總結一下幾種ExchangeTypes。

之前寫發布/訂閱模式時第一次提到了exchange type。即PRoducer不是將消息直接放到隊列中，而是先到exchange中，exchange主要用于控制消息到隊列的路由,根據具體的exchange type將消息傳給需要的隊列或者直接廢棄。在這一篇中總結一下那些用到的exchange type。

一.Direct Exchangedirect exchange算是最基本的了。direct exchange用于將帶上routing key的消息傳值擁有相同routing key的隊列中。

當我們想用一個簡單的標識符區別所有傳入同一個exchange中的消息時direct exchange就非常合適。

private static String DIRECT_EXCHANGE = "DIRECT_EXCHAGNE";     static class FanoutProducer {        public static void main(String[] args) throws IOException {            ConnectionFactory connectionFactory = new ConnectionFactory();            Connection connection = connectionFactory.newConnection();            Channel channel= connection.createChannel();;             String content = "I miss the conversation";            channel.exchangeDeclare(DIRECT_EXCHANGE, ExchangeTypes.DIRECT);            channel.basicPublish(DIRECT_EXCHANGE, "alvez", null, content.getBytes());        }    }     static class FanoutConsumer {        public static void main(String[] args) throws IOException, InterruptedException {            ConnectionFactory connectionFactory = new ConnectionFactory();            Connection connection = connectionFactory.newConnection();            Channel channel= connection.createChannel();             String queueName = channel.queueDeclare().getQueue();            channel.queueBind(queueName, DIRECT_EXCHANGE, "alvez");             QueueingConsumer consumer = new QueueingConsumer(channel);            String s = channel.basicConsume(queueName, true, consumer);            System.out.println(s);            while (true) {                QueueingConsumer.Delivery delivery = consumer.nextDelivery();                String message = new String(delivery.getBody());                String routingKey = delivery.getEnvelope().getRoutingKey();                 System.out.println("From:" + routingKey + "':'" + message + "'");            }         }　　}

二.Fanout Exchangefanout和routing key無關，它將消息無差別地(indiscriminately)傳送給所有隊列。fanout exchange通常用于發布/訂閱模式。將消息傳送給不同的隊列，不同的隊列對同一種消息采取不同的行為。比如，現在有一個客戶訂單消息被三個隊列接收，隊列1完成該訂單，隊列2將訂單寫入日志，隊列3將訂單發給別的部門什么的。比如下面的代碼，消費者可以獲得routing key并輸出，但能否獲取與routing key無關：

    private static String FANOUT_EXCHANGE = "FANOUT_EXCHANGE";     static class DirectProducer {        public static void main(String[] args) throws IOException {            ConnectionFactory connectionFactory = new ConnectionFactory();            Connection connection = connectionFactory.newConnection();            Channel channel= connection.createChannel();;             String content = "I miss the conversation";            channel.exchangeDeclare(FANOUT_EXCHANGE, ExchangeTypes.FANOUT);            channel.basicPublish(FANOUT_EXCHANGE, "alvez", null, content.getBytes());        }    }     static class DirectConsumer {        public static void main(String[] args) throws IOException, InterruptedException {            ConnectionFactory connectionFactory = new ConnectionFactory();            Connection connection = connectionFactory.newConnection();            Channel channel= connection.createChannel();             String queueName = channel.queueDeclare().getQueue();            channel.queueBind(queueName, FANOUT_EXCHANGE, "");             QueueingConsumer consumer = new QueueingConsumer(channel);            String s = channel.basicConsume(queueName, true, consumer);            System.out.println(s);            while (true) {                QueueingConsumer.Delivery delivery = consumer.nextDelivery();                String message = new String(delivery.getBody());                String routingKey = delivery.getEnvelope().getRoutingKey();                 System.out.println("From:" + routingKey + "':'" + message + "'");            }         }     }

三.Topic Exchange如果根據topic exchange用法說明其特征的話反而更麻煩。topic exchange正如其名，就是根據某種主題而不是特定的標題，也就是可以匹配routing key的一部分或者全部。topic exchange的routing key可以有多個詞組成，詞用'.'分隔。routing key中可以包括'*'或者'#'，'*'表示一個詞，'#'表示0～N個詞。

比如消息發布時routing key為"honda.civic.navy"，能接收消息的隊列的routing key可以是"honda.civic.navy"或"*.civic.*"或"honda.#"或"#"，但不能是"honda.accord.navy"或"honda.accord.silver"或"*.accord.*"或"ford.#"。

    private static String TOPIC_EXCHANGE = "TOPIC_EXCHAGNE";     static class TopicProducer {        public static void main(String[] args) throws IOException {            ConnectionFactory connectionFactory = new ConnectionFactory();            Connection connection = connectionFactory.newConnection();            Channel channel= connection.createChannel();;             String content = "I miss the conversation";            channel.exchangeDeclare(TOPIC_EXCHANGE, ExchangeTypes.TOPIC);            channel.basicPublish(TOPIC_EXCHANGE, "alvez.dep.FBI.map", null, content.getBytes());        }    }     static class TopicConsumer {        public static void main(String[] args) throws IOException, InterruptedException {            ConnectionFactory connectionFactory = new ConnectionFactory();            Connection connection = connectionFactory.newConnection();            Channel channel= connection.createChannel();             String queueName = channel.queueDeclare().getQueue();            channel.queueBind(queueName, TOPIC_EXCHANGE, "alvez.#");             QueueingConsumer consumer = new QueueingConsumer(channel);            String s = channel.basicConsume(queueName, true, consumer);            System.out.println(s);            while (true) {                QueueingConsumer.Delivery delivery = consumer.nextDelivery();                String message = new String(delivery.getBody());                String routingKey = delivery.getEnvelope().getRoutingKey();                 System.out.println("From:" + routingKey + "':'" + message + "'");            }         }　　}

四.Headers Exchange即消息頭和隊列中聲明的消息頭匹配時可以通信，似乎不是很常用。就可以定義多個條件進行匹配這一點來說，headers exchange和topic exchange有些相似。有時候會給人"為什么會有這種東西?"的感覺，相比topic exchage有什么優勢?如果僅僅說"headers exchange是基于headers的，topic exchange是基于routing key的"，這種回答沒什么意義。

代碼如下，可以看到producer和consumer的routing key是不同的，producer的header通過properties對象傳輸：

　　private static String HEADERS_EXCHANGE = "HEADERS_EXCHANGE";     static class HeadersProducer {        public static void main(String[] args) throws IOException {            ConnectionFactory connectionFactory = new ConnectionFactory();            Connection connection = connectionFactory.newConnection();            Channel channel = connection.createChannel();             String content = "I miss the conversation";            channel.exchangeDeclare(HEADERS_EXCHANGE, ExchangeTypes.HEADERS);            AMQP.BasicProperties properties = new AMQP.BasicProperties();            Map<String,Object> map = new HashMap<>();            map.put("key1","val1");            properties.setHeaders(map);             channel.basicPublish(HEADERS_EXCHANGE, "alvez", properties, content.getBytes());         }    }     static class HeadersConsumer {        public static void main(String[] args) throws IOException, InterruptedException {            ConnectionFactory connectionFactory = new ConnectionFactory();            Connection connection = connectionFactory.newConnection();            Channel channel = connection.createChannel();             String queueName = channel.queueDeclare().getQueue();            Map<String, Object> headers = new HashMap<>();            headers.put("key1","val1");            headers.put("key2","val2");            headers.put("key3","val3");            headers.put("key4","val4");            channel.queueBind(queueName, HEADERS_EXCHANGE, "",headers);             QueueingConsumer consumer = new QueueingConsumer(channel);            String s = channel.basicConsume(queueName, true, consumer);            System.out.println(s);            while (true) {                QueueingConsumer.Delivery delivery = consumer.nextDelivery();                String message = new String(delivery.getBody());                String routingKey = delivery.getEnvelope().getRoutingKey();                 System.out.println("From:" + routingKey + "':'" + message + "'");            }         }     }

(ps:圖不錯，感謝圖片作者。)