Spring #Async #Autowired null in only one method - spring

Question

I am encountering a NullPointerException in my Spring application when calling a method on an autowired object. The class in question looks like the following:
#Component
public class Listener {
#Autowired
TemplateService templateService;
#Async
#EventListener
private Future<String> listener1(Event1 event) {
System.out.println(templateService);
return new AsyncResult<>(null);
}
#Async
#EventListener
public Future<String> listener2(Event2 event) {
System.out.println(templateService);
return new AsyncResult<>(null);
}
}
When I publish an event that triggers listener1, a null value is printed, but when I publish an event that triggers listener2, the toString() method of TemplateService is called (as I would expect). I'm probably misunderstanding some aspect of how #Async affects #Autowired objects, but I haven't been able to determine what that would be. Am I misusing #Async? Am I misunderstanding how to use #Autowired objects in a multithreaded environment?

Answer 1

Change the visibility of your listener1 method to be at least protected (package visibility , protected or public). This is because Spring creates a proxy which is a subclass of your component. It overrides your #Async annotated methods in order to add new logic to execute your code in a separate thread. However because it uses inheritance it can only override methods which are visible to the subclass.
This explains why listener2 method which is public works.
Change your method to
#Async
#EventListener
public Future<String> listener1(Event1 event) {
System.out.println(templateService);
return new AsyncResult<>(null);
}

Answer 2

Spring needs an interface to create a proxy class. It's this proxy class that gets called every time you call the method, and it's through this method that the whole asynchronous execution happens. Without an interface Spring can't autowire, scan or make methods execute asynchronously.
public interface Listener {
public Future<String> listener1(Event1 event);
public Future<String> listener2(Event2 event);
}
#Component
public class ListenerImpl {
#Autowired
private TemplateService templateService;
#Async
#Override
public Future<String> listener1(Event1 event) {
System.out.println(templateService);
return new AsyncResult<>(null);
}
#Async
#Override
public Future<String> listener2(Event2 event) {
System.out.println(templateService);
return new AsyncResult<>(null);
}
}
It's also worth noting that Spring can't run private methods asynchronously.