The connection is returned to the free pool when sendMessage() exits.
当 sendMessage() 退出时,该连接返回到空闲池。
The sendMessage() function creates an AsyncMessage, which is an implementation of the IMessage interface, and then sends the message using the producer’s send() method.
sendMessage()函数先创建一个AsyncMessage,其是IMessage接口的一个实现,然后使用producer的send()方法发送此消息。