com.google.api.gax.rpc.FirstElementCallable Maven / Gradle / Ivy

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 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
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package com.google.api.gax.rpc;

import com.google.api.core.ApiFuture;

/**
 * Wraps a {@link ServerStreamingCallable} in a {@link UnaryCallable} that returns the first
 * element. After returning the first element, the underlying stream will be gracefully cancelled.
 *
 * Package-private for internal use.
 *
 * @param  The type of the request.
 * @param  The type of the item in the stream.
 */
final class FirstElementCallable extends UnaryCallable {
  private final ServerStreamingCallable streamingCallable;

  FirstElementCallable(ServerStreamingCallable streamingCallable) {
    this.streamingCallable = streamingCallable;
  }

  /**
   * Starts the RPC and returns a future wrapping the result. If the stream is empty, the result
   * will be null. If a request is cancelled, the future will be rejected with a {@link
   * java.util.concurrent.CancellationException}.
   *
   * @param request The request.
   * @param context {@link ApiCallContext} to make the call with
   * @return A {@link ApiFuture} wrapping a possible first element of the stream.
   */
  @Override
  public ApiFuture futureCall(RequestT request, ApiCallContext context) {
    FirstElementResponseObserver observer = new FirstElementResponseObserver<>();

    streamingCallable.call(request, observer, context);
    // NOTE: Since onStart must be called synchronously on this thread, the observer is now fully
    // initialized and the future can be safely returned to the caller.
    return observer.getFuture();
  }
}