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🧣 JMS 客戶端基礎

本章節從程式端出發，解析 JMS 客戶端與 ActiveMQ Broker 互動的核心物件與生命週期。掌握 ConnectionFactory → Session → Producer/Consumer 這條鏈路，是撰寫任何 ActiveMQ 應用的起點。

環境​

1. JMS 核心物件 —— 四層結構​

物件職責
ConnectionFactory建立與 Broker 的連線工廠，通常以 Singleton 持有
Connection代表一條 TCP 連線，開啟後才能建立 Session
Session訊息收發的工作單元，控制 ACK 模式與 Transaction
Producer / Consumer訊息的生產者與消費者，綁定目的地

2. Maven 依賴​

<dependency>
  <groupId>org.apache.activemq</groupId>
  <artifactId>activemq-client</artifactId>
  <version>5.16.6</version>
</dependency>

3. Queue 點對點範例​

3.1 Producer — 發送訊息​

ActiveMQConnectionFactory factory =
    new ActiveMQConnectionFactory("tcp://localhost:61616");
Connection connection = factory.createConnection();
connection.start();

Session session = connection.createSession(false, Session.AUTO_ACKNOWLEDGE);
Destination queue = session.createQueue("ORDER.QUEUE");
MessageProducer producer = session.createProducer(queue);

TextMessage message = session.createTextMessage("New order #1001");
message.setStringProperty("category", "urgent");
producer.send(message);

producer.close();
session.close();
connection.close();

3.2 Consumer — 同步接收​

ActiveMQConnectionFactory factory =
    new ActiveMQConnectionFactory("tcp://localhost:61616");
Connection connection = factory.createConnection();
connection.start();

Session session = connection.createSession(false, Session.AUTO_ACKNOWLEDGE);
Destination queue = session.createQueue("ORDER.QUEUE");
MessageConsumer consumer = session.createConsumer(queue);

Message message = consumer.receive(5000); // 等待 5 秒
if (message instanceof TextMessage) {
    System.out.println("Received: " + ((TextMessage) message).getText());
}

consumer.close();
session.close();
connection.close();

3.3 Consumer — 異步監聽​

connection.start();
Session session = connection.createSession(false, Session.AUTO_ACKNOWLEDGE);
MessageConsumer consumer = session.createConsumer(session.createQueue("ORDER.QUEUE"));

consumer.setMessageListener(message -> {
    try {
        if (message instanceof TextMessage) {
            System.out.println("Async received: " + ((TextMessage) message).getText());
        }
    } catch (JMSException e) {
        e.printStackTrace();
    }
});

// 主執行緒保持運行，等待訊息到達
Thread.sleep(Long.MAX_VALUE);

4. Topic 發布/訂閱範例​

Topic 與 Queue 的程式差異在於目的地類型與消費模式：同一則訊息會被所有訂閱者接收。

4.1 Publisher​

Session session = connection.createSession(false, Session.AUTO_ACKNOWLEDGE);
Topic topic = session.createTopic("STOCK.PRICE");
MessageProducer publisher = session.createProducer(topic);

TextMessage message = session.createTextMessage("AAPL: 185.50");
publisher.send(message);

4.2 Subscriber​

Session session = connection.createSession(false, Session.AUTO_ACKNOWLEDGE);
Topic topic = session.createTopic("STOCK.PRICE");
MessageConsumer subscriber = session.createConsumer(topic);

subscriber.setMessageListener(msg -> {
    System.out.println("Price update: " + ((TextMessage) msg).getText());
});
caution

Topic 的 Subscriber 必須在 Publisher 發送之前就上線，否則非持久化訊息會錯過。需要離線接收請使用持久訂閱（參見 durableSubscription）。

5. 連線與認證​

若 Broker 啟用了 Simple Authentication（參見 setUser），連線字串需帶入帳密：

ActiveMQConnectionFactory factory = new ActiveMQConnectionFactory(
    "tcp://localhost:61616");
factory.setUserName("admin");
factory.setPassword("admin1pwd");

6. 資源釋放與最佳實踐​

實踐說明
依序關閉資源Producer → Consumer → Session → Connection
ConnectionFactory 單例避免重複建立連線工廠
生產環境用連線池使用 PooledConnectionFactory 包裹
避免在 Listener 中阻塞長時間處理應交由工作執行緒
PooledConnectionFactory pooledFactory = new PooledConnectionFactory();
pooledFactory.setConnectionFactory(new ActiveMQConnectionFactory("tcp://localhost:61616"));
pooledFactory.setMaxConnections(10);

7. Queue vs Topic 程式對照​

項目QueueTopic
建立方式session.createQueue(name)session.createTopic(name)
消費者數量多消費者競爭，每則訊息只一人收到所有訂閱者都收到
典型用途任務分配、訂單處理事件廣播、價格推送

8. 與其他文章的關聯​