Exception handling in async in Spring Boot - Mini Project: Build & Apply

Exception handling in async

📖 Scenario: You are building a Spring Boot application that performs a task asynchronously. Sometimes, errors happen during this task. You want to catch these errors properly so your app can handle them without crashing.

🎯 Goal: Create a Spring Boot service that runs a method asynchronously and handles exceptions thrown inside that method using a custom async exception handler.

📋 What You'll Learn

Create a service class with an asynchronous method

Configure a custom AsyncConfigurer to handle exceptions

Throw an exception inside the async method

Log the exception using the custom handler

💡 Why This Matters

🌍 Real World

Handling exceptions in asynchronous tasks is important in real-world apps to avoid crashes and to log errors for debugging.

💼 Career

Knowing how to manage async exceptions is a key skill for backend developers working with Spring Boot and microservices.

1

Create the async service class

Create a Spring service class called AsyncService with a method runAsyncTask annotated with @Async. Inside the method, throw a new RuntimeException with the message "Async error occurred".

Spring Boot

@Service
public class AsyncService {
    // Create the async method here
}

Hint

Use @Service on the class and @Async on the method. Throw the exception inside the method body.

2

Add async configuration class

Create a configuration class called AsyncConfig that implements AsyncConfigurer. Override the method getAsyncUncaughtExceptionHandler() to return a new instance of SimpleAsyncUncaughtExceptionHandler.

Spring Boot

@Service
public class AsyncService {
    @Async
    public void runAsyncTask() {
        throw new RuntimeException("Async error occurred");
    }
}

// Create AsyncConfig class here

Hint

Use @Configuration and @EnableAsync on the class. Implement AsyncConfigurer and override getAsyncUncaughtExceptionHandler().

3

Customize exception handler to log errors

Inside AsyncConfig, replace SimpleAsyncUncaughtExceptionHandler with a custom class CustomAsyncExceptionHandler that implements AsyncUncaughtExceptionHandler. In the handleUncaughtException method, log the exception message using System.out.println with the prefix "Async exception caught:".

Spring Boot

@Service
public class AsyncService {
    @Async
    public void runAsyncTask() {
        throw new RuntimeException("Async error occurred");
    }
}

@Configuration
@EnableAsync
public class AsyncConfig implements AsyncConfigurer {
    @Override
    public AsyncUncaughtExceptionHandler getAsyncUncaughtExceptionHandler() {
        // Return custom exception handler here
    }

    // Define CustomAsyncExceptionHandler class here
}

Hint

Create a static inner class implementing AsyncUncaughtExceptionHandler. Override handleUncaughtException to print the exception message.

4

Call async method from main application

In your main Spring Boot application class, inject AsyncService and call runAsyncTask() inside the run method of CommandLineRunner to trigger the async method and see the exception handling in action.

Spring Boot

@Service
public class AsyncService {
    @Async
    public void runAsyncTask() {
        throw new RuntimeException("Async error occurred");
    }
}

@Configuration
@EnableAsync
public class AsyncConfig implements AsyncConfigurer {
    @Override
    public AsyncUncaughtExceptionHandler getAsyncUncaughtExceptionHandler() {
        return new CustomAsyncExceptionHandler();
    }

    public static class CustomAsyncExceptionHandler implements AsyncUncaughtExceptionHandler {
        @Override
        public void handleUncaughtException(Throwable ex, Method method, Object... params) {
            System.out.println("Async exception caught: " + ex.getMessage());
        }
    }
}

@SpringBootApplication
public class DemoApplication implements CommandLineRunner {

    private final AsyncService asyncService;

    public DemoApplication(AsyncService asyncService) {
        this.asyncService = asyncService;
    }

    @Override
    public void run(String... args) throws Exception {
        // Call async method here
    }

    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }
}

Hint

Inject AsyncService via constructor. Call runAsyncTask() inside run method.

Practice

1. What is the main purpose of using AsyncUncaughtExceptionHandler in Spring Boot asynchronous methods?

easy

A. To handle exceptions thrown by async methods that return void

B. To automatically retry failed async methods

C. To log successful completion of async methods

D. To convert async methods into synchronous ones

Solution

1. Step 1: Understand async method return types

   Async methods returning void cannot return exceptions directly to the caller.

2. Step 2: Role of AsyncUncaughtExceptionHandler

   This handler catches exceptions from void async methods to prevent app crashes.

3. Final Answer:

   To handle exceptions thrown by async methods that return void -> Option A

4. Quick Check:

   Void async exceptions need AsyncUncaughtExceptionHandler [OK]

Hint: Void async methods need special exception handler [OK]

Common Mistakes:

• Thinking AsyncUncaughtExceptionHandler retries failed tasks
• Assuming it handles exceptions for Future-returning methods
• Confusing it with synchronous exception handling

2. Which of the following is the correct way to declare an async method that returns a result and handles exceptions properly?

easy

A. @Async public void processData() throws Exception { ... }

B. @Async public Future<String> fetchData() throws InterruptedException { return null; }

C. @Async public String fetchData() { return "data"; }

D. @Async public CompletableFuture<String> fetchData() { return CompletableFuture.completedFuture("data"); }

Solution

1. Step 1: Identify async method return types

   Async methods returning results should return Future or CompletableFuture to handle async results and exceptions.

2. Step 2: Check method signatures

   @Async public CompletableFuture<String> fetchData() { return CompletableFuture.completedFuture("data"); } returns CompletableFuture<String> correctly and does not declare checked exceptions, which is proper for async methods.

3. Final Answer:

   @Async public CompletableFuture<String> fetchData() { return CompletableFuture.completedFuture("data"); } -> Option D

4. Quick Check:

   Async methods with results use CompletableFuture [OK]

Hint: Async methods with results return CompletableFuture or Future [OK]

Common Mistakes:

• Declaring async methods with void when result is needed
• Throwing checked exceptions directly in async methods
• Returning raw types instead of Future or CompletableFuture

3. Given the following async method, what will be the output if an exception occurs inside the CompletableFuture?

medium

A. The exception is caught inside exceptionally and a fallback value is returned

B. The exception is automatically logged and the method returns null

C. The exception crashes the application immediately

D. The exception is ignored and the method returns normally

Solution

1. Step 1: Analyze CompletableFuture exception handling

   The exceptionally method handles exceptions and provides fallback values.

2. Step 2: Understand method behavior on exception

   If an exception occurs, exceptionally returns the fallback value instead of propagating the exception.

3. Final Answer:

   The exception is caught inside exceptionally and a fallback value is returned -> Option A

4. Quick Check:

   CompletableFuture.exceptionally handles exceptions [OK]

Hint: Use exceptionally to catch async exceptions with fallback [OK]

Common Mistakes:

• Assuming exceptions crash the app without handling
• Expecting null return instead of fallback value
• Ignoring the need for exceptionally or handle methods

4. Identify the error in this async method that uses Future and explain why exceptions are not handled properly:

@Async
public Future<String> loadData() {
  throw new RuntimeException("Failed");
}

medium

A. Future cannot be used with @Async methods

B. Method should not be annotated with @Async when returning Future

C. Throwing exception directly in async method causes it to be lost; should return Future with exception

D. RuntimeException must be caught inside the method before throwing

Solution

1. Step 1: Understand async method exception handling

   Exceptions thrown directly in async methods returning Future are not propagated properly.

2. Step 2: Proper way to handle exceptions in Future

   Return a Future that completes exceptionally instead of throwing directly.

3. Final Answer:

   Throwing exception directly in async method causes it to be lost; should return Future with exception -> Option C

4. Quick Check:

   Throwing exceptions directly in async Future methods is incorrect [OK]

Hint: Return Future with exception, don't throw directly in async methods [OK]

Common Mistakes:

• Throwing exceptions directly in async methods returning Future
• Ignoring exception handling in async methods
• Misusing @Async annotation with incompatible return types

5. You have an async method returning CompletableFuture<String> that may throw exceptions. How can you combine exception handling and fallback logic so the caller always gets a valid string without blocking?

hard

A. Use get() on CompletableFuture and catch exceptions synchronously

B. Use handle method on CompletableFuture to catch exceptions and provide fallback value

C. Wrap the async method call in a try-catch block in the caller thread

D. Declare the async method to throw checked exceptions and let caller handle them

Solution

1. Step 1: Understand async exception handling with CompletableFuture

   The handle method allows processing both result and exception without blocking.

2. Step 2: Implement fallback logic in handle

   Inside handle, check if exception is present and return fallback string if so.

3. Final Answer:

   Use handle method on CompletableFuture to catch exceptions and provide fallback value -> Option B

4. Quick Check:

   CompletableFuture.handle combines result and exception handling [OK]

Hint: Use CompletableFuture.handle for async fallback without blocking [OK]

Common Mistakes:

• Blocking caller thread with get() defeats async purpose
• Trying to catch exceptions outside async context
• Declaring checked exceptions in async methods unnecessarily