ActiveMQ 的连接和会话

要了解 connection 和 session 的概念，可以先从 ConnectionState 和 SessionState 入手：

// 省略部分代码
public class ConnectionState {
    ConnectionInfo info;
    private final ConcurrentHashMap<TransactionId, TransactionState> transactions = new ConcurrentHashMap<TransactionId, TransactionState>();
    private final ConcurrentHashMap<SessionId, SessionState> sessions = new ConcurrentHashMap<SessionId, SessionState>();
    private final List<DestinationInfo> tempDestinations = Collections.synchronizedList(new ArrayList<DestinationInfo>());
    private final AtomicBoolean shutdown = new AtomicBoolean(false);
    private boolean connectionInterruptProcessingComplete = true;
    private HashMap<ConsumerId, ConsumerInfo> recoveringPullConsumers;

    public ConnectionState(ConnectionInfo info) {
        this.info = info;
        // Add the default session id.
        addSession(new SessionInfo(info, -1));
    }
}

从代码可以看出，连接里有事务集合、会话集合、临时队列集合等，这说明：
1. 事务属于一个连接； 2. 会话属于一个连接； 3. 临时队列的生存期是连接的有效期

// 省略部分代码
public class SessionState {
    final SessionInfo info;

    private final Map<ProducerId, ProducerState> producers = new ConcurrentHashMap<ProducerId, ProducerState>();
    private final Map<ConsumerId, ConsumerState> consumers = new ConcurrentHashMap<ConsumerId, ConsumerState>();
    private final AtomicBoolean shutdown = new AtomicBoolean(false);

    public SessionState(SessionInfo info) {
        this.info = info;
    }
}

从上面能看出，producer 和 consumer 是属于某个会话的，producer 和 consumer 都有唯一的 ID 。

// 省略部分代码
public class ProducerState {
    final ProducerInfo info;
    private TransactionState transactionState;
}

public class ConsumerState {        
    final ConsumerInfo info;
}

ProducerState 和 ConsumerState 只是做了简单的封装。

其中 ConnectionInfo, SessionInfo, ProducerInfo, ConsumerInfo 都是消息类型，均继承自 BaseCommand 接口。