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1。同步和异步的方式
public static void main(String[] args) {
// ConnectionFactory ：连接工厂，JMS 用它创建连接
ConnectionFactory connectionFactory;
// Connection ：JMS 客户端到JMS Provider 的连接
Connection connection = null;
// Session： 一个发送或接收消息的线程
Session session;
// Destination ：消息的目的地;消息发送给谁.
Destination destination;
// 消费者，消息接收者
MessageConsumer consumer;
connectionFactory = new ActiveMQConnectionFactory(
ActiveMQConnection.DEFAULT_USER,
ActiveMQConnection.DEFAULT_PASSWORD,
"tcp://localhost:61616");
try {
// 构造从工厂得到连接对象
connection = connectionFactory.createConnection();
// 启动
connection.start();
// 获取操作连接
session = connection.createSession(Boolean.FALSE,
Session.AUTO_ACKNOWLEDGE);
//
destination = session.createQueue("testQueue1");
consumer = session.createConsumer(destination);
// 同步的方式接收消息
while (true) {
//
TextMessage message = (TextMessage) consumer.receive(100000);
if (null != message) {
System.out.println("收到消息" + message.getText());
} else {
break;
}
}
// 异步的方式接收消息
consumer.setMessageListener(new MessageListener(){
@Override
public void onMessage(Message arg0) {
// TODO Auto-generated method stub
try{
if(arg0 instanceof TextMessage)
{
TextMessage txtMsg = (TextMessage) arg0;
System.out.println("consumer异步 recive:"+txtMsg.getText());
Thread.sleep(500);
}
}catch(Exception e)
{
e.printStackTrace();
}
}
}); //(异步接收)
} catch (Exception e) {
e.printStackTrace();
} finally {
try {
if (null != connection)
connection.close();
} catch (Throwable ignore) {
}
}
}
现在讲讲consumer.receive和consumer.setMessageListener方式的不同；
1。 consumer.receive方式
public Message receive() throws JMSException {
checkClosed();
checkMessageListener();
sendPullCommand(0);
MessageDispatch md = dequeue(-1);
if (md == null) {
return null;
}
beforeMessageIsConsumed(md);
afterMessageIsConsumed(md, false);
return createActiveMQMessage(md);
}可以看到sendPullCommand，其会先发送一个pull命令，然后然后从接收的消息列表中dequeue一条消息，dequeue的参数－1表示，如果没有消息返回，会阻塞在这儿，如果有消息便会进行消费。
下面讲解异步消费消息过程。
@Override
public void setMessageListener(MessageListener listener) throws JMSException {
checkClosed();
if (info.getPrefetchSize() == 0) {
throw new JMSException("Illegal prefetch size of zero. This setting is not supported for asynchronous consumers please set a value of at least 1");
}
if (listener != null) {
boolean wasRunning = session.isRunning();
if (wasRunning) {
session.stop();
}
this.messageListener.set(listener);
session.redispatch(this, unconsumedMessages);
if (wasRunning) {
session.start();
}
} else {
this.messageListener.set(null);
}
}有一句session.start();然后进入ActiveMQSession类，看下这个start方法
/**
* Start this Session.
*
* @throws JMSException
*/
protected void start() throws JMSException {
started.set(true);
for (Iterator<ActiveMQMessageConsumer> iter = consumers.iterator(); iter.hasNext();) {
ActiveMQMessageConsumer c = iter.next();
c.start();
}
executor.start();
}在看这句c.start();
public void start() throws JMSException {
if (unconsumedMessages.isClosed()) {
return;
}
started.set(true);
unconsumedMessages.start();
session.executor.wakeup();
}在看ActiveMQSessionExecutor中的wakeup方法
public void wakeup() {
if (!dispatchedBySessionPool) {
if (session.isSessionAsyncDispatch()) {
try {
TaskRunner taskRunner = this.taskRunner;
if (taskRunner == null) {
synchronized (this) {
if (this.taskRunner == null) {
if (!isRunning()) {
// stop has been called
return;
}
this.taskRunner = session.connection.getSessionTaskRunner().createTaskRunner(this,
"ActiveMQ Session: " + session.getSessionId());
}
taskRunner = this.taskRunner;
}
}
taskRunner.wakeup();
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
}
} else {
while (iterate()) {
}
}
}
}其创建了一个线程运行的类；
public TaskRunner createTaskRunner(Task task, String name) {
init();
if (executor != null) {
return new PooledTaskRunner(executor, task, maxIterationsPerRun);
} else {
return new DedicatedTaskRunner(task, name, priority, daemon);
}
}看其构造方法
public DedicatedTaskRunner(final Task task, String name, int priority, boolean daemon) {
this.task = task;
thread = new Thread(name) {
@Override
public void run() {
try {
runTask();
} finally {
LOG.trace("Run task done: {}", task);
}
}
};
thread.setDaemon(daemon);
thread.setName(name);
thread.setPriority(priority);
thread.start();
}在看runtask方法：
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final void runTask() {
try {
while (true) {
synchronized (mutex) {
pending = false;
if (shutdown) {
return;
}
}
LOG.trace("Running task {}", task);
if (!task.iterate()) {
// wait to be notified.
synchronized (mutex) {
if (shutdown) {
return;
}
while (!pending) {
mutex.wait();
}
}
}
}
} catch (InterruptedException e) {
// Someone really wants this thread to die off.
Thread.currentThread().interrupt();
} finally {
// Make sure we notify any waiting threads that thread
// has terminated.
synchronized (mutex) {
threadTerminated = true;
mutex.notifyAll();
}
}
}其中有这句话task.iterate()。。就是执行到ActiveMQSessionExecutor的iterate方法
public boolean iterate() {
// Deliver any messages queued on the consumer to their listeners.
for (ActiveMQMessageConsumer consumer : this.session.consumers) {
if (consumer.iterate()) {
return true;
}
}
// No messages left queued on the listeners.. so now dispatch messages
// queued on the session
MessageDispatch message = messageQueue.dequeueNoWait();
if (message == null) {
return false;
} else {
dispatch(message);
return !messageQueue.isEmpty();
}
}查看dispatch方法
void dispatch(MessageDispatch message) {
// TODO - we should use a Map for this indexed by consumerId
for (ActiveMQMessageConsumer consumer : this.session.consumers) {
ConsumerId consumerId = message.getConsumerId();
if (consumerId.equals(consumer.getConsumerId())) {
consumer.dispatch(message);
break;
}
}
}然后看看consumer.dispatch(message);这句话。最终是执行到了ActiveMQMessageConsumer的dispatch方法。。
@Override
public void dispatch(MessageDispatch md) {
MessageListener listener = this.messageListener.get();
try {
clearMessagesInProgress();
clearDeliveredList();
synchronized (unconsumedMessages.getMutex()) {
if (!unconsumedMessages.isClosed()) {
if (this.info.isBrowser() || !session.connection.isDuplicate(this, md.getMessage())) {
if (listener != null && unconsumedMessages.isRunning()) {
if (redeliveryExceeded(md)) {
posionAck(md, "dispatch to " + getConsumerId() + " exceeds redelivery policy limit:" + redeliveryPolicy);
return;
}
ActiveMQMessage message = createActiveMQMessage(md);
beforeMessageIsConsumed(md);
try {
boolean expired = isConsumerExpiryCheckEnabled() && message.isExpired();
if (!expired) {
listener.onMessage(message);
}
afterMessageIsConsumed(md, expired);
} catch (RuntimeException e) {
LOG.error("{} Exception while processing message: {}", getConsumerId(), md.getMessage().getMessageId(), e);
md.setRollbackCause(e);
if (isAutoAcknowledgeBatch() || isAutoAcknowledgeEach() || session.isIndividualAcknowledge()) {
// schedual redelivery and possible dlq processing
rollback();
} else {
// Transacted or Client ack: Deliver the next message.
afterMessageIsConsumed(md, false);
}
}
} else {
if (!unconsumedMessages.isRunning()) {
// delayed redelivery, ensure it can be re delivered
session.connection.rollbackDuplicate(this, md.getMessage());
}
if (md.getMessage() == null) {
// End of browse or pull request timeout.
unconsumedMessages.enqueue(md);
} else {
if (!consumeExpiredMessage(md)) {
unconsumedMessages.enqueue(md);
if (availableListener != null) {
availableListener.onMessageAvailable(this);
}
} else {
beforeMessageIsConsumed(md);
afterMessageIsConsumed(md, true);
// Pull consumer needs to check if pull timed out and send
// a new pull command if not.
if (info.getCurrentPrefetchSize() == 0) {
unconsumedMessages.enqueue(null);
}
}
}
}
} else {
// deal with duplicate delivery
ConsumerId consumerWithPendingTransaction;
if (redeliveryExpectedInCurrentTransaction(md, true)) {
LOG.debug("{} tracking transacted redelivery {}", getConsumerId(), md.getMessage());
if (transactedIndividualAck) {
immediateIndividualTransactedAck(md);
} else {
session.sendAck(new MessageAck(md, MessageAck.DELIVERED_ACK_TYPE, 1));
}
} else if ((consumerWithPendingTransaction = redeliveryPendingInCompetingTransaction(md)) != null) {
LOG.warn("{} delivering duplicate {}, pending transaction completion on {} will rollback", getConsumerId(), md.getMessage(), consumerWithPendingTransaction);
session.getConnection().rollbackDuplicate(this, md.getMessage());
dispatch(md);
} else {
LOG.warn("{} suppressing duplicate delivery on connection, poison acking: {}", getConsumerId(), md);
posionAck(md, "Suppressing duplicate delivery on connection, consumer " + getConsumerId());
}
}
}
}
if (++dispatchedCount % 1000 == 0) {
dispatchedCount = 0;
Thread.yield();
}
} catch (Exception e) {
session.connection.onClientInternalException(e);
}
}
然后查看上述方法。你会发现最终执行到了。consumer上的listener。