Error Handling and Recovery: Subsequent operators in the reactive chain can handle error notifications using various methods like onErrorResume or onErrorReturn. These operators allow you to define custom behavior when errors occur, potentially recovering from the error or providing an alternative value.

Java

// Assuming MyCustomException is in the same package
import reactor.core.publisher.Mono;

public class MonoErrorExample {

    public static Mono<String> getData() {
        // Simulate an error scenario
        if (Math.random() < 0.5) {
            return Mono.error(new MyCustomException("Error occurred!"));
        } else {
            return Mono.just("Success!");
        }
    }

    public static void main(String[] args) {
        getData()
                .subscribe(
                        data -> System.out.println("Data: " + data),
                        error -> System.err.println("Error: " + error.getMessage())
                );
    }
}

Explanation of the above Program:

• The getData() method simulates an error scenario by throwing a MyCustomException with a probability of 50%.
• If no error occurs, it returns a Mono that emits the value “Success!”.
• In the main method, we call getData() and subscribe to the returned Mono.
• The subscribe method takes two arguments: a consumer for receiving data and a consumer for handling errors.
• If data is emitted (“Success!”), the first consumer prints it.
• If an error occurs (the exception is thrown), the second consumer prints the error message.

Throwing Exceptions

While throwing exceptions can be convenient in imperative programming. It is generally discouraged in Spring Webflux due to several potential drawbacks:

• Disruption of Reactive Flow: Throwing exceptions can disrupt the asynchronous nature of reactive streams, potentially causing resource leaks or unexpected behavior.
• Lack of Fine-Grained Error Handling: Exceptions often carry limited information about the error, making it difficult to implement detailed error handling strategies.
• Incompatibility with Reactive Operators: Many reactive operators in Spring Webflux are designed to work with error notifications through methods like onErrorResume or onErrorReturn. Throwing exceptions might bypass these error handling mechanisms.

Best Practices for Error Handling in Spring Webflux

• Mono.error() and Error Notifications: Signal errors explicitly using Mono.error() to adhere to the Reactive Streams specification and enable more flexible error handling capabilities.
• Utilize Reactive Operators for Error Handling: Use operators like onErrorResume, onErrorReturn, onErrorMap, and others to define custom behavior when errors occur. These operators can help you recover from errors gracefully or provide alternative values.
• Consider Exception Wrapping: If using custom exceptions is necessary, consider wrapping them within a Reactive Exception to ensure compatibility with reactive operators and avoid disrupting the reactive flow.

Difference between Throwing an Exceptions and Mono.error()

Features	Throwing Exceptions	Mono.error()
Usage	Gives signal errors.	Gives signal errors in reactive streams.
Compatibility	Disrupts reactive flow.	Adheres to Reactive Streams.
Error handling	Limited information, potential resource leaks.	Enables fine-grained error handling, avoids resource leaks.
Operator usage	May bypass operators.	Compatible with reactive operators.
Best practice	Generally discouraged.	Preferred approach in Spring Webflux.