api/src/main/java/io/grpc/Contexts.java

/*
 * Copyright 2015 The gRPC Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package io.grpc;

import com.google.common.base.Preconditions;
import java.util.concurrent.TimeoutException;

/**
 * Utility methods for working with {@link Context}s in GRPC.
 */
public final class Contexts {

  private Contexts() {
  }

  /**
   * Make the provided {@link Context} {@link Context#current()} for the creation of a listener
   * to a received call and for all events received by that listener.
   *
   * <p>This utility is expected to be used by {@link ServerInterceptor} implementations that need
   * to augment the {@link Context} in which the application does work when receiving events from
   * the client.
   *
   * @param context to make {@link Context#current()}.
   * @param call used to send responses to client.
   * @param headers received from client.
   * @param next handler used to create the listener to be wrapped.
   * @return listener that will receive events in the scope of the provided context.
   */
  public static <ReqT, RespT> ServerCall.Listener<ReqT> interceptCall(
        Context context,
        ServerCall<ReqT, RespT> call,
        Metadata headers,
        ServerCallHandler<ReqT, RespT> next) {
    Context previous = context.attach();
    try {
      return new ContextualizedServerCallListener<>(
          next.startCall(call, headers),
          context);
    } finally {
      context.detach(previous);
    }
  }

  /**
   * Implementation of {@link io.grpc.ForwardingServerCallListener} that attaches a context before
   * dispatching calls to the delegate and detaches them after the call completes.
   */
  private static class ContextualizedServerCallListener<ReqT> extends
      ForwardingServerCallListener.SimpleForwardingServerCallListener<ReqT> {
    private final Context context;

    public ContextualizedServerCallListener(ServerCall.Listener<ReqT> delegate, Context context) {
      super(delegate);
      this.context = context;
    }

    @Override
    public void onMessage(ReqT message) {
      Context previous = context.attach();
      try {
        super.onMessage(message);
      } finally {
        context.detach(previous);
      }
    }

    @Override
    public void onHalfClose() {
      Context previous = context.attach();
      try {
        super.onHalfClose();
      } finally {
        context.detach(previous);
      }
    }

    @Override
    public void onCancel() {
      Context previous = context.attach();
      try {
        super.onCancel();
      } finally {
        context.detach(previous);
      }
    }

    @Override
    public void onComplete() {
      Context previous = context.attach();
      try {
        super.onComplete();
      } finally {
        context.detach(previous);
      }
    }

    @Override
    public void onReady() {
      Context previous = context.attach();
      try {
        super.onReady();
      } finally {
        context.detach(previous);
      }
    }
  }

  /**
   * Returns the {@link Status} of a cancelled context or {@code null} if the context
   * is not cancelled.
   */
  @ExperimentalApi("https://github.com/grpc/grpc-java/issues/1975")
  public static Status statusFromCancelled(Context context) {
    Preconditions.checkNotNull(context, "context must not be null");
    if (!context.isCancelled()) {
      return null;
    }

    Throwable cancellationCause = context.cancellationCause();
    if (cancellationCause == null) {
      return Status.CANCELLED.withDescription("io.grpc.Context was cancelled without error");
    }
    if (cancellationCause instanceof TimeoutException) {
      return Status.DEADLINE_EXCEEDED
          .withDescription(cancellationCause.getMessage())
          .withCause(cancellationCause);
    }
    Status status = Status.fromThrowable(cancellationCause);
    if (Status.Code.UNKNOWN.equals(status.getCode())
        && status.getCause() == cancellationCause) {
      // If fromThrowable could not determine a status, then
      // just return CANCELLED.
      return Status.CANCELLED.withDescription("Context cancelled").withCause(cancellationCause);
    }
    return status.withCause(cancellationCause);
  }
}