Update RocketMQ_Example.md

docs/cn/RocketMQ_Example.md

@@ -66,7 +66,11 @@ public class AsyncProducer {
     	// 启动Producer实例
         producer.start();
         producer.setRetryTimesWhenSendAsyncFailed(0);
-    	for (int i = 0; i < 100; i++) {
+	
+	int messageCount = 100;
+        // 根据消息数量实例化倒计时计算器
+	final CountDownLatch2 countDownLatch = new CountDownLatch2(messageCount);
+    	for (int i = 0; i < messageCount; i++) {
                 final int index = i;
             	// 创建消息，并指定Topic，Tag和消息体
                 Message msg = new Message("TopicTest",
@@ -87,6 +91,8 @@ public class AsyncProducer {
                     }
             	});
     	}
+	// 等待5s
+	countDownLatch.await(5, TimeUnit.SECONDS);
     	// 如果不再发送消息，关闭Producer实例。
     	producer.shutdown();
     }