@Async for async methods in Spring Boot - Mini Project: Build & Apply

Using @Async for Async Methods in Spring Boot

📖 Scenario: You are building a Spring Boot application that needs to perform a time-consuming task without blocking the main thread. To improve user experience, you want to run this task asynchronously.

🎯 Goal: Learn how to use the @Async annotation in Spring Boot to run methods asynchronously, so the main thread can continue working while the task runs in the background.

📋 What You'll Learn

Create a Spring Boot service class with a method that simulates a long-running task

Enable asynchronous processing in the Spring Boot application

Annotate the long-running method with @Async to make it run asynchronously

Call the asynchronous method from a controller and return a response immediately

💡 Why This Matters

🌍 Real World

Asynchronous methods help improve responsiveness in web applications by running slow tasks in the background.

💼 Career

Knowing how to use @Async is important for backend developers working with Spring Boot to build scalable and efficient applications.

1

Create a service class with a long-running method

Create a Spring service class called TaskService with a method public void runLongTask() that simulates a long task by sleeping for 3000 milliseconds inside a try-catch block.

Spring Boot

import org.springframework.stereotype.Service;

@Service
public class TaskService {
    // Your code here
}

Hint

Use Thread.sleep(3000) inside runLongTask() to simulate a delay.

2

Enable asynchronous processing in the application

In your main Spring Boot application class, add the @EnableAsync annotation above the class declaration to enable async support.

Spring Boot

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
// Add import for EnableAsync

@SpringBootApplication
// Your code here
public class AsyncDemoApplication {
    public static void main(String[] args) {
        SpringApplication.run(AsyncDemoApplication.class, args);
    }
}

Hint

Import and add @EnableAsync above your main application class.

3

Make the long-running method asynchronous

In the TaskService class, add the @Async annotation above the runLongTask() method to make it run asynchronously.

Spring Boot

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

@Service
public class TaskService {
    // Add @Async annotation here
    public void runLongTask() {
        try {
            Thread.sleep(3000);
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
    }
}

// Rest of the code unchanged

Hint

Import @Async and place it just above runLongTask().

4

Call the async method from a controller

Create a Spring REST controller class called TaskController with a GET endpoint /start-task that calls taskService.runLongTask() and immediately returns the string "Task started".

Spring Boot

import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.beans.factory.annotation.Autowired;

@RestController
public class TaskController {
    @Autowired
    private TaskService taskService;

    // Add GET endpoint /start-task here
}

Hint

Create a controller with a GET method that calls the async method and returns immediately.

Practice

1.

What is the main purpose of using @Async in a Spring Boot application?

easy

A. To secure methods with authentication

B. To make methods run faster by optimizing CPU usage

C. To automatically retry failed methods

D. To run methods in the background without blocking the main thread

Solution

1. Step 1: Understand what @Async does

   @Async allows methods to run in a separate thread, so they don't block the main thread.

2. Step 2: Identify the main benefit

   This helps keep the app responsive by running long tasks in the background.

3. Final Answer:

   To run methods in the background without blocking the main thread -> Option D

4. Quick Check:

   Background execution = To run methods in the background without blocking the main thread [OK]

Hint: Async means background task, not faster or secure [OK]

Common Mistakes:

• Thinking @Async speeds up method execution
• Confusing @Async with security annotations
• Assuming @Async retries failed methods automatically

2.

Which annotation must be added to a Spring Boot configuration class to enable @Async support?

easy

A. @EnableAsync

B. @EnableScheduling

C. @AsyncSupport

D. @EnableAsyncSupport

Solution

1. Step 1: Recall the enabling annotation for async

   Spring Boot requires @EnableAsync on a configuration class to activate async processing.

2. Step 2: Confirm the exact annotation name

   The correct annotation is @EnableAsync, not any other variant.

3. Final Answer:

   @EnableAsync -> Option A

4. Quick Check:

   Enable async = @EnableAsync [OK]

Hint: Enable async with @EnableAsync, not similar names [OK]

Common Mistakes:

• Using @EnableScheduling instead of @EnableAsync
• Guessing annotation names without checking docs
• Confusing @Async with enabling annotations

3.

Consider this Spring Boot method annotated with @Async:

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

What will be the output if you call fetchData().get()?

medium

A. It blocks until the async task completes and returns "Done"

B. It throws a NullPointerException

C. It returns "Done" immediately

D. It returns null because async methods don't return values

Solution

1. Step 1: Understand CompletableFuture with @Async

   The method returns a CompletableFuture that completes immediately with "Done".

2. Step 2: Calling get() on CompletableFuture

   Calling get() blocks the caller until the async task finishes and then returns the result.

3. Final Answer:

   It blocks until the async task completes and returns "Done" -> Option A

4. Quick Check:

   CompletableFuture.get() blocks and returns result [OK]

Hint: CompletableFuture.get() waits for async result [OK]

Common Mistakes:

• Assuming async methods return immediately without blocking on get()
• Thinking async methods can't return values
• Confusing exceptions with null returns

4.

Given this Spring Boot service method:

  @Async
  public void process() {
    System.out.println("Start");
    throw new RuntimeException("Error");
  }

What happens when process() is called?

medium

A. The exception is thrown on the caller thread immediately

B. The exception is logged but does not affect the caller thread

C. The method runs synchronously and throws the exception normally

D. The exception is ignored silently

Solution

1. Step 1: Understand exception handling in @Async methods

   Exceptions in async methods run in a separate thread and do not propagate to the caller.

2. Step 2: What happens to the exception?

   The exception is logged by Spring but does not crash or throw on the caller thread.

3. Final Answer:

   The exception is logged but does not affect the caller thread -> Option B

4. Quick Check:

   Async exceptions logged, caller unaffected [OK]

Hint: Async exceptions don't throw on caller thread [OK]

Common Mistakes:

• Expecting exceptions to propagate to caller immediately
• Thinking async methods run synchronously
• Assuming exceptions are ignored silently

5.

You want to run two independent tasks asynchronously and wait concurrently for both to finish before proceeding. Which approach using @Async and CompletableFuture is correct?

  @Async
  public CompletableFuture<String> taskOne() { ... }

  @Async
  public CompletableFuture<String> taskTwo() { ... }

  public void runTasks() throws Exception {
    CompletableFuture<String> future1 = taskOne();
    CompletableFuture<String> future2 = taskTwo();
    // What to do here?
  }

hard

A. Just call taskOne() and taskTwo() without waiting

B. Call future1.get() and future2.get() sequentially to wait

C. Use CompletableFuture.allOf(future1, future2).join() to wait both

D. Call future1.complete() and future2.complete() manually

Solution

1. Step 1: Understand waiting for multiple async tasks

   Calling get() sequentially works but is inefficient and blocks one after another.

2. Step 2: Use CompletableFuture.allOf for parallel waiting

   CompletableFuture.allOf(future1, future2).join() waits for both tasks to complete concurrently.

3. Final Answer:

   Use CompletableFuture.allOf(future1, future2).join() to wait both -> Option C

4. Quick Check:

   Wait all async tasks with allOf().join() [OK]

Hint: Use allOf().join() to wait multiple async tasks together [OK]

Common Mistakes:

• Calling get() sequentially causing unnecessary blocking
• Not waiting at all and proceeding too early
• Trying to manually complete futures which is incorrect