java - 使用异步拉取持续从 Google PubSub 接收消息

Question

借助这个link ，我成功创建了一个小型 Java 应用程序，可以在一分钟内提取已发布的消息。我的实现看起来像这样。

    public static void eventListener() throws InterruptedException {

    MessageReceiver receiver = new MessageReceiver() {
      @Override
      public void receiveMessage(PubsubMessage message, AckReplyConsumer consumer) {
        System.out.println("Received message: " + message.getData().toStringUtf8());
        consumer.ack();
      }
    };
    //Subscriber subscriber = null;
    try {
      subscriber = Subscriber.newBuilder(subscription, receiver)
          .setCredentialsProvider(FixedCredentialsProvider.create(creds)).build();
      subscriber.addListener(new Subscriber.Listener() {
        @Override
        public void failed(Subscriber.State from, Throwable failure) {
          // Handle failure. This is called when the Subscriber encountered a fatal error
          // and is
          // shutting down.
          System.err.println(failure);
        }
      }, MoreExecutors.directExecutor());
      subscriber.startAsync().awaitRunning();

      // In this example, we will pull messages for one minute (60,000ms) then stop.
      // In a real application, this sleep-then-stop is not necessary.
      // Simply call stopAsync().awaitTerminated() when the server is shutting down,
      // etc.
      Thread.sleep(60000);
    } finally {
      if (subscriber != null) {
        subscriber.stopAsync().awaitTerminated();
      }
    }
  }

当我在main中调用这个方法时

public static void main(String[] args) throws InterruptedException {
  eventListener();

}

并将对象上传到我的 Google 云存储，程序会打印发布者发送的消息，如下所示

Received message: {
  "kind": "storage#object",
  "id": "roshanbucket/stones.jpg/1553765105996166",
  "selfLink": "https://www.googleapis.com/storage/v1/b/roshanbucket/o/stones.jpg",
  "name": "stones.jpg",
  "bucket": "roshanbucket",
  "generation": "1553765105996166",
  "metageneration": "1",
  "contentType": "image/jpeg",
  "timeCreated": "2019-03-28T09:25:05.995Z",
  "updated": "2019-03-28T09:25:05.995Z",
  "storageClass": "STANDARD",
  "timeStorageClassUpdated": "2019-03-28T09:25:05.995Z",
  "size": "137256",
  "md5Hash": "1GmpUnGeiW+/KU+0U8c8Wg==",
  "mediaLink": "https://www.googleapis.com/download/storage/v1/b/roshanbucket/o/stones.jpg?generation=1553765105996166&alt=media",
  "crc32c": "FMaEGg==",
  "etag": "CIaj1InCpOECEAE="
}

程序执行后的一分钟内，它会打印对象上传帐户上收到的所有消息，然后停止。要在一分钟后接收事件消息，我需要重新启动应用程序。现在，我想做的是连续运行监听器，因此，我尝试在 main 方法内的无限循环内运行方法 eventListener() ，如下所示

    public static void main(String[] args) throws InterruptedException {
    while(true) {
      eventListener();
    }
  }

有了这个，我似乎能够在每次上传后立即收到事件消息，无论我何时上传对象。但是，每隔一段时间，它就会抛出这个堆栈跟踪。

    Mar 28, 2019 12:56:34 PM io.grpc.internal.ManagedChannelOrphanWrapper$ManagedChannelReference cleanQueue
SEVERE: *~*~*~ Channel ManagedChannelImpl{logId=6, target=pubsub.googleapis.com:443} was not shutdown properly!!! ~*~*~*
    Make sure to call shutdown()/shutdownNow() and wait until awaitTermination() returns true.
java.lang.RuntimeException: ManagedChannel allocation site
    at io.grpc.internal.ManagedChannelOrphanWrapper$ManagedChannelReference.<init>(ManagedChannelOrphanWrapper.java:103)
    at io.grpc.internal.ManagedChannelOrphanWrapper.<init>(ManagedChannelOrphanWrapper.java:53)
    at io.grpc.internal.ManagedChannelOrphanWrapper.<init>(ManagedChannelOrphanWrapper.java:44)
    at io.grpc.internal.AbstractManagedChannelImplBuilder.build(AbstractManagedChannelImplBuilder.java:440)
    at com.google.api.gax.grpc.InstantiatingGrpcChannelProvider.createSingleChannel(InstantiatingGrpcChannelProvider.java:223)
    at com.google.api.gax.grpc.InstantiatingGrpcChannelProvider.createChannel(InstantiatingGrpcChannelProvider.java:164)
    at com.google.api.gax.grpc.InstantiatingGrpcChannelProvider.getTransportChannel(InstantiatingGrpcChannelProvider.java:156)
    at com.google.api.gax.rpc.ClientContext.create(ClientContext.java:157)
    at com.google.cloud.pubsub.v1.stub.GrpcSubscriberStub.create(GrpcSubscriberStub.java:260)
    at com.google.cloud.pubsub.v1.Subscriber.doStart(Subscriber.java:268)
    at com.google.api.core.AbstractApiService$InnerService.doStart(AbstractApiService.java:148)
    at com.google.common.util.concurrent.AbstractService.startAsync(AbstractService.java:225)
    at com.google.api.core.AbstractApiService.startAsync(AbstractApiService.java:120)
    at com.google.cloud.pubsub.v1.Subscriber.startAsync(Subscriber.java:260)
    at listener.AsynchronousPull.eventListener(AsynchronousPull.java:57)
    at listener.AsynchronousPull.main(AsynchronousPull.java:74)

但是，它仍然在每次上传后打印消息，同时偶尔抛出堆栈跟踪。我对线程没有太多经验，非常感谢您提供解决此问题的帮助。

Answer 1

在紧密循环中调用 eventListener() 并不是您想要在此处执行的操作。这将创建许多新的订阅者实例，这些实例接收消息，每个实例的生存时间为 60 秒。您想要的是让您创建的订阅者的单个实例一直存在，直到您想要将其关闭为止。通常，您可以通过创建订阅者并通过 awaitTermulated() 等待其终止来完成此操作。

上面的代码将被修改为这样:

public static void eventListener() throws InterruptedException {
  MessageReceiver receiver = new MessageReceiver() {
    @Override
    public void receiveMessage(PubsubMessage message, AckReplyConsumer consumer) {
      System.out.println("Received message: " + message.getData().toStringUtf8());
      consumer.ack();
    }
  };
  Subscriber subscriber = null;
  try {
    subscriber = Subscriber.newBuilder(subscription, receiver)
        .setCredentialsProvider(FixedCredentialsProvider.create(creds)).build();
    subscriber.addListener(new Subscriber.Listener() {
      @Override
      public void failed(Subscriber.State from, Throwable failure) {
        // Handle failure. This is called when the Subscriber encountered a fatal error
        // and is
        // shutting down.
        System.err.println(failure);
      }
    }, MoreExecutors.directExecutor());
    subscriber.startAsync().awaitRunning();
    subscriber.awaitTerminated();
  } finally {
    if (subscriber != null) {
      subscriber.stopAsync().awaitTerminated();
    }
  }
}

public static void main(String[] args) throws InterruptedException {
  eventListener();
}

如果您只是希望订阅者在应用程序终止时停止，并且不想进行任何额外的清理，那么上面的代码将起作用，允许订阅者运行并接收消息，直到发生错误或应用程序停止运行。关闭。如果您想在应用程序的干净终止时进行一些清理，例如，您想确保已由 receiveMessage 处理的任何消息运行完成，那么您可以 attach a shutdown hook捕获此类终止(尽管它不会在所有情况下运行)。在此 Hook 中，您将调用 stopAsync()。例如，您可以在 try block 之前插入以下内容:

Runtime.getRuntime().addShutdownHook(new Thread() {
    public void run() {
      subscriber.stopAsync().awaitTerminated();
    }
});