Java Mina解码中遇到的问题及解决方案

最近一个项目中用到了Java解码，主要采用的是Mina框架，现将遇到的问题总结一下，以备后查：
终端是用C编码，通过CAN中转，最后转成TCP送出，用Java写了个服务端，接收并解析入库


一、字节序的问题
关于字节序，请见 http://zh.wikipedia.org/wiki/%E5%AD%97%E8%8A%82%E5%BA%8F
C和Java不一样，所以代码中要这样设置一下

	@Override
	protected boolean doDecode(IoSession session, IoBuffer in, ProtocolDecoderOutput out) throws Exception {
		in.order(ByteOrder.LITTLE_ENDIAN);
		//...
}

二、数据类型的问题
C，C++中有unsigned类型，Java中没有，所以对于一个字节，C中可以存储unsigned类型，即0到255，而java对应的是byte，即-128到127
所以要做转换

byte cur_b = in.get(); //这个是byte
int cur_i = cur_b & 0xff; //做运算时要转成整形

三、丰富的字节转换函数
除了上面的强转之外，Mina的IoBuffer中提供了很多方便的转换api，能将一个或多个字节的二进制数据方便地转成这种类型

get()
get(byte[])
get(byte[], int, int)
get(int)
getChar()
getChar(int)
getDouble()
getDouble(int)
getEnum(int, Class)
getEnum(Class)
getEnumInt(int, Class)
getEnumInt(Class)
getEnumSet(int, Class)
getEnumSet(Class)
getEnumSetInt(int, Class)
getEnumSetInt(Class)
getEnumSetLong(int, Class)
getEnumSetLong(Class)
getEnumSetShort(int, Class)
getEnumSetShort(Class)
getEnumShort(int, Class)
getEnumShort(Class)
getFloat()
getFloat(int)
getHexDump()
getHexDump(int)
getInt()
getInt(int)
getLong()
getLong(int)
getMediumInt()
getMediumInt(int)
getObject()
getObject(ClassLoader)
getPrefixedString(int, CharsetDecoder)
getPrefixedString(CharsetDecoder)
getShort()
getShort(int)
getSlice(int)
getSlice(int, int)
getString(int, CharsetDecoder)
getString(CharsetDecoder)
getUnsigned()
getUnsigned(int)
getUnsignedInt()
getUnsignedInt(int)
getUnsignedMediumInt()
getUnsignedMediumInt(int)
getUnsignedShort()
getUnsignedShort(int)

基本上足够使用了


四、处理断包等问题，只要解码器Decoder extends CumulativeProtocolDecoder即可
/*
A ProtocolDecoder that cumulates the content of received buffers to a cumulative buffer to help users implement decoders. 
If the received IoBuffer is only a part of a message. decoders should cumulate received buffers to make a message complete or to postpone decoding until more buffers arrive. 
Here is an example decoder that decodes CRLF terminated lines into Command objects:
 */

public class CrLfTerminatedCommandLineDecoder
         extends CumulativeProtocolDecoder {


     private Command parseCommand(IoBuffer in) {
         // Convert the bytes in the specified buffer to a
         // Command object.
         ...
     }


     protected boolean doDecode(
             IoSession session, IoBuffer in, ProtocolDecoderOutput out)
             throws Exception {


         // Remember the initial position.
         int start = in.position();


         // Now find the first CRLF in the buffer.
         byte previous = 0;
         while (in.hasRemaining()) {
             byte current = in.get();


             if (previous == '\r' && current == '\n') {
                 // Remember the current position and limit.
                 int position = in.position();
                 int limit = in.limit();
                 try {
                     in.position(start);
                     in.limit(position);
                     // The bytes between in.position() and in.limit()
                     // now contain a full CRLF terminated line.
                     out.write(parseCommand(in.slice()));
                 } finally {
                     // Set the position to point right after the
                     // detected line and set the limit to the old
                     // one.
                     in.position(position);
                     in.limit(limit);
                 }
                 // Decoded one line; CumulativeProtocolDecoder will
                 // call me again until I return false. So just
                 // return true until there are no more lines in the
                 // buffer.
                 return true;
             }


             previous = current;
         }


         // Could not find CRLF in the buffer. Reset the initial
         // position to the one we recorded above.
         in.position(start);


         return false;
     }
 }

/*
Please note that this decoder simply forward the call to doDecode(IoSession, IoBuffer, ProtocolDecoderOutput) if the underlying transport doesn't have a packet fragmentation. Whether the transport has fragmentation or not is determined by querying TransportMetadata.

*/

除此之外，还要将未解析完的数据和标志位等放到IoSession中

Context ctx = getContext(session);


private Context getContext(IoSession session) {
	Context context = (Context) session.getAttribute(CONTEXT);
	if (context == null) {
		context = new Context();
		session.setAttribute(CONTEXT, context);
	}
	return context;
}

五、多个服务（端口）共享Session问题

应用场景比如，一个端口9090专门负责数据接收，另外一个端口8080接收来自web端的指令并传送给终端并返回数据。

原理是在一个主函数中开两个mina服务（端口），但此时两个服务中的IoSession是不能互访的，所以要在主进程中管理他们

public class Main {
	private static final Set sessions = Collections.synchronizedSet(new HashSet());//这个至关重要
	private static final Logger LOGGER = LoggerFactory.getLogger(Main.class);
	private static final int PORT_DTU = 9090;
	private static final int PORT_WEB = 8080;
	private static final ProtocolCodecFactory textLineCodecFactory = new TextLineCodecFactory(Charset.forName("UTF-8"));

	public static void main(String[] args) {
		try {
			new MinaxServer(PORT_DTU, textLineCodecFactory, new MinaxDtuIoHandler()).start();
			LOGGER.info("Server started at port {}.", PORT_DTU);

			new MinaxServer(PORT_WEB, textLineCodecFactory, new MinaxWebIoHandler()).start();
			LOGGER.info("Server started at port {}.", PORT_WEB);

		} catch (IOException ioe) {
			LOGGER.error("Can't start server!", ioe);
		}

		while (true) {
			System.out.printf("session count:%d\n", Main.getSessions().size());
			try {
				Thread.sleep(10000);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
	}

	public static Set getSessions() {
		return sessions;
	}
}

之后在传送端的IoHandler中加以管理，即在SessionCreated时，加入到Set中，sessionClosed的时候从Set中Remove掉

	@Override
	public void sessionCreated(IoSession session) throws Exception {
		// super.sessionCreated(session);
		Main.getSessions().add(session);
		// String message = String.format("IP:%s, Welcome to dtu server\n", session.getRemoteAddress());
		// session.write(message);
		logger.debug("{}:session[{}]Created...", session.getRemoteAddress(), session.getId());
	}

	@Override
	public void sessionOpened(IoSession session) throws Exception {
		// super.sessionOpened(session);
		logger.debug("sessionOpened...");
	}

	@Override
	public void sessionClosed(IoSession session) throws Exception {
		Main.getSessions().remove(session);
		// super.sessionClosed(session);
		logger.debug("sessionClosed...");
	}

在Web端访问IoSession

@Override
	public void messageReceived(IoSession session, Object message) throws Exception {
		logger.debug("messageReceived...");
		logger.debug("...message:{}", message);
		String jsonMessage = message.toString();

		JSONObject o = JSON.parseObject(jsonMessage);
		Integer dtu_id = o.getInteger("dtu_id");
		Long session_id = o.getLong("session_id");
		String action = o.getString("action");
		String params = o.getString("params");

		action = null == action ? "" : action.toLowerCase();
		JSONObject p = JSON.parseObject(params);

		Set sessions = Main.getSessions();//从主线程中取得session
		switch (action) {
		case "quit":
			session.close(true);
			break;
		case "get_session_count":
			session.write(sessions.size());
			break;
		case "broadcast":
			String msg_bc = null == p ? null : p.getString("message");
			if (null == msg_bc || msg_bc.length() == 0) {
				msg_bc = "hello dtu!";
			}
			synchronized (sessions) {//注意同步
				for (IoSession sess : sessions) {
					// if (session.hashCode() == sess.hashCode()) {
					// continue;
					// }
					if (sess.isConnected()) {
						sess.write(msg_bc);
					}
				}
			}
			break;
		default:
			session.write("UNKOWN COMMAND");
			break;
		}

六、Web端的处理

对于Server接收端，一般采用长连接，http是无状态的，例如我发送了一个指令以取得在线的终端数据，发送到Server后，通过IoHandler处理，返回数据后在页面显示，这个连接就没有必要再保持了，可以直接关掉。相关代码

	@RequestMapping(value = "/send", produces = "text/html;charset=UTF-8", method = RequestMethod.POST)
	@ResponseBody
	public String send(@RequestParam String cmd) {
		logger.debug("...cmd:{}", cmd);

		// 创建客户端连接器
		IoConnector connector = new NioSocketConnector();
		// 设置事件处理器
		connector.setHandler(new ClientIoHandler());
		// 设置编码过滤器和按行读取数据模式
		connector.getFilterChain().addLast("codec",
				new ProtocolCodecFilter(new TextLineCodecFactory(Charset.forName("UTF-8"))));
		// 创建连接
		ConnectFuture future = connector.connect(new InetSocketAddress("localhost", 8080));
		// 等待连接创建完成
		future.awaitUninterruptibly();
		// 获取连接会话
		IoSession session = future.getSession();
		// 产生当次发送请求标识,标识由客户端地址＋随机数
		// 这里的标识只是一个例子，标识可以任何方法产生，只要保持在系统中的唯一性
		String flag = UUID.randomUUID().toString();
		logger.debug("...flag:{}", flag);
		// 将标识保存到当前session中
		session.setAttribute(ClientIoHandler.SESSION_KEY, flag);
		// 向服务器发送命令信息
		session.write(cmd);
		// 等待连接断开
		session.getCloseFuture().awaitUninterruptibly();
		connector.dispose();

		// 通过标识获取保存的结果
		Object result = ClientIoHandler.RESULTS.get(flag);
		// 清除标识内容
		ClientIoHandler.RESULTS.remove(flag);
		// 将结果返回客户端
		return result.toString();
	}

ClientIoHandler：

public class ClientIoHandler extends IoHandlerAdapter {
	private final Logger logger = LoggerFactory.getLogger(ClientIoHandler.class);
	public static final String SESSION_KEY = "com.company.project.client.clientiohandle.SESSION_KEY";
	public static final Map RESULTS = new ConcurrentHashMap();

	public void messageReceived(IoSession session, Object message) throws Exception {
		logger.debug("messageReceived...");
		// 从session中取到标识
		String flag = (String) session.getAttribute(SESSION_KEY);
		logger.debug("...flag:{}", flag);
		// 将从服务端接收到的message和标识绑定保存，可以保存到内存、文件、数据库等等
		// 在这里简单的以标识为key,message为value保存到Map中
		RESULTS.put(flag, message);
		// 关闭session
		session.close(true);
	}
}

后继再补充...