## Why is this an issue?

The Spring framework provides the annotation `Async` to mark a method (or all methods of a type) as a candidate for asynchronous execution.

Asynchronous methods do not necessarily, by their nature, return the result of their calculation immediately. Hence, it is unexpected and in clear breach of the `Async` contract for such methods to have a return type that is neither `void` nor a `Future` type.

## How to fix it

Use `void` as the return type if the method is not expected to return a result. Otherwise, a `Future` should be returned, allowing the caller to retrieve the result once it is ready. It is permitted to return more specific subtypes that inherit from `Future`.

### Code examples

#### Noncompliant code example

    @Async
    public String asyncMethod() {
      ...
    }

#### Compliant solution

    @Async
    public Future<String> asyncMethod() {
      ...
    }

Alternatively, if the method does not need to return a result:

    @Async
    public void asyncMethod() {
      ...
    }

## Resources

### Documentation

- Spring Framework Documentation - Annotation Interface Async