ActiveMQ源码分析

    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);
        }
    }

    /**
     * Receives the next message produced for this message consumer.
     * <P>
     * This call blocks indefinitely until a message is produced or until this
     * message consumer is closed.
     * <P>
     * If this <CODE>receive</CODE> is done within a transaction, the consumer
     * retains the message until the transaction commits.
     * 
     * @return the next message produced for this message consumer, or null if
     *         this message consumer is concurrently closed
     */
    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);
    }