/*
 * Copyright 2015-2016 the original author or 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 org.springframework.integration.channel.interceptor;

import org.springframework.integration.support.MessageDecorator;
import org.springframework.messaging.Message;
import org.springframework.messaging.MessageChannel;
import org.springframework.messaging.MessageHandler;
import org.springframework.messaging.MessageHeaders;
import org.springframework.messaging.support.ChannelInterceptorAdapter;
import org.springframework.messaging.support.ExecutorChannelInterceptor;

/**
 * The {@link ExecutorChannelInterceptor} implementation responsible for
 * the {@link Thread} (any?) state propagation from one message flow's thread to another
 * through the {@link MessageChannel}s involved in the flow.
 * <p>
 * The propagation is done from the {@link #preSend(Message, MessageChannel)}
 * implementation using some internal {@link Message} extension which keeps the message
 * to send and the state to propagate.
 * <p>
 * The propagated state context extraction and population is done from the {@link #postReceive}
 * implementation for the {@link org.springframework.messaging.PollableChannel}s, and from
 * the {@link #beforeHandle} for the
 * {@link org.springframework.integration.channel.AbstractExecutorChannel}s and
 * {@link org.springframework.messaging.support.ExecutorSubscribableChannel}s
 * <p>
 * Important. Any further interceptor, which modifies the message to send
 * (e.g. {@code MessageBuilder.withPayload(...)...build()}), may drop the state to propagate.
 * Such kind of interceptors combination should be revised properly.
 * In most cases the interceptors reordering is enough to overcome the issue.
 *
 * @param <S> the propagated state object type.
 *
 * @author Artem Bilan
 * @author Gary Russell
 * @since 4.2
 */
public abstract class ThreadStatePropagationChannelInterceptor<S>
		extends ChannelInterceptorAdapter implements ExecutorChannelInterceptor {

	@Override
	public final Message<?> preSend(Message<?> message, MessageChannel channel) {
		S threadContext = obtainPropagatingContext(message, channel);
		if (threadContext != null) {
			return new MessageWithThreadState<S>(message, threadContext);
		}
		else {
			return message;
		}
	}

	@Override
	@SuppressWarnings("unchecked")
	public final Message<?> postReceive(Message<?> message, MessageChannel channel) {
		if (message instanceof MessageWithThreadState) {
			MessageWithThreadState<S> messageWithThreadState = (MessageWithThreadState<S>) message;
			Message<?> messageToHandle = messageWithThreadState.message;
			populatePropagatedContext(messageWithThreadState.state, messageToHandle, channel);
			return messageToHandle;
		}
		return message;
	}

	@Override
	public final Message<?> beforeHandle(Message<?> message, MessageChannel channel, MessageHandler handler) {
		return postReceive(message, channel);
	}

	@Override
	public void afterMessageHandled(Message<?> message, MessageChannel channel, MessageHandler handler,
			Exception ex) {
		// No-op
	}

	protected abstract S obtainPropagatingContext(Message<?> message, MessageChannel channel);

	protected abstract void populatePropagatedContext(S state, Message<?> message, MessageChannel channel);


	private static final class MessageWithThreadState<S> implements Message<Object>, MessageDecorator {

		private final Message<Object> message;

		private final S state;

		@SuppressWarnings("unchecked")
		MessageWithThreadState(Message<?> message, S state) {
			this.message = (Message<Object>) message;
			this.state = state;
		}

		@Override
		public Object getPayload() {
			return this.message.getPayload();
		}

		@Override
		public MessageHeaders getHeaders() {
			return this.message.getHeaders();
		}

		@Override
		public Message<?> decorateMessage(Message<?> message) {
			return new MessageWithThreadState<>(message, this.state);
		}

		@Override
		public String toString() {
			return "MessageWithThreadState{" +
					"message=" + this.message +
					", state=" + this.state +
					'}';
		}

	}

}