在上一篇博文中，有介绍到用换行符分割消息的方法。但是这种方法有个小问题，如果消息中本身就包含换行符，那将会将这条消息分割成两条，结果就不对了。

本文介绍另外一种消息分割方式，即上一篇博文中讲的第2条：use a fixed length header that indicates the length of the body，用一个固定字节数的Header前缀来指定Body的字节数，以此来分割消息。

上面图中Header固定为4字节，Header中保存的是一个4字节（32位）的整数，例如12即为0x0000000C，这个整数用来指定Body的长度（字节数）。当读完这么多字节的Body之后，又是下一条消息的Header。

下面分别用MINA、Netty、Twisted来实现对这种消息的切合和解码。

MINA：

MINA提供了PrefixedStringCodecFactory来对这种类型的消息进行编码解码，PrefixedStringCodecFactory默认Header的大小是4字节，当然也可以指定成1或2。

public class TcpServer {

	public static void main(String[] args) throws IOException {
		IoAcceptor acceptor = new NioSocketAcceptor();
		
		// 4字节的Header指定Body的字节数，对这种消息的处理
		acceptor.getFilterChain().addLast("codec", 
				new ProtocolCodecFilter(new PrefixedStringCodecFactory(Charset.forName("UTF-8"))));
		
		acceptor.setHandler(new TcpServerHandle());
		acceptor.bind(new InetSocketAddress(8080));
	}

}

class TcpServerHandle extends IoHandlerAdapter {

	@Override
	public void exceptionCaught(IoSession session, Throwable cause)
			throws Exception {
		cause.printStackTrace();
	}

	// 接收到新的数据
	@Override
	public void messageReceived(IoSession session, Object message)
			throws Exception {

		String msg = (String) message;
		System.out.println("messageReceived:" + msg);
		
	}

	@Override
	public void sessionCreated(IoSession session) throws Exception {
		System.out.println("sessionCreated");
	}

	@Override
	public void sessionClosed(IoSession session) throws Exception {
		System.out.println("sessionClosed");
	}
}

Netty使用LengthFieldBasedFrameDecoder来处理这种消息。下面代码中的new LengthFieldBasedFrameDecoder(80, 0, 4, 0, 4)中包含5个参数，分别是int maxFrameLength, int lengthFieldOffset, int lengthFieldLength, int lengthAdjustment, int initialBytesToStrip。maxFrameLength为消息的最大长度，lengthFieldOffset为Header的位置，lengthFieldLength为Header的长度，lengthAdjustment为长度调整（默认Header中的值表示Body的长度，并不包含Header自己），initialBytesToStrip为去掉字节数（默认解码后返回Header+Body的全部内容，这里设为4表示去掉4字节的Header，只留下Body）。

public class TcpServer {

	public static void main(String[] args) throws InterruptedException {
		EventLoopGroup bossGroup = new NioEventLoopGroup();
		EventLoopGroup workerGroup = new NioEventLoopGroup();
		try {
			ServerBootstrap b = new ServerBootstrap();
			b.group(bossGroup, workerGroup)
					.channel(NioServerSocketChannel.class)
					.childHandler(new ChannelInitializer<SocketChannel>() {
						@Override
						public void initChannel(SocketChannel ch)
								throws Exception {
							ChannelPipeline pipeline = ch.pipeline();
							
							// LengthFieldBasedFrameDecoder按行分割消息，取出body
							pipeline.addLast(new LengthFieldBasedFrameDecoder(80, 0, 4, 0, 4));
							// 再按UTF-8编码转成字符串
							pipeline.addLast(new StringDecoder(CharsetUtil.UTF_8));
							
							pipeline.addLast(new TcpServerHandler());
						}
					});
			ChannelFuture f = b.bind(8080).sync();
			f.channel().closeFuture().sync();
		} finally {
			workerGroup.shutdownGracefully();
			bossGroup.shutdownGracefully();
		}
	}

}

class TcpServerHandler extends ChannelInboundHandlerAdapter {

	// 接收到新的数据
	@Override
	public void channelRead(ChannelHandlerContext ctx, Object msg) {
		
		String message = (String) msg;
		System.out.println("channelRead:" + message);
	}

	@Override
	public void channelActive(ChannelHandlerContext ctx) {
		System.out.println("channelActive");
	}

	@Override
	public void channelInactive(ChannelHandlerContext ctx) {
		System.out.println("channelInactive");
	}

	@Override
	public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
		cause.printStackTrace();
		ctx.close();
	}
}

在Twisted中需要继承Int32StringReceiver，不再继承Protocol。Int32StringReceiver表示固定32位（4字节）的Header，另外还有Int32StringReceiver、Int8StringReceiver等。而需要实现的接受数据事件的方法不再是dataReceived，也不是lineReceived，而是stringReceived。

# -*- coding:utf-8 –*-

from twisted.protocols.basic import Int32StringReceiver
from twisted.internet.protocol import Factory
from twisted.internet import reactor

class TcpServerHandle(Int32StringReceiver):

    # 新的连接建立
    def connectionMade(self):
        print ‘connectionMade‘

    # 连接断开
    def connectionLost(self, reason):
        print ‘connectionLost‘

    # 接收到新的数据
    def stringReceived(self, data):
        print ‘stringReceived:‘ + data

factory = Factory()
factory.protocol = TcpServerHandle
reactor.listenTCP(8080, factory)
reactor.run()

public class TcpClient {

	public static void main(String[] args) throws IOException {

		Socket socket = null;
		DataOutputStream out = null;

		try {

			socket = new Socket("localhost", 8080);
			out = new DataOutputStream(socket.getOutputStream());

			// 请求服务器
			String data1 = "牛顿";
			byte[] outputBytes1 = data1.getBytes("UTF-8");
			out.writeInt(outputBytes1.length); // write header
			out.write(outputBytes1); // write body
			
			String data2 = "爱因斯坦";
			byte[] outputBytes2 = data2.getBytes("UTF-8");
			out.writeInt(outputBytes2.length); // write header
			out.write(outputBytes2); // write body

		} finally {
			// 关闭连接
			out.close();
			socket.close();
		}

	}

}

sessionCreated
messageReceived:牛顿
messageReceived:爱因斯坦
sessionClosed

Netty服务器输出结果：

channelActive
channelRead:牛顿
channelRead:爱因斯坦
channelInactive

Twisted服务器输出结果：

connectionMade
stringReceived:牛顿
stringReceived:爱因斯坦
connectionLost

作者：叉叉哥   转载请注明出处：http://blog.csdn.net/xiao__gui/article/details/38752105