The Complete Guide to CompletionException in Java

Have you ever encountered a CompletionException while working with Java? Don’t worry, you’re not alone! In this comprehensive guide, we’ll explore what a CompletionException is, why it occurs, and how to handle it. So, fasten your seatbelts and let’s dive into the world of CompletionException!

What is a CompletionException?

A CompletionException is an unchecked exception that extends the java.util.concurrent.ExecutionException class in Java. It is thrown when a Future computation has been completed with an exception and that exception cannot be propagated directly.

In simpler terms, when you’re working with CompletableFuture or any other asynchronous computation in Java, any exception thrown by the asynchronous task will be wrapped in a CompletionException.

Why Do CompletionExceptions Occur?

CompletionExceptions typically occur in scenarios where an exception is thrown within an asynchronous task, and that exception is not properly handled. Let’s consider an example to illustrate this:

CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> {
    // Some task that may throw an exception
    throw new RuntimeException("Oops! Something went wrong.");
});

future.join();

In this example, we have a CompletableFuture that performs a computation asynchronously. In the supplyAsync method, we have a task that intentionally throws a RuntimeException to simulate an error. When we call future.join(), a CompletionException is thrown, wrapping the original exception.

Handling CompletionExceptions

To handle a CompletionException, we need to unwrap it and retrieve the original, root cause exception. This can be achieved by using the exceptionally method provided by the CompletableFuture class:

CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> {
    // Some task that may throw an exception
    throw new RuntimeException("Oops! Something went wrong.");
});

future.exceptionally(ex -> {
    if (ex instanceof CompletionException) {
        ex = ex.getCause();
    }
    
    // Handle the exception
    System.out.println("Error: " + ex.getMessage());
    return null;
}).join();

In the above code snippet, we chain a exceptionally method to the CompletableFuture instance. Inside the exceptionally block, we check if the exception is a CompletionException and then retrieve the root cause using ex.getCause(). Finally, we handle the exception by printing an error message and returning a default value (null in this case).

Note: It’s important to handle CompletionExceptions effectively to avoid suppressing the root cause exception. In the example above, we simply print the error message, but you can choose to log it or take appropriate action based on your use case.

Chaining Multiple CompletionExceptions

In real-world scenarios, you may need to perform a sequence of asynchronous tasks that depend on each other. In such cases, it’s essential to handle CompletionExceptions safely, as they can occur at any stage in the chain.

Let’s take a look at an example where we chain multiple CompletableFuture instances, each potentially throwing an exception:

CompletableFuture<Integer> future1 = CompletableFuture.supplyAsync(() -> {
    // Some task that may throw an exception
    throw new RuntimeException("Oops! Something went wrong in future1.");
});

CompletableFuture<Integer> future2 = CompletableFuture.supplyAsync(() -> {
    // Some task that may throw an exception
    throw new RuntimeException("Oops! Something went wrong in future2.");
});

CompletableFuture<Integer> future3 = future1.thenCombine(future2, (result1, result2) -> {
    // Perform some computation with the results
    return result1 + result2;
});

future3.exceptionally(ex -> {
    if (ex instanceof CompletionException) {
        ex = ex.getCause();
    }
    
    // Handle the exception
    System.out.println("Error: " + ex.getMessage());
    return null;
}).join();

In this example, we have three CompletableFuture instances: future1, future2, and future3. We intentionally throw exceptions in the first two futures to simulate errors. We then chain future1 and future2 using the thenCombine method to perform a computation when both futures complete. Finally, we handle any CompletionExceptions that may occur during the execution of future3.

Conclusion

CompletionException is a powerful construct in Java that helps handle exceptions raised during asynchronous computations. By properly handling CompletionExceptions, you can gracefully handle asynchronous errors and ensure your code remains robust.

In this article, we explored what a CompletionException is, why it occurs, and how to handle it effectively using CompletableFuture. By following the provided examples, you should now be able to confidently handle CompletionExceptions in your own Java code.

So, the next time you encounter a CompletionException, don’t panic! Instead, embrace it as an opportunity to enhance your code resilience.

Happy coding!

Reference Links

• JavaDocs - CompletionException
• JavaDocs - CompletableFuture

Note: The examples provided in this article have minimal error handling for brevity. In a real-world scenario, it is recommended to handle exceptions more robustly and take appropriate actions based on your application’s requirements.

This article is inspired by the Stack Overflow question: How to properly handle CompletionException in Java?