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

Question

我开始在 Spring Boot 中使用 CompletableFuture，我在某些地方看到通常的存储库方法 return CompletableFuture <Entity> 而不是 Entity.

我不知道发生了什么，但是当我在存储库中 return 个 CompletableFuture 实例时，代码运行完美无缺。但是，当我 return 实体时，代码不会异步工作并且总是 returns 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())
        );
    }
}

前面的代码运行很完美，但下面的代码不是运行异步的，结果总是 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

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.

在您的第二个示例中，您的 @Async 注释方法不 return 和 Future（或 ListenableFuture 和 CompletableFuture 也受支持） .但是，Spring 必须异步地运行您的方法。因此它只能表现得好像你的方法有一个 void return 类型，因此它 returns null.

附带说明一下，当您使用 @Async 时，您的方法已经运行异步，因此您不应在方法内部使用 CompletableFuture.supplyAsync()。你应该简单地计算你的结果和 return 它，包裹在 CompletableFuture.completedFuture() if necessary. If your method is only composing futures (like your service that simply composes asynchronous repository results), then you probably don't need the @Async annotation. See also the example from the Getting Started guide.

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

How to use CompletableFuture.thenCompose() when returning entities from repositories?

java

return

completable-future

spring-async