1. Overview

In this article, we'll explore the asynchronous execution support in Spring – and the @Async annotation.

Simply put – annotating a method of a bean with @Async will make it execute in a separate thread i.e. the caller will not wait for the completion of the called method.

One interesting aspect in Spring is that the event support in the framework also has support for async processing if you want to go that route.

2. Enable Async Support

Let's start by enabling asynchronous processing with Java configuration – by simply adding the @EnableAsync to a configuration class:

@Configuration
@EnableAsync
public class SpringAsyncConfig { ... }

The enable annotation is enough, but as you'd expect, there are also a few simple options for configuration as well:

• annotation – by default, @EnableAsync detects Spring's @Async annotation and the EJB 3.1 javax.ejb.Asynchronous; this option can be used to detect other, user-defined annotation types as well
• mode – indicates the type of advice that should be used – JDK proxy-based or AspectJ weaving
• proxyTargetClass – indicates the type of proxy that should be used – CGLIB or JDK; this attribute has effect only if the mode is set to AdviceMode.PROXY
• order – sets the order in which AsyncAnnotationBeanPostProcessor should be applied; by default, it runs last, just so that it can take into account all existing proxies

Asynchronous processing can also be enabled using XML configuration – by using the task namespace:

<task:executor id="myexecutor" pool-size="5"  />
<task:annotation-driven executor="myexecutor"/>

3. The @Async Annotation

First – let's go over the rules – @Async has two limitations:

• it must be applied to public methods only
• self-invocation – calling the async method from within the same class – won't work

The reasons are simple – the method needs to be public so that it can be proxied. And self-invocation doesn't work because it bypasses the proxy and calls the underlying method directly.

3.1. Methods with Void Return Type

Following is the simple way to configure a method with void return type to run asynchronously:

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

3.2. Methods With Return Type

@Async can also be applied to a method with return type – by wrapping the actual return in the Future:

@Async
public Future<String> asyncMethodWithReturnType() {
    System.out.println("Execute method asynchronously - " 
      + Thread.currentThread().getName());
    try {
        Thread.sleep(5000);
        return new AsyncResult<String>("hello world !!!!");
    } catch (InterruptedException e) {
        //
    }

    return null;
}

Spring also provides an AsyncResult class which implements Future. This can be used to track the result of asynchronous method execution.

Now, let's invoke the above method and retrieve the result of the asynchronous process using the Future object.

public void testAsyncAnnotationForMethodsWithReturnType()
  throws InterruptedException, ExecutionException {
    System.out.println("Invoking an asynchronous method. " 
      + Thread.currentThread().getName());
    Future<String> future = asyncAnnotationExample.asyncMethodWithReturnType();

    while (true) {
        if (future.isDone()) {
            System.out.println("Result from asynchronous process - " + future.get());
            break;
        }
        System.out.println("Continue doing something else. ");
        Thread.sleep(1000);
    }
}

4. The Executor

By default, Spring uses a SimpleAsyncTaskExecutor to actually run these methods asynchronously. The defaults can be overridden at two levels – at the application level or at the individual method level.

4.1. Override the Executor at the Method Level

The required executor needs to be declared in a configuration class:

@Configuration
@EnableAsync
public class SpringAsyncConfig {
    
    @Bean(name = "threadPoolTaskExecutor")
    public Executor threadPoolTaskExecutor() {
        return new ThreadPoolTaskExecutor();
    }
}

Then the executor name should be provided as an attribute in @Async:

@Async("threadPoolTaskExecutor")
public void asyncMethodWithConfiguredExecutor() {
    System.out.println("Execute method with configured executor - "
      + Thread.currentThread().getName());
}

4.2. Override the Executor at the Application Level

The configuration class should implement the AsyncConfigurer interface – which will mean that it has the implement the getAsyncExecutor() method. It's here that we will return the executor for the entire application – this now becomes the default executor to run methods annotated with @Async:

@Configuration
@EnableAsync
public class SpringAsyncConfig implements AsyncConfigurer {
    
    @Override
    public Executor getAsyncExecutor() {
        return new ThreadPoolTaskExecutor();
    }
    
}

5. Exception Handling

When a method return type is a Future, exception handling is easy – Future.get() method will throw the exception.

But, if the return type is void, exceptions will not be propagated to the calling thread. Hence we need to add extra configurations to handle exceptions.

We'll create a custom async exception handler by implementing AsyncUncaughtExceptionHandler interface. The handleUncaughtException() method is invoked when there are any uncaught asynchronous exceptions:

public class CustomAsyncExceptionHandler
  implements AsyncUncaughtExceptionHandler {

    @Override
    public void handleUncaughtException(
      Throwable throwable, Method method, Object... obj) {
 
        System.out.println("Exception message - " + throwable.getMessage());
        System.out.println("Method name - " + method.getName());
        for (Object param : obj) {
            System.out.println("Parameter value - " + param);
        }
    }
    
}

In the previous section, we looked at the AsyncConfigurer interface implemented by the configuration class. As part of that, we also need to override the getAsyncUncaughtExceptionHandler() method to return our custom asynchronous exception handler:

@Override
public AsyncUncaughtExceptionHandler getAsyncUncaughtExceptionHandler() {
    return new CustomAsyncExceptionHandler();
}

6. Conclusion

In this tutorial, we looked at running asynchronous code with Spring. We started with the very basic configuration and annotation to make it work but also looked at more advanced configs such as providing our own executor, or exception handling strategies.

And, as always, the full code presented in this article is available over on Github.