需要简单的纯 Java 11 WebSocket 客户端示例的帮助

Question

网络上似乎很少有 Java 11（纯 Java 非基于框架的）WebSocket 客户端代码示例，所以我希望 StackOverflow 可以再次为我提供帮助。

This 是我找到的最接近的，但不幸的是，在我的（新手）眼中，它似乎不是展示如何使用来自 WebSocket 侦听器的数据的完整解决方案。

查看WebSocket.Listener implementation, the onText 回调方法，我认为可以提供我需要的东西，但我正在努力弄清楚如何从套接字返回 CompletionStage 对象和某种字符串数据。

这是我目前的一些测试代码。

不胜感激。谢谢

    public class Main {

        public static void main(String[] args) {

           WebSocketClient wsc = new WebSocketClient();
           wsc.startSocket("ws://demos.kaazing.com/echo");

           int i = 0;   

           // Bad, very bad
           do {} while (i == 0);
        }
    }


    public class WebSocketClient implements WebSocket.Listener {

        @Override
        public void onOpen(WebSocket webSocket) {
           //...
            System.out.println("Go...Open".concat(
                    webSocket.getSubprotocol()));
        }

        @Override
        public CompletionStage<?> onText(WebSocket webSocket, CharSequence data, boolean last) {
           //...
            System.out.println(data.toString());

            // How do I return the CompletionStage object
            // return CompletionStage<String>
        }

        @Override
        public void onError(WebSocket webSocket, Throwable error) {
           //..
            System.out.println("Bad day! ".concat(webSocket.toString()));
        }

        void startSocket(String connection) {
            CompletableFuture<WebSocket> server_cf = HttpClient.
                    newHttpClient().
                    newWebSocketBuilder().
                    buildAsync(URI.create(connection),
                            new WebSocketClient());
            WebSocket server = server_cf.join();
            server.sendText("Hello!", true);
        }
    }

Answer 1

管理返回 CompletionStage 的 WebSocket 响应的模式是：

@Override
public CompletionStage<?> onText(WebSocket webSocket, CharSequence data, boolean last) {
    // return inmmediately but response is geenrated lazyly.
    return CompletableFuture.supplyAsync(() -> {
        String response = "Received  ...";
        // do slow task. Access to database or access to a server.
        return response;
    });
}

仅在快速生成响应时才建议使用这种简单的实现方式。

@Override
public CompletionStage<?> onText(WebSocket webSocket, CharSequence data, boolean last) {
    // fast response.
    String response = "The text has " + data.length() + " chars";
    return CompletableFuture.completedFuture(response);
}

Answer 2

您可以在下面找到一个工作示例。我对上面的代码做了一些修改：

• onOpen 需要在 websocket 上调用 request(1)（调用默认实现）才能接收进一步的调用。
• 将方法startSocket移动到主方法中
• 用倒计时锁代替忙等待
• 将类WebSocketClient 声明为（静态）内部类

但除了这些（外观）更改之外，程序会遵循您的想法，即首先构建一个 websocket 连接，然后在成功构建后将文本 Hello! 发送到 echo 服务器。这也可以直接在方法onOpen 中完成。

    import java.net.URI;
    import java.net.http.HttpClient;
    import java.net.http.WebSocket;
    import java.util.concurrent.CompletionStage;
    import java.util.concurrent.CountDownLatch;

    public class Main {

        public static void main(String[] args) throws Exception {
            CountDownLatch latch = new CountDownLatch(1);

            WebSocket ws = HttpClient
                    .newHttpClient()
                    .newWebSocketBuilder()
                    .buildAsync(URI.create("ws://demos.kaazing.com/echo"), new WebSocketClient(latch))
                    .join();
            ws.sendText("Hello!", true);
            latch.await();
        }

        private static class WebSocketClient implements WebSocket.Listener {
            private final CountDownLatch latch;

            public WebSocketClient(CountDownLatch latch) { this.latch = latch; }

            @Override
            public void onOpen(WebSocket webSocket) {
                System.out.println("onOpen using subprotocol " + webSocket.getSubprotocol());
                WebSocket.Listener.super.onOpen(webSocket);
            }

            @Override
            public CompletionStage<?> onText(WebSocket webSocket, CharSequence data, boolean last) {
                System.out.println("onText received " + data);
                latch.countDown();
                return WebSocket.Listener.super.onText(webSocket, data, last);
            }

            @Override
            public void onError(WebSocket webSocket, Throwable error) {
                System.out.println("Bad day! " + webSocket.toString());
                WebSocket.Listener.super.onError(webSocket, error);
            }
        }
    }

顺便说一句，没有协商 supprotocol，因此方法 webSocket.getSubprotocol() 返回一个空字符串。控制台中的输出是

    onOpen using subprotocol 
    onText received Hello!