activemq持久订阅工作原理

那么持久订阅到底是如何实现的呢，笔者在这里将展现其中的奥秘：

先来看下TopicRegion的addConsumer方法

public Subscription addConsumer(ConnectionContext context, ConsumerInfo info) throws Exception {

if (info.isDurable()) {

//看该消息是否是持久化订阅

ActiveMQDestination destination = info.getDestination();

if (!destination.isPattern()) {

// Make sure the destination is created.

lookup(context, destination,true);

}

**String clientId = context.getClientId();

String subscriptionName = info.getSubscriptionName();

SubscriptionKey key = new SubscriptionKey(clientId, subscriptionName);

DurableTopicSubscription sub = durableSubscriptions.get(key);**

if (sub != null) {

if (sub.isActive()) {

throw new JMSException("Durable consumer is in use for client: " + clientId + " and subscriptionName: " + subscriptionName);

}

// 看下该订阅者的消息筛选项是否变化

if (hasDurableSubChanged(info, sub.getConsumerInfo())) {

// 如果变化了那么首先移除该订阅者对应的DurableTopicSubscription，然后再追加最新创建的DurableTopicSubscription

durableSubscriptions.remove(key);

destinationsLock.readLock().lock();

try {

for (Destination dest : destinations.values()) {

//Account for virtual destinations

if (dest instanceof Topic){

Topic topic = (Topic)dest;

topic.deleteSubscription(context, key);

}

}

} finally {

destinationsLock.readLock().unlock();

}

**super.removeConsumer(context, sub.getConsumerInfo());

super.addConsumer(context, info);

sub = durableSubscriptions.get(key);**

} else {

// 如果消息筛选项没有变化，那么直接将刚恢复连接的订阅者id与之前的DurableTopicSubscription 关联起来

if (sub.getConsumerInfo().getConsumerId() != null) {

subscriptions.remove(sub.getConsumerInfo().getConsumerId());

}

subscriptions.put(info.getConsumerId(), sub);

}

} else {

super.addConsumer(context, info);

sub = durableSubscriptions.get(key);

if (sub == null) {

throw new JMSException("Cannot use the same consumerId: " + info.getConsumerId() + " for two different durable subscriptions clientID: " + key.getClientId()

+ " subscriberName: " + key.getSubscriptionName());

}

}

sub.activate(usageManager, context, info, broker);

return sub;

} else {

return super.addConsumer(context, info);

}

}

上面代码是订阅者连接到消息提供者时的处理代码，下面看下更核心的持久订阅与消息提供者断开连接时的处理：

@Override

public void removeConsumer(ConnectionContext context, ConsumerInfo info) throws Exception {

if (info.isDurable()) {

SubscriptionKey key = new SubscriptionKey(context.getClientId(), info.getSubscriptionName());

DurableTopicSubscription sub = durableSubscriptions.get(key);