@EnableAsync annotation in Spring Boot - Mini Project: Build & Apply

Using @EnableAsync Annotation in Spring Boot

📖 Scenario: You are building a Spring Boot application that needs to perform some tasks asynchronously to improve performance and user experience. For example, sending emails or processing files without blocking the main thread.

🎯 Goal: Learn how to enable asynchronous method execution in Spring Boot using the @EnableAsync annotation and create a simple asynchronous method.

📋 What You'll Learn

Create a Spring Boot application class

Enable asynchronous processing with @EnableAsync

Create a service class with an asynchronous method using @Async

Call the asynchronous method from the main application

💡 Why This Matters

🌍 Real World

Many web applications need to perform tasks like sending emails, processing files, or calling external APIs without blocking the main thread. Using @EnableAsync allows these tasks to run in the background improving responsiveness.

💼 Career

Understanding asynchronous processing with @EnableAsync is important for backend developers working with Spring Boot to build scalable and efficient applications.

1

Create the Spring Boot application class

Create a class called AsyncApplication annotated with @SpringBootApplication and add the main method that runs SpringApplication.run(AsyncApplication.class, args).

Spring Boot

@SpringBootApplication
public class AsyncApplication {
    public static void main(String[] args) {
        // Your code here
    }
}

Hint

Use @SpringBootApplication on the class and inside main call SpringApplication.run with the class name and args.

2

Enable asynchronous processing with @EnableAsync

Add the @EnableAsync annotation to the AsyncApplication class above the @SpringBootApplication annotation to enable asynchronous method execution.

Spring Boot

// Add @EnableAsync annotation above @SpringBootApplication
@SpringBootApplication
public class AsyncApplication {
    public static void main(String[] args) {
        SpringApplication.run(AsyncApplication.class, args);
    }
}

Hint

Place @EnableAsync directly above @SpringBootApplication on the class.

3

Create a service with an asynchronous method

Create a class called EmailService annotated with @Service. Inside it, create a method called sendEmail that takes a String recipient parameter and annotate this method with @Async. The method body can be empty or have a comment.

Spring Boot

// Create EmailService class with @Service annotation
// Add @Async annotation to sendEmail method
@Service
public class EmailService {
    public void sendEmail(String recipient) {
        // Your code here
    }
}

Hint

Use @Service on the class and @Async on the method sendEmail.

4

Call the asynchronous method from the application

In the AsyncApplication class, add a @Bean method that returns a CommandLineRunner. Inside the runner, inject EmailService and call sendEmail("[e-mail]") to test asynchronous execution.

Spring Boot

// Add a @Bean method returning CommandLineRunner
// Inject EmailService and call sendEmail("[e-mail]") inside run method
@Bean
public CommandLineRunner runner(EmailService emailService) {
    return args -> {
        // Your code here
    };
}

Hint

Create a @Bean method that returns a CommandLineRunner. Inside its run method, call sendEmail on the injected EmailService.

Practice

1. What is the main purpose of the @EnableAsync annotation in a Spring Boot application?

easy

A. To enable automatic database transactions

B. To enable REST API endpoint scanning

C. To enable running methods asynchronously in the background

D. To enable security features like authentication

Solution

1. Step 1: Understand what @EnableAsync does

   This annotation activates Spring's ability to run methods asynchronously, meaning in the background without blocking the main thread.

2. Step 2: Compare with other options

   Other options relate to transactions, REST scanning, or security, which are unrelated to @EnableAsync.

3. Final Answer:

   To enable running methods asynchronously in the background -> Option C

4. Quick Check:

   @EnableAsync = run methods asynchronously [OK]

Hint: Remember: Enable async means run tasks in background [OK]

Common Mistakes:

• Confusing @EnableAsync with transaction management
• Thinking it enables REST endpoints
• Assuming it controls security features

2. Which of the following is the correct way to enable asynchronous method execution in a Spring Boot application?

easy

A. Add @Async on configuration class and @EnableAsync on methods

B. Add @Async on the main application class only

C. Add @EnableAsync on every method you want async

D. Add @EnableAsync on a configuration class and @Async on methods

Solution

1. Step 1: Identify correct annotation placement

   @EnableAsync must be on a configuration or main class to activate async support globally.

2. Step 2: Mark methods with @Async

   Methods that should run asynchronously need the @Async annotation.

3. Final Answer:

   Add @EnableAsync on a configuration class and @Async on methods -> Option D

4. Quick Check:

   Enable async globally + mark methods async = Add @EnableAsync on a configuration class and @Async on methods [OK]

Hint: Enable async on config class, async on methods [OK]

Common Mistakes:

• Putting @Async on the main class only
• Using @EnableAsync on methods instead of config
• Mixing up @Async and @EnableAsync roles

3. Given the code below, what will be the output when mainMethod() is called?

@EnableAsync
@Configuration
public class AppConfig {}

@Service
public class MyService {
  @Async
  public void asyncMethod() {
    System.out.println("Async task started");
  }

  public void mainMethod() {
    System.out.println("Main method start");
    asyncMethod();
    System.out.println("Main method end");
  }
}

medium

A. Main method start
Main method end
Async task started

B. Main method start
Async task started
Main method end

C. Async task started
Main method start
Main method end

D. Main method start
Main method end

Solution

1. Step 1: Understand async method behavior

   The @Async method runs asynchronously only when called from a different Spring-managed bean proxy. Calling asyncMethod() directly within the same class does NOT run asynchronously, so it executes synchronously.

2. Step 2: Trace output order

   All print statements execute in order: "Main method start", "Async task started", then "Main method end".

3. Final Answer:

   Main method start Async task started Main method end -> Option B

4. Quick Check:

   Direct self-invocation bypasses async proxy, so method runs synchronously [OK]

Hint: Async methods called within same class run synchronously [OK]

Common Mistakes:

• Assuming async prints immediately in order
• Thinking async blocks main thread
• Ignoring that async runs in background thread only when called via proxy

4. Consider this Spring Boot setup:

@Configuration
public class AsyncConfig {
  @Async
  public void runAsync() {
    System.out.println("Running async");
  }
}

What is the main problem with this code?

medium

A. Missing @EnableAsync annotation to activate async support

B. Methods annotated with @Async must return a value

C. @Async cannot be used inside @Configuration classes

D. The method must be static to run asynchronously

Solution

1. Step 1: Check if async support is enabled

   Without @EnableAsync on a configuration or main class, Spring will not run @Async methods asynchronously.

2. Step 2: Validate other options

   @Async methods can be void, can be in any Spring bean, and do not need to be static.

3. Final Answer:

   Missing @EnableAsync annotation to activate async support -> Option A

4. Quick Check:

   Enable async missing = async won't run [OK]

Hint: Always add @EnableAsync to activate async methods [OK]

Common Mistakes:

• Thinking @Async needs return value
• Believing @Async can't be in config classes
• Assuming method must be static

5. You want to run multiple slow tasks in parallel in a Spring Boot app. Which setup correctly uses @EnableAsync and @Async to achieve this?

@Configuration
@EnableAsync
public class AsyncConfig {}

@Service
public class TaskService {
  @Async
  public CompletableFuture<String> slowTask(String name) {
    Thread.sleep(2000);
    return CompletableFuture.completedFuture(name + " done");
  }
}

// In some controller or service
public void runTasks() {
  CompletableFuture<String> t1 = taskService.slowTask("Task1");
  CompletableFuture<String> t2 = taskService.slowTask("Task2");
  CompletableFuture.allOf(t1, t2).join();
  System.out.println("All tasks completed");
}

hard

A. This setup runs slowTask methods in parallel without blocking main thread

B. This setup runs slowTask methods sequentially blocking main thread

C. Missing @Async on slowTask method, so runs sequentially

D. Missing @EnableAsync, so async calls run synchronously

Solution

1. Step 1: Check annotations and method return type

   @EnableAsync is present to enable async support, and @Async marks slowTask to run asynchronously returning CompletableFuture.

2. Step 2: Understand method calls and joining

   Calling slowTask twice starts both tasks in parallel threads. CompletableFuture.allOf(...).join() waits for both to finish without blocking main thread execution during tasks.

3. Final Answer:

   This setup runs slowTask methods in parallel without blocking main thread -> Option A

4. Quick Check:

   EnableAsync + Async + CompletableFuture = parallel tasks [OK]

Hint: Use @EnableAsync + @Async + CompletableFuture for parallel tasks [OK]

Common Mistakes:

• Forgetting @EnableAsync disables async behavior
• Not returning CompletableFuture causes sync execution
• Calling async methods without waiting causes premature exit