Nats消息中间件Android集成

门禁人脸识别项目，项目会议中讨论终端与后台信息通讯消息中间件，考虑到常用的消息通讯系ActiveMQ（Java编写）、KafKa（Scala编写）、RabbitMq（Erlang编写）、Nats（之前是Ruby编写现已修改为Go）、Redis（C语言编写）、Kestrel（Scala编写不常用）、NSQ（Go语言编写）、Nats（Go）都能满足基本的需求，但公司后台开发人员使用go语言，于是选择了Nats框架。

Nats 官方与社区版本均无对应的客户端，Android 使用 Java 开发，在 Java 上能运行即可移植在 Android 客户端

Java 客户端sample：https://github.com/nats-io/nats.java

Android 终端集成问题

Android 工程中引入 nats 框架，开发过程中遇到时间类文件在 Android 8.0 版本才引入，但门禁终端为 7.0 系统，需要找个折中办法改造 Java 客户端 sdk，万能 Github 已有个人改造nats java sdk，引入时间类 Duration ：

Android 客户端sample：https://github.com/SpotOnInc/nats-android

Android 工程中引入改造后 JDK：implementation 'com.spoton:nats-android:2.4.6'

Java 工程中无UI主线程，但 Android 客户端 UI 主线程不能执行耗时任务，客户端发起请求连接服务端为耗时任务，直接在主线程中请求，触发设备 ANR，必须在子线程中去请求连接服务器。

Basic Usage

Connecting「连接」

There are four different ways to connect using the Java library:

1. Connect to a local server on the default port:

   Connection nc = Nats.connect();
   

2. Connect to a server using a URL:「推荐」

   Connection nc = Nats.connect("nats://myhost:4222");
   

3. Connect to one or more servers with a custom configuration:

   Options o = new Options.Builder().server("nats://serverone:4222").server("nats://servertwo:4222").maxReconnects(-1).build();
   Connection nc = Nats.connect(o);
   

   See the javadoc for a complete list of configuration options.

4. Connect asynchronously, this requires a callback to tell the application when the client is connected:

    Options options = new  Options.Builder().server(Options.DEFAULT_URL).connectionListener(handler).build();
    Nats.connectAsynchronously(options, true);
   

Publishing「发布」

Once connected, publishing is accomplished via one of three methods:

1. With a subject and message body:「推荐」

   nc.publish("subject", "hello world".getBytes(StandardCharsets.UTF_8));
   

2. With a subject and message body, as well as a subject for the receiver to reply to:

   nc.publish("subject", "replyto", "hello world".getBytes(StandardCharsets.UTF_8));
   

3. As a request that expects a reply. This method uses a Future to allow the application code to wait for the response. Under the covers a request/reply pair is the same as a publish/subscribe only the library manages the subscription for you.「私有发布，只有服务器能接收消息」

   Future incoming = nc.request("subject", "hello world".getBytes(StandardCharsets.UTF_8));
   Message msg = incoming.get(500, TimeUnit.MILLISECONDS);
   String response = new String(msg.getData(), StandardCharsets.UTF_8);
   

Listening for Incoming Messages

The Java NATS library provides two mechanisms to listen for messages, three if you include the request/reply discussed above.

1. Synchronous subscriptions where the application code manually asks for messages and blocks until they arrive. Each subscription is associated with a single subject, although that subject can be a wildcard.

   Subscription sub = nc.subscribe("subject");
   Message msg = sub.nextMessage(Duration.ofMillis(500));
   
   String response = new String(msg.getData(), StandardCharsets.UTF_8);
   

2. A Dispatcher that will call application code in a background thread. Dispatchers can manage multiple subjects with a single thread and single callback.「推荐，Java 8 Lambda表达式，工程中JDK 必选选择为JDK 8，回调监听模式」

   Dispatcher d = nc.createDispatcher((msg) -> {
       String response = new String(msg.getData(), StandardCharsets.UTF_8);
       ...
   });
   
   d.subscribe("subject");