How to Handle Exceptions in an Async Block in Java

What are the best practices for handling exceptions within an async block in Java?

CompletableFuture.supplyAsync(() -> {
    // Your async code here
}).exceptionally(ex -> {
    // Handle exception
    return null;
});

Handling exceptions in asynchronous programming, such as when using Java's CompletableFuture, is crucial to ensure robust, error-free applications. Asynchronous methods can fail without throwing exceptions in the traditional sense, which means special care must be taken to manage these errors effectively.

CompletableFuture.supplyAsync(() -> {
    // Some long-running task
    int result = riskyOperation();
    return result;
}).handle((result, ex) -> {
    if (ex != null) {
        // Handle the exception
        System.out.println("An error occurred: " + ex.getMessage());
        return -1;
    }
    return result;
});

Causes

• Exceptions thrown by the asynchronous operation itself.
• Failure of the CompletableFuture to complete due to cancellation.
• Execution of exceptions in another thread context.

Solutions

• Use the `exceptionally` method of CompletableFuture to handle exceptions gracefully.
• Adopt try-catch blocks within lambda expressions where you define your async tasks.
• Utilize the `handle` method to manage both successful and exceptional results in one place.

Mistake: Forgetting to handle the exception cases, leading to unhandled errors during the async tasks.

Solution: Always implement exception handling by using either `exceptionally` or `handle` methods.

Mistake: Using thread blocking methods within the async block, which defeats the purpose of asynchronous programming.

Solution: Ensure that all code within the async block is non-blocking and returns a value directly.