@Async for async methods in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a Spring Boot method annotated with @Async returns void?

Consider a Spring Boot service method annotated with @Async that returns void. What is the behavior when this method is called from another bean?

Spring Boot

import org.springframework.scheduling.annotation.Async;
import org.springframework.stereotype.Service;

@Service
public class AsyncService {
    @Async
    public void asyncVoidMethod() {
        System.out.println("Start asyncVoidMethod");
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
        System.out.println("End asyncVoidMethod");
    }
}

A The method runs asynchronously in a separate thread, and the caller does not wait for it to finish.

B The method runs synchronously, blocking the caller until it finishes.

C The method throws an exception because @Async cannot be used with void return type.

D The method runs asynchronously but the caller receives a Future object to track completion.

💡 Hint

Think about how @Async handles methods without a return value.

When a method annotated with @Async returns void, Spring runs it asynchronously in a separate thread. The caller does not wait for the method to complete and does not receive any result or Future object.

❓ state_output

intermediate

What is the output order when calling two @Async methods sequentially?

Given two @Async methods called one after another in a Spring Boot application, what is the expected order of their printed output?

Spring Boot

import org.springframework.scheduling.annotation.Async;
import org.springframework.stereotype.Service;

@Service
public class AsyncService {
    @Async
    public void firstAsync() {
        System.out.println("First start");
        try { Thread.sleep(500); } catch (InterruptedException e) { Thread.currentThread().interrupt(); }
        System.out.println("First end");
    }

    @Async
    public void secondAsync() {
        System.out.println("Second start");
        try { Thread.sleep(300); } catch (InterruptedException e) { Thread.currentThread().interrupt(); }
        System.out.println("Second end");
    }
}

// In some caller bean
asyncService.firstAsync();
asyncService.secondAsync();

A First start, First end, Second start, Second end

B Second start, Second end, First start, First end

C Second start, First start, First end, Second end

D First start, Second start, Second end, First end

💡 Hint

Remember that @Async methods run in separate threads and may overlap.

Both methods start asynchronously almost immediately. Since secondAsync sleeps less, its end prints before firstAsync ends. The starts print in call order.

📝 Syntax

advanced

Which option correctly enables @Async support in a Spring Boot application?

To use @Async annotation in Spring Boot, which configuration is required to enable asynchronous processing?

A @EnableAsync annotation on a configuration class

B @AsyncConfiguration annotation on the main application class

C Adding async=true in application.properties

D Using @EnableScheduling annotation on a service class

💡 Hint

Look for the official annotation that activates async processing.

The @EnableAsync annotation on a configuration or main class activates Spring's asynchronous method execution capability.

🔧 Debug

advanced

Why does @Async not work when calling an async method from the same class?

In Spring Boot, a method annotated with @Async is called from another method in the same class, but it runs synchronously. Why?

Spring Boot

import org.springframework.scheduling.annotation.Async;
import org.springframework.stereotype.Service;

@Service
public class AsyncService {
    @Async
    public void asyncMethod() {
        System.out.println("Async method running");
    }

    public void callerMethod() {
        asyncMethod();
    }
}

A The @Async annotation requires the method to be public, and asyncMethod is private.

B Spring proxies do not intercept calls within the same class, so @Async is ignored.

C The asyncMethod must return a Future for @Async to work.

D The @Async annotation only works on methods in interfaces, not classes.

💡 Hint

Think about how Spring creates proxies for @Async methods.

Spring uses proxies to intercept calls to @Async methods. Calls from within the same class bypass the proxy, so the annotation has no effect.

🧠 Conceptual

expert

What is the effect of returning CompletableFuture from an @Async method?

In Spring Boot, if an @Async method returns a CompletableFuture, what advantage does this provide compared to returning void?

A It disables the asynchronous behavior and runs the method in the caller thread.

B It forces the method to run synchronously to produce the CompletableFuture.

C It allows the caller to track completion and get the result asynchronously.

D It causes a runtime error because @Async methods cannot return CompletableFuture.

💡 Hint

Think about how CompletableFuture can be used to handle async results.

Returning CompletableFuture from an @Async method lets the caller receive a handle to the future result, enabling non-blocking result processing.