java - 从存储库返回实体时如何使用 CompletableFuture.thenCompose() ？

Question

我开始使用CompletableFuture在 Spring Boot 中，我在某些地方看到常用的存储库方法返回 CompletableFuture <Entity>而不是Entity .

我不知道发生了什么，但是当我返回 CompletableFuture 的实例时在存储库中，代码运行完美。但是，当我返回实体时，代码不会异步工作，并且始终返回 null .

这是一个例子:

@Service
public class AsyncServiceImpl{
    /** .. Init repository instances .. **/

    @Async(AsyncConfiguration.TASK_EXECUTOR_SERVICE)
    public CompletableFuture<Token> getTokenByUser(Credential credential) {
        return userRepository.getUser(credential)
            .thenCompose(s -> TokenRepository.getToken(s));
    }
}

@Repository
public class UserRepository {

    @Async(AsyncConfiguration.TASK_EXECUTOR_REPOSITORY)
    public CompletableFuture<User> getUser(Credential credentials) {
        return CompletableFuture.supplyAsync(() -> 
            new User(credentials.getUsername())
        );
    }       
}

@Repository
public class TokenRepository {

    @Async(AsyncConfiguration.TASK_EXECUTOR_REPOSITORY)
    public CompletableFuture<Token> getToken(User user) {
        return CompletableFuture.supplyAsync(() -> 
            new Token(user.getUserId())
        );
    }
}

前面的代码运行完美，但是后面的代码没有异步运行，结果始终是 null .

@Service
public class AsyncServiceImpl {
    /** .. Init repository instances .. **/

    @Async(AsyncConfiguration.TASK_EXECUTOR_SERVICE)
    public CompletableFuture<Token> requestToken(Credential credential) {
        return CompletableFuture.supplyAsync(() -> userRepository.getUser(credential))
            .thenCompose(s -> 
                CompletableFuture.supplyAsync(() -> TokenRepository.getToken(s)));
    }
}

@Repository
public class UserRepository {
    @Async(AsyncConfiguration.TASK_EXECUTOR_REPOSITORY)
    public User getUser(Credential credentials) {
        return new User(credentials.getUsername());
    }       
}

@Repository
public class TokenRepository {
    @Async(AsyncConfiguration.TASK_EXECUTOR_SERVICE)
    public Token getToken(User user) {
        return new Token(user.getUserId());
    }
}

为什么第二个代码不起作用？

Answer 1

根据 the Spring @Async Javadoc :

the return type is constrained to either void or Future

它也是further detailed in the reference documentation :

In the simplest case, the annotation may be applied to a void-returning method.

[…]

Even methods that return a value can be invoked asynchronously. However, such methods are required to have a Future typed return value. This still provides the benefit of asynchronous execution so that the caller can perform other tasks prior to calling get() on that Future.

在第二个示例中，您的 @Async 注释方法不会返回 Future (或 ListenableFuture 和 CompletableFuture code> 也受支持)。但是，Spring 必须异步运行您的方法。因此，它只能表现为您的方法具有 void 返回类型，因此它返回 null。

顺便说一句，当您使用 @Async 时，您的方法已经异步运行，因此您不应在方法内使用 CompletableFuture.supplyAsync() 。您应该简单地计算结果并将其返回，包装在 CompletableFuture.completedFuture() 中如果需要的话。如果您的方法仅组合 future(例如您的服务仅组合异步存储库结果)，那么您可能不需要 @Async 注释。另请参阅the example from the Getting Started guide .