@Async spoils references to @Value variables from final methods of the class

[taqqanori]

When a method is @Async, @Value variable becomes null in any other final methods (and, the @Async method itself if it's final) of the class.

Is this unavoidable due to Java Reflection spec?

Reproduction code here: https://github.com/taqqanori/spring-async-bug.

Reproduction code in short

@Service
public class AsyncService {

	@Value("${spring.application.name}")
	private String value;

	@Async
	public void async() {
		// just declared, never called
	}

	public final void _final() {
		// becomes null
		System.out.println("async final: " + value);
	}

	public void nonFinal() {
		// becomes "demo"
		System.out.println("async non-final: " + value);
	}

}

@Service
public class NonAsyncService {

	@Value("${spring.application.name}")
	private String value;

	public final void _final() {
		// becomes "demo"
		System.out.println("non-async final: " + value);
	}

	public void nonFinal() {
		// becomes "demo"
		System.out.println("non-async non-final: " + value);
	}

}

spring.application.name=demo

Output

async final: null
async non-final: demo
non-async final: demo
non-async non-final: demo

Environment

• OS: Ubuntu-20.04 on WSL2 on Windows11
• Java: openjdk-21
• Spring: Spring Boot 3.4.0 initialized from https://start.spring.io/ with "Spring Web" dependency

Thanks.

[taqqanori]

I found @Retryable has the same problem spring-projects/spring-retry#478

[ZLATAN628]

Hi @taqqanori,

Here’s my understanding: When you use the @async annotation, Spring creates a proxy object for the bean of the target class. Since the proxy is created using CGLIB, it generates a subclass of the target class and overrides the non-final methods to enable interception. As a result, when you invoke a final method, the call is handled directly by the proxy object itself, rather than being delegated to the original (target) object. Additionally, the proxy object does not copy the field values from the original target object.

[taqqanori]

Hi @ZLATAN628

Thank you for the detailed explanation. Very interesting.
If this issue is unavoidable or too hard to fix, I'd like Spring framework to raise an Exception or warn in log, when @Async/@Retryable and @Value and final is used together in a class (or its ascendant and descendant).
I wasted hours wondering why my value is null...

[lucky8987]

@taqqanori Generating proxy objects through cglib will skip the final modified class or method, but the log level is: trace, so you may not be able to see it. The corresponding code location is: org.springframework.context.annotation.ConfigurationClassEnhancer.BeanMethodInterceptor#enhanceFactoryBean

[taqqanori]

@lucky8987
Thank you for investigation.
I wanted to see that log at default log level, so that I could have noticed that final was doing something, before messing around with my code.