Java - 使用CompletableFuture链中断执行序列

Question

I am new to Java and am using CompletableFutures to perform async operations such as below:

public CompletionStage<Either<ErrorResponse, Response>> insertOrUpdate(String actor, String key) {
    return this.objectDAO.getByKey(key)
            .thenApply(mapDOToContainer(key))
            .thenApply(mergeContainerToDO(key, actor))
            .thenComposeAsync(this.objectDAO.UpdateFn())
            .thenApply(DBResult::finished)
            .thenApply(finished -> {
                if (finished) {
                    Response response = Response.ok().build();
                    return Either.right(response);
                } else {
                    return Either.left(ErrorResponse.create("Error", 400));
                }
            });
}

Now I need to modify this so that if the get fails then I perform the above chain, but if it succeeds then I need to break this chain and return from the function with an Either object containing an ErrorResponse.

How can I break this processing chain? I know I can pass a flag to each function in the chain and achieve this by performing the actions in the functions based on the value of the flag. I was hoping there is a better way to do this.

Thanks!!

Answer 1

I would rewrite your code.

• Don't use Either for errors, Java has exception
• Don't return a CompletionStage from your DAO
• Use exceptionally from CompletableFuture, it is designed for this

Then do this:

public CompletionStage<Response> insertOrUpdate(String actor, String key) {
    return CompletableFuture.supplyAsync(() -> this.objectDAO.getByKey(key))
            .thenApply(mapDOToContainer(key))
            .thenApply(mergeContainerToDO(key, actor))
            .thenComposeAsync(this.objectDAO.UpdateFn())
            .thenApply(DBResult::finished)
            .thenApply(finished -> {
                Response response = Response.ok().build();
                return response;
            })
            .exceptionally(e -> ErrorResponse.create("Error", 400));
}

The DAO should be something like this:

class ObjectDAO {
    public Object getByKey(String key) {
        if (keyNotFound) {
            throw new NoSuchElementException();
        }

        return new Object();
    }
}

You may have to make sure that ErrorResponse is a subclass of Response to make this work.