core/src/main/java/com/google/net/stubby/ClientInterceptors.java

package com.google.net.stubby;

import com.google.common.base.Preconditions;
import com.google.common.collect.ImmutableList;
import com.google.common.util.concurrent.ListenableFuture;

import java.util.Arrays;
import java.util.Iterator;
import java.util.List;

/**
 * Utility class for {@link ClientInterceptor}s
 */
public class ClientInterceptors {

  // Prevent instantiation
  private ClientInterceptors() {}

  /**
   * Create a new {@code Channel} that will call {@code interceptors} before starting an RPC on the
   * given channel.
   */
  public static Channel intercept(Channel channel, ClientInterceptor... interceptors) {
    return intercept(channel, Arrays.asList(interceptors));
  }

  /**
   * Create a new {@code Channel} that will call {@code interceptors} before starting an RPC on the
   * given channel.
   */
  public static Channel intercept(Channel channel, List<ClientInterceptor> interceptors) {
    Preconditions.checkNotNull(channel);
    if (interceptors.isEmpty()) {
      return channel;
    }
    return new InterceptorChannel(channel, interceptors);
  }

  private static class InterceptorChannel implements Channel {
    private final Channel channel;
    private final Iterable<ClientInterceptor> interceptors;

    private InterceptorChannel(Channel channel, List<ClientInterceptor> interceptors) {
      this.channel = channel;
      this.interceptors = ImmutableList.copyOf(interceptors);
    }

    @Override
    public <ReqT, RespT> Call<ReqT, RespT> newCall(MethodDescriptor<ReqT, RespT> method) {
      return new ProcessInterceptorChannel(channel, interceptors).newCall(method);
    }
  }

  private static class ProcessInterceptorChannel implements Channel {
    private final Channel channel;
    private Iterator<ClientInterceptor> interceptors;

    private ProcessInterceptorChannel(Channel channel, Iterable<ClientInterceptor> interceptors) {
      this.channel = channel;
      this.interceptors = interceptors.iterator();
    }

    @Override
    public <ReqT, RespT> Call<ReqT, RespT> newCall(MethodDescriptor<ReqT, RespT> method) {
      if (interceptors != null && interceptors.hasNext()) {
        return interceptors.next().interceptCall(method, this);
      } else {
        Preconditions.checkState(interceptors != null,
            "The channel has already been called. "
            + "Some interceptor must have called on \"next\" twice.");
        interceptors = null;
        return channel.newCall(method);
      }
    }
  }

  /**
   * A {@link Call} which forwards all of it's methods to another {@link Call}.
   */
  public static class ForwardingCall<ReqT, RespT> extends Call<ReqT, RespT> {

    private final Call<ReqT, RespT> delegate;

    public ForwardingCall(Call<ReqT, RespT> delegate) {
      this.delegate = delegate;
    }

    @Override
    public void start(Listener<RespT> responseListener, Metadata.Headers headers) {
      this.delegate.start(responseListener, headers);
    }

    @Override
    public void cancel() {
      this.delegate.cancel();
    }

    @Override
    public void halfClose() {
      this.delegate.halfClose();
    }

    @Override
    public void sendPayload(ReqT payload) {
      this.delegate.sendPayload(payload);
    }
  }

  /**
   * A {@link com.google.net.stubby.Call.Listener} which forwards all of its methods to another
   * {@link com.google.net.stubby.Call.Listener}.
   */
  public static class ForwardingListener<T> extends Call.Listener<T> {

    private final Call.Listener<T> delegate;

    public ForwardingListener(Call.Listener<T> delegate) {
      this.delegate = delegate;
    }

    @Override
    public ListenableFuture<Void> onHeaders(Metadata.Headers headers) {
      return delegate.onHeaders(headers);
    }

    @Override
    public ListenableFuture<Void> onPayload(T payload) {
      return delegate.onPayload(payload);
    }

    @Override
    public void onClose(Status status, Metadata.Trailers trailers) {
      delegate.onClose(status, trailers);
    }
  }
}