Yes.<\/p>\n
The code tree is as follows:<\/p>\n
Receive call on the ActiveMQMessageConsumer class, which implements the javax.jms.MessageConsumer interface…<\/p>\n
\r\n public javax.jms.Message receive()\r\n throws JMSException\r\n {\r\n checkClosed();\r\n checkMessageListener();\r\n \r\n sendPullCommand(0L);\r\n MessageDispatch md = dequeue(-1L);\r\n if (md == null) {\r\n return null;\r\n }\r\n beforeMessageIsConsumed(md);\r\n afterMessageIsConsumed(md, false);\r\n \r\n return createActiveMQMessage(md);\r\n }\r\n<\/pre>\nThe sendPullCommand() method, in the same class, is shown below…<\/p>\n
\r\nprotected void sendPullCommand(long timeout)\r\n throws JMSException\r\n {\r\n clearDeliveredList();\r\n if ((this.info.getCurrentPrefetchSize() == 0) && (this.unconsumedMessages.isEmpty()))\r\n {\r\n MessagePull messagePull = new MessagePull();\r\n messagePull.configure(this.info);\r\n messagePull.setTimeout(timeout);\r\n this.session.asyncSendPacket(messagePull);\r\n }\r\n }\r\n<\/pre>\nThe MessagePull{} class is configured and sent to the broker with the asyncSendPacket() method call.<\/p>\n
For empirical evidence, an actual strace of a consumer registered with ActiveMQ shows what is below.<\/p>\n
We identified the socket associated with the file descriptor 430 using strace against the Consumer. What is below is the strace against ActiveMQ itself. We start our consumer and create a Queue connection to the broker at 9:17:05. At 9:17:17, we send a message from our producer to the queue. There is no other traffic between the consumer and ActiveMQ until ActiveMQ sends it the message at 9:17:17. It doesn’t even contain a poll call every so often to see if there is work to do. As a result, it is truly asynchronous.<\/p>\n
\r\n15032 09:17:05.722665 fcntl(430, F_GETFL \r\n15032 09:17:05.722741 fcntl(430, F_SETFL, O_RDWR \r\n30525 09:17:05.724339 setsockopt(430, SOL_SOCKET, SO_RCVBUF, [65536], 4) = 0\r\n30525 09:17:05.724409 setsockopt(430, SOL_SOCKET, SO_SNDBUF, [65536], 4) = 0\r\n30525 09:17:05.725299 sendto(430, \"ActiveMQ MaxFrameSize CacheSize CacheEnabled SizePrefixDisabled MaxInactivityDurationInitalDelay TcpNoDelayEnabled MaxInactivityDuration TightEncodingEnabled StackTraceEnabled\", 244, 0, NULL, 0 \r\n30657 09:17:05.731351 setsockopt(430, SOL_TCP, TCP_NODELAY, [1], 4 \r\n30658 09:17:05.738382 sendto(430, \"ID:cmhlcarchapp01.foo.com-18813-1481762261285-0:1 (tcp:\/\/cmhlcarchapp01.foo.com:61616 amq2\", 121, 0, NULL, 0) = 121\r\n30658 09:17:05.807018 sendto(430, \"\", 16, 0, NULL, 0) = 16\r\n30657 09:17:05.807576 sendto(430, \"\", 15, 0, NULL, 0) = 15\r\n30657 09:17:05.807658 recvfrom(430, \"ID:cmhlcarchapp01.foo.com-15477-1481897825526-1:1 ActiveMQ.Advisory.TempQueue,ActiveMQ.Advisory.TempTopic\", 8192, 0, NULL, NULL) = 153\r\n30657 09:17:05.817916 sendto(430, \"\", 15, 0, NULL, 0) = 15\r\n30657 09:17:05.852143 recvfrom(430, \"} ID:cmhlcarchapp01.foo.com-15477-1481897825526-1:1 requestQ ID:cmhlcarchapp01.expressco.com-15477-1481897825526-0:1\", 8192, 0, NULL, NULL) = 157\r\n30657 09:17:05.865215 sendto(430, \"\", 15, 0, NULL, 0 \r\n30657 09:17:05.874460 sendto(430, \"\", 15, 0, NULL, 0 \r\n30658 09:17:17.179415 sendto(430, \"ID:cmhlcarchapp01.foo.com-15477-1481897825526-1:1 requestQ {ID:cmhlcarchapp01.foo.com-63166-1481897836921-1:1 replyQ Y Hello world. Y Y {\", 247, 0, NULL, 0) = 247\r\n30657 09:17:17.202377 sendto(430, \"\", 15, 0, NULL, 0) = 15\r\n30657 09:17:17.202469 recvfrom(430, \"ID:cmhlcarchapp01.foo.com-63166-1481897836921-1:1:1:1:1 Y Hello world.\", 8192, 0, NULL, NULL) = 133\r\n30657 09:17:17.206053 sendto(430, \"\", 15, 0, NULL, 0 \r\n<\/pre>\n","protected":false},"excerpt":{"rendered":"Yes. The code tree is as follows: Receive call on the ActiveMQMessageConsumer class, which implements the javax.jms.MessageConsumer interface… public javax.jms.Message receive() throws JMSException { checkClosed(); checkMessageListener(); sendPullCommand(0L); MessageDispatch md = dequeue(-1L); if (md == null) { return null; } beforeMessageIsConsumed(md);…<\/p>\n