Exception handling in async in Spring Boot - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to annotate a method for asynchronous execution.

Spring Boot

@[1]
public void processAsync() {
    // async logic
}

Drag options to blanks, or click blank then click option'

AAsyncTask

BAsync

CAsyncExecution

DAsyncMethod

Attempts:

3 left

💡 Hint

Common Mistakes

Using a non-existent annotation like @AsyncMethod

Forgetting the @ symbol before the annotation

The @Async annotation marks a method to run asynchronously in Spring Boot.

2fill in blank

medium

Complete the code to return a Future object from an async method.

Spring Boot

@Async
public [1]<String> fetchData() {
    return new AsyncResult<>("data");
}

Drag options to blanks, or click blank then click option'

ACompletableFuture

BListenableFuture

CFuture

DDeferredResult

Attempts:

3 left

💡 Hint

Common Mistakes

Using CompletableFuture without proper async support

Returning void instead of a Future type

The method returns a Future object to represent the result of an async computation.

3fill in blank

hard

Fix the error in the async exception handler method signature.

Spring Boot

public void handleError([1] ex) {
    // handle exception
}

Drag options to blanks, or click blank then click option'

AThrowable

BException

CRuntimeException

DError

Attempts:

3 left

💡 Hint

Common Mistakes

Using Exception only misses Errors

Using Error only misses Exceptions

Using Throwable allows catching all errors and exceptions in async handlers.

4fill in blank

hard

Fill both blanks to correctly define an async exception handler method.

Spring Boot

public void [1]([2] ex) {
    // log or handle exception
}

Drag options to blanks, or click blank then click option'

AhandleAsyncException

BhandleError

CThrowable

DException

Attempts:

3 left

💡 Hint

Common Mistakes

Using Exception as parameter misses Errors

Using unrelated method names

The method name can be handleError and it should accept a Throwable parameter to catch all async exceptions.

5fill in blank

hard

Fill all three blanks to create a complete async method with exception handling.

Spring Boot

@Async
public [1]<String> [2]() {
    try {
        // async logic
        return new AsyncResult<>("success");
    } catch ([3] ex) {
        // handle exception
        return new AsyncResult<>("failure");
    }
}

Drag options to blanks, or click blank then click option'

AFuture

BfetchDataAsync

CException

DCompletableFuture

Attempts:

3 left

💡 Hint

Common Mistakes

Using CompletableFuture without proper async support

Catching Throwable instead of Exception in try-catch

Using generic method names that don't indicate async

The method returns a Future, is named fetchDataAsync, and catches Exception to handle errors.

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