public void setTopic(final String topic) {
        this.checkState();
        this.topic = topic;
    }
Set message's topic,if you want to send it,you must publish it at first
with MessageProducer.

@see com.taobao.metamorphosis.client.producer.MessageProducer#publish(String)
@param topic
metamorphosis-commons/src/main/java/com/taobao/metamorphosis/Message.java