@Async annotation

About

The @Async annotation in Spring allows to run methods asynchronously in a separate thread, rather than executing them in the main thread. It’s commonly used to improve application performance by offloading long-running tasks or non-blocking operations.

What is Asynchronous Programming?

Asynchronous programming allows to execute tasks in the background without blocking the main thread. This can significantly improve the responsiveness of an application, especially in web applications where blocking the main thread can delay response times.

Some Points to Note

Calling @Async Methods within the Same Class

If we call an @Async method from within the same class, it won’t work as expected because the AOP proxy mechanism Spring uses will not intercept the call. A workaround is to call the async method from a different bean or through self-injection.

Handling Return Values Properly

Since @Async methods return immediately, we should be careful when expecting a return value. Ensure that the calling code handles Future or CompletableFuture appropriately, especially for exception handling and timeout scenarios.

5.3. Overloading the Thread Pool

Ensure that the thread pool configuration matches the application's load. An improperly configured thread pool can either lead to resource exhaustion (too many threads) or poor performance (too few threads).

How to use it?

1. Basic Setup

To enable asynchronous processing, we need to configure our Spring application.

1. Enable Asynchronous Processing:

@Configuration
@EnableAsync
public class AsyncConfig {
}

1. Use @Async in Methods:

@Service
public class MyAsyncService {

    @Async
    public void asyncMethod() {
        System.out.println("Execute method asynchronously - "
            + Thread.currentThread().getName());
    }
}

1. Calling Asynchronous Methods:

When we call asyncMethod(), it will run in a different thread than the caller thread.

2. Return Type of Asynchronous Methods

• Void Return Type: If we don't need to return anything, the method can simply have a void return type.

• Future Return Type: If we need to return a result, can use java.util.concurrent.Future.

  @Async
  public Future<String> asyncMethodWithReturnType() {
      // Simulate a long-running task
      return new AsyncResult<>("Hello from Async!");
  }

• CompletableFuture: As of Java 8, CompletableFuture can be used as a more flexible alternative to Future.

  @Async
  public CompletableFuture<String> asyncMethodWithCompletableFuture() {
      // Simulate a long-running task
      return CompletableFuture.completedFuture("Hello from Async!");
  }

3. Using Custom Thread Pools

By default, Spring uses a SimpleAsyncTaskExecutor, but in production, we may want to customize the thread pool to control the number of threads, queue size, etc.

1. Define a Custom Executor:

   @Configuration
   @EnableAsync
   public class AsyncConfig {
   
       @Bean(name = "taskExecutor")
       public Executor taskExecutor() {
           ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
           executor.setCorePoolSize(2);
           executor.setMaxPoolSize(5);
           executor.setQueueCapacity(500);
           executor.setThreadNamePrefix("Async-");
           executor.initialize();
           return executor;
       }
   }

2. Specify the Executor: We can specify which executor to use for a particular method.

   @Async("taskExecutor")
   public void asyncMethodWithCustomExecutor() {
       // method implementation
   }

4. Exception Handling in Async Methods

Unhandled exceptions in asynchronous methods are not propagated to the calling thread. We need to handle exceptions within the method or provide a custom AsyncUncaughtExceptionHandler.

1. Custom Async Exception Handler:

   @Configuration
   @EnableAsync
   public class AsyncConfig implements AsyncConfigurer {
   
       @Override
       public Executor getAsyncExecutor() {
           return new ThreadPoolTaskExecutor();
       }
   
       @Override
       public AsyncUncaughtExceptionHandler getAsyncUncaughtExceptionHandler() {
           return new CustomAsyncExceptionHandler();
       }
   }
   
   public class CustomAsyncExceptionHandler implements AsyncUncaughtExceptionHandler {
   
       @Override
       public void handleUncaughtException(Throwable ex, Method method, Object... params) {
           System.out.println("Exception message - " + ex.getMessage());
           System.out.println("Method name - " + method.getName());
           for (Object param : params) {
               System.out.println("Parameter value - " + param);
           }
       }
   }

5. Chaining Asynchronous Calls

Using CompletableFuture, we can chain asynchronous calls, making it easier to compose complex asynchronous workflows.

@Async
public CompletableFuture<String> findUser() {
    // Simulate a long-running task
    return CompletableFuture.supplyAsync(() -> "User1");
}

@Async
public CompletableFuture<String> findUserOrders(String user) {
    // Simulate a long-running task
    return CompletableFuture.supplyAsync(() -> "Order1, Order2");
}

@Async
public CompletableFuture<String> findUserOrderDetails(String order) {
    // Simulate a long-running task
    return CompletableFuture.supplyAsync(() -> "OrderDetails");
}

public void getUserOrderDetails() {
    findUser()
        .thenCompose(this::findUserOrders)
        .thenCompose(this::findUserOrderDetails)
        .thenAccept(System.out::println);
}

6. Integration with Spring Events

We can combine @Async with Spring’s event-driven programming model. By using @Async on event listeners, we can handle events asynchronously.

@Component
public class MyEventListener {

    @Async
    @EventListener
    public void handleEvent(MyCustomEvent event) {
        // handle event asynchronously
    }
}

When to Use @Async ?

1. Simple Asynchronous Execution:

• Background Tasks: Use @Async when you want to offload simple, non-blocking tasks to a background thread. For example, sending an email, logging, or processing a file in the background without blocking the main thread.

• Fire-and-Forget: If the method doesn't need to return a result or you don't care about the outcome (like sending notifications or logging), @Async is a good fit.

2. Declarative Asynchronous Programming:

• Spring Integration: If you are already using Spring and want a simple declarative approach to make a method asynchronous, @Async is an easy-to-use annotation that fits naturally within the Spring ecosystem.

3. Event-Driven Asynchrony:

• Handling Events Asynchronously: When combined with Spring's event mechanism, @Async can make event listeners execute asynchronously, which is useful in systems where you need to decouple event processing from the main application flow.

4. Simple Configurations:

• Thread Pool Management: If you need basic thread pool management but don't want to deal with the complexity of manually managing threads, @Async with a custom executor is a straightforward solution.

5. Exception Handling:

• Custom Async Exception Handling: @Async allows you to define custom AsyncUncaughtExceptionHandler to handle uncaught exceptions in async methods, making it easier to handle exceptions without disrupting the main application flow.

When to Use CompletableFuture ?

1. Complex Asynchronous Workflows:

• Chaining Asynchronous Tasks: If your application requires a sequence of asynchronous tasks where each task depends on the result of the previous one, CompletableFuture is ideal. It allows you to chain tasks together using methods like thenApply, thenCompose, and thenAccept.

• Combining Results from Multiple Tasks: If you need to run multiple asynchronous tasks in parallel and then combine their results, CompletableFuture provides methods like allOf, anyOf, and join to handle such scenarios.

2. Fine-Grained Control:

• Custom Execution Logic: CompletableFuture provides more control over how tasks are executed. You can specify different executors for different tasks, handle exceptions at each step, and control how results are combined.

• Handling Timeouts and Delays: With CompletableFuture, you can handle timeouts, delays, and retries more flexibly. For instance, you can specify a timeout for a task or retry a task if it fails.

3. Non-Blocking Async Calls:

• Reactive and Non-Blocking Systems: In reactive or non-blocking architectures, CompletableFuture is often used to handle async calls without blocking the main thread. This is especially useful in highly concurrent systems where blocking threads can lead to performance bottlenecks.

4. Combining with Other Java 8+ Features:

• Streams and Parallelism: CompletableFuture integrates well with other Java 8+ features like Streams and parallel processing, making it easier to build complex data processing pipelines that operate asynchronously.

5. Better Control Over Exceptions:

• Exception Handling: CompletableFuture provides mechanisms like exceptionally, handle, and whenComplete to manage exceptions at various stages of the asynchronous workflow, offering more granular control compared to @Async.

Impact on Thread Context When Main Thread Exits

• Thread Context:

  • The thread context includes things like security context, transaction context, and any thread-local variables.

  • When we return immediately and the main thread handling the HTTP request exits, the thread context associated with that main thread is no longer available.

• Impact with @Async:

  • Spring will execute the @Async method in a different thread. The new thread will not have the same context as the original main thread unless we explicitly propagate the context (e.g., using DelegatingSecurityContextAsyncTaskExecutor for security context).

  • If our asynchronous task relies on the main thread's context (e.g., security context or transaction management), we need to ensure that this context is either propagated or re-established in the new thread.

• Impact with CompletableFuture:

  • Similar to @Async, when using CompletableFuture, the task is executed in a different thread (unless we explicitly provide an executor with the same context).

  • Any context tied to the original request thread will not be present in the thread executing the CompletableFuture. This means if our task relies on the original thread's context, we may need to manually pass the necessary information or use context-aware executors.

Example - User Registration and Notification Service

Imagine we are building a user registration system for an e-commerce platform. When a user registers, the system performs the following actions:

1. Save the User Details in the database.

2. Send a Welcome Email to the user.

3. Notify Admins of the new user registration.

4. Generate a Welcome Gift Voucher for the new user.

Some of these tasks, like sending an email or notifying admins, can be done asynchronously to improve the overall performance of the application.

package com.example.config;

import org.springframework.context.annotation.Configuration;
import org.springframework.scheduling.annotation.EnableAsync;

@Configuration
@EnableAsync
public class AsyncConfig {
    // We'll add a custom thread pool later
}

package com.example.service;

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

import java.util.concurrent.CompletableFuture;

@Service
public class VoucherService {

    @Async
    public CompletableFuture<String> generateVoucher(String username) {
        // Simulate voucher generation
        return CompletableFuture.completedFuture("Voucher1234");
    }

    @Async
    public CompletableFuture<Void> sendVoucher(String username, String voucherCode) {
        // Simulate sending voucher
        System.out.println("Sent voucher " + voucherCode + " to " + username);
        return CompletableFuture.completedFuture(null);
    }

    @Async
    public void processVoucher(String username) {
        generateVoucher(username)
            .thenCompose(voucherCode -> sendVoucher(username, voucherCode))
            .join();  // Wait for the entire process to complete
    }
}