带有RECEIVE_TIMEOUT_NO_WAIT的JmsTemplate不会从JMS队列中检索消息 [英] JmsTemplate with RECEIVE_TIMEOUT_NO_WAIT doesn't retrieve messages from JMS queue

问题描述

鉴于我有ActiveMQ队列，其中已经存在许多消息.

Given I have ActiveMQ queue where many messages are already present.

当我将JmsTemplate上的接收超时设置为RECEIVE_TIMEOUT_NO_WAIT时，它等于-1:

When I set receive timeout on JmsTemplate to RECEIVE_TIMEOUT_NO_WAIT which is equal to -1:

jmsTemplate.setReceiveTimeout(JmsTemplate.RECEIVE_TIMEOUT_NO_WAIT); 

并尝试接收这些消息之一:

and try to receive one of those messages:

Message msg = jmsTemplate.receive(queueName);

然后msg是null，但是它不符合JavaDoc的要求:

then msg is null, but it should not be according JavaDoc:

/**
 * Timeout value indicating that a receive operation should
 * check if a message is immediately available without blocking.
 */
public static final long RECEIVE_TIMEOUT_NO_WAIT = -1;

那是为什么?

当我这样做时:

jmsTemplate.setReceiveTimeout(1000);

然后检索邮件.

推荐答案

它与JmsTemplate完全没有任何关系，因为它只是委派给基础JMS Consumer对象:

It has absolutely nothing at all to do with the JmsTemplate since it simply delegates to the underlying JMS Consumer object:

protected Message receiveFromConsumer(MessageConsumer consumer, long timeout) throws JMSException {
    if (timeout > 0) {
        return consumer.receive(timeout);
    }
    else if (timeout < 0) {
        return consumer.receiveNoWait();
    }
    else {
        return consumer.receive();
    }
}

我会说它的工作完全符合JMS设计师的意图:

I would say it is working exactly as the JMS designers intended:

/** Receives the next message if one is immediately available.
  *
  * @return the next message produced for this message consumer, or 
  * null if one is not available
  *  
  * @exception JMSException if the JMS provider fails to receive the next
  *                         message due to some internal error.
  */ 

Message receiveNoWait() throws JMSException;

换句话说，这是在用例中，如果当前没有消息已经由经纪人发送给使用者，则您绝对绝对不想在任何时候阻塞线程-甚至没有等待网络I/O完成，这正是ActiveMQ实施的方式-启动I/O，但如果该I/O没有立即完成则返回null(如果涉及到网络，则很可能是这种情况)

In other words, it is for use cases where you absolutely don't want to block the thread for any time at all, if there is not currently a message that has already been sent to the consumer by the broker - not even waiting for a network I/O to complete, which is exactly how ActiveMQ has implemented it - initiates an I/O but returns null if that I/O doesn't complete immediately (which is most likely the case if there's a network involved).

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