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Spring Messaging
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/*
 * Copyright 2002-2014 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.messaging;

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.UUID;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import org.springframework.util.AlternativeJdkIdGenerator;
import org.springframework.util.IdGenerator;

/**
 * The headers for a {@link Message}.
 *
 * IMPORTANT: This class is immutable. Any mutating operation such as
 * {@code put(..)}, {@code putAll(..)} and others will throw
 * {@link UnsupportedOperationException}.
 * 
Subclasses do have access to the raw headers, however, via {@link #getRawHeaders()}.
 *
 * 
One way to create message headers is to use the
 * {@link org.springframework.messaging.support.MessageBuilder MessageBuilder}:
 * 
 * MessageBuilder.withPayload("foo").setHeader("key1", "value1").setHeader("key2", "value2");
 * 
 *
 * A second option is to create {@link org.springframework.messaging.support.GenericMessage}
 * passing a payload as {@link Object} and headers as a {@link Map java.util.Map}:
 *  * Map headers = new HashMap();
 * headers.put("key1", "value1");
 * headers.put("key2", "value2");
 * new GenericMessage("foo", headers);
 * 
 *
 * A third option is to use {@link org.springframework.messaging.support.MessageHeaderAccessor}
 * or one of its subclasses to create specific categories of headers.
 *
 * @author Arjen Poutsma
 * @author Mark Fisher
 * @author Gary Russell
 * @since 4.0
 * @see org.springframework.messaging.support.MessageBuilder
 * @see org.springframework.messaging.support.MessageHeaderAccessor
 */
public final class MessageHeaders implements Map, Serializable {

	private static final long serialVersionUID = -4615750558355702881L;

	private static final Log logger = LogFactory.getLog(MessageHeaders.class);

	private static volatile IdGenerator idGenerator = null;

	private static final IdGenerator defaultIdGenerator = new AlternativeJdkIdGenerator();

	/**
	 * The key for the Message ID. This is an automatically generated UUID and
	 * should never be explicitly set in the header map except in the
	 * case of Message deserialization where the serialized Message's generated
	 * UUID is being restored.
	 */
	public static final String ID = "id";

	public static final String TIMESTAMP = "timestamp";

	public static final String REPLY_CHANNEL = "replyChannel";

	public static final String ERROR_CHANNEL = "errorChannel";

	public static final String CONTENT_TYPE = "contentType";


	private final Map headers;


	public MessageHeaders(Map headers) {
		this.headers = (headers != null) ? new HashMap(headers) : new HashMap();
		this.headers.put(ID, (idGenerator != null ? idGenerator : defaultIdGenerator).generateId());
		this.headers.put(TIMESTAMP, System.currentTimeMillis());
	}


	public UUID getId() {
		return get(ID, UUID.class);
	}

	public Long getTimestamp() {
		return get(TIMESTAMP, Long.class);
	}

	public Object getReplyChannel() {
		return get(REPLY_CHANNEL);
	}

	public Object getErrorChannel() {
		return get(ERROR_CHANNEL);
	}

	@SuppressWarnings("unchecked")
	public  T get(Object key, Class type) {
		Object value = this.headers.get(key);
		if (value == null) {
			return null;
		}
		if (!type.isAssignableFrom(value.getClass())) {
			throw new IllegalArgumentException("Incorrect type specified for header '" +
					key + "'. Expected [" + type + "] but actual type is [" + value.getClass() + "]");
		}
		return (T) value;
	}


	@Override
	public boolean equals(Object other) {
		return (this == other ||
				(other instanceof MessageHeaders && this.headers.equals(((MessageHeaders) other).headers)));
	}

	@Override
	public int hashCode() {
		return this.headers.hashCode();
	}

	@Override
	public String toString() {
		Map map = new LinkedHashMap(this.headers);
		map.put(ID,  map.remove(ID)); // remove and add again at the end
		map.put(TIMESTAMP, map.remove(TIMESTAMP));
		return map.toString();
	}


	// Delegating Map implementation

	public boolean containsKey(Object key) {
		return this.headers.containsKey(key);
	}

	public boolean containsValue(Object value) {
		return this.headers.containsValue(value);
	}

	public Set> entrySet() {
		return Collections.unmodifiableSet(this.headers.entrySet());
	}

	public Object get(Object key) {
		return this.headers.get(key);
	}

	public boolean isEmpty() {
		return this.headers.isEmpty();
	}

	public Set keySet() {
		return Collections.unmodifiableSet(this.headers.keySet());
	}

	public int size() {
		return this.headers.size();
	}

	public Collection