使用 completableFuture 编写来自 doPOST 的 doGET 响应

Question

我有 2 个 API（doGET 和 doPOST），我正在尝试使用异步机制让 doPOST 为 doGET 请求写入 httpServletResponse。

我的控制流程 -

1. 客户端发出 requestA (getData) 调用
2. Java 服务器进行一些处理并调用外部环境 3rd 方 API
3. 第 3 方 API 不返回响应但调用我的另一个端点 doPOST
4. doPOST 现在需要将 httpServletResponse 的对象写入 doGET
5. doGET 在 doPOST 完成后立即返回此对象。

为了解决这个问题，我发现我可以在java中使用一些异步编程机制，比如CompletableFuture。但是我对如何在我的代码中准确设置这个机制感到困惑。这是我到目前为止所做的 -

doGET

public void doGET(HttpServletRequest request, HttpServletResponse response) {
    // some processing
    // Call 3rd Party API
    CompletableFuture<HttpServletRequest> completableFuture = CompletableFuture.supplyAsync(() -> doPOST());
    while (!completableFuture.isDone()) {
        System.out.println("CompletableFuture is not finished yet...");
    }
    HttpServletRequest result = completableFuture.get();
    
    response = result;
}

我无法弄清楚如何为此设置 completableFuture。在这里需要帮助。

doPOST

public HttpServletResponse doPOST(HttpServletRequest request, HttpServletResponse response) {
    // receive 3rd party request 
    // add data from 3rd party request into a new response object 
    // add response object into hashmap
}

我怎样才能正确地完成这项工作？

Answer 1

您可能有一个请求 id 与一个对象的映射作为您的类的属性，公开这两种方法：

private final Map<String, HttpRequestResponse> requests = new HashMap<>();

...其中HttpRequestResponse 类是请求（您在doGET 上收到）和响应（将由doPOST) 提供）的简单包装器：

class HttpRequestResponse {
    private final HttpServletRequest request;
    private final CompletableFuture<HttpServletResponse> responseSupplier;

    public HttpRequestResponse(HttpServletRequest request, CompletableFuture<HttpServletResponse> responseSupplier) {
        this.request = request;
        this.responseSupplier = responseSupplier;
    }

    public void supplyResponse(HttpServletResponse response) {
        this.responseSupplier.complete(response); //<-- this will release the .get()
    }

    //getters
    public CompletableFuture<HttpServletResponse> getSupplier() {
        return responseSupplier;
    }
}

在doGET收到请求后，您将创建实例并将其放入地图中，然后等待结果：

public void doGET(HttpServletRequest request, HttpServletResponse response) {
    HttpRequestResponse responseSupplier = new HttpRequestResponse(request, new CompletableFuture<>());
    requests.put(yourId, responseSupplier); //add supplier to the map (so that doPOST can retrieve it later)
    //perform request to your 3rd party API
    response = responseSupplier.getSupplier().get(); //<- wait until someone completes the future
}

另一方面，在收到来自doPOST 的第 3 方 API 的响应后，您需要通过其 id 获取未来，将其从地图中移除并完成：

public void doPOST(HttpServletRequest request, HttpServletResponse response) {
    HttpRequestResponse responseSupplier = requests.remove(yourId); //<-- removes the supplier from the map and returns it to you
    responseSupplier.getSupplier().complete(<your response>); //<-- once you complete the future with a result, the .get() which is hanging on doGET will return
}

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问题：如果doGET 在继续之前一直等待响应准备好，为什么要使用异步模式？ 我想一旦你把它放在适当的位置，doGET 也可以变成异步的并返回执行 id（然后客户端可以监听结果）。