在TCP连接开始到结束连接，之间可能会多次传输数据，也就是服务器和客户端之间可能会在连接过程中互相传输多条消息。理想状况是一方每发送一条消息，另一方就立即接收到一条，也就是一次write对应一次read。但是，现实不总是按照剧本来走。
MINA官方文档节选：
TCP guarantess delivery of all packets in the correct order. But there is no guarantee that one write operation on the sender-side will result in one read event on the receiving side. One call of IoSession.write(Object message) by the sender can result in multiple messageReceived(IoSession session, Object message) events on the receiver; and multiple calls of IoSession.write(Object message) can lead to a single messageReceived event.
Netty官方文档节选：
In a stream-based transport such as TCP/IP, received data is stored into a socket receive buffer. Unfortunately, the buffer of a stream-based transport is not a queue of packets but a queue of bytes. It means, even if you sent two messages as two independent packets, an operating system will not treat them as two messages but as just a bunch of bytes. Therefore, there is no guarantee that what you read is exactly what your remote peer wrote.
上面两段话表达的意思相同：TCP是基于字节流的协议，它只能保证一方发送和另一方接收到的数据的字节顺序一致，但是，并不能保证一方每发送一条消息，另一方就能完整的接收到一条信息。有可能发送了两条对方将其合并成一条，也有可能发送了一条对方将其拆分成两条。所以在上一篇博文中的Demo，可以说是一个错误的示范。不过服务器和客户端在同一台机器上或者在局域网等网速很好的情况下，这种问题还是很难测试出来。
举个简单了例子（这个例子来源于Netty官方文档）：
消息发送方发送了三个字符串：
但是接收方收到的可能是这样的：
那么问题就很严重了，接收方没法分开这三条信息了，也就没法解析了。
对此，MINA的官方文档提供了以下几种解决方案：
1、use fixed length messages
使用固定长度的消息。比如每个长度4字节，那么接收的时候按每条4字节拆分就可以了。
2、use a fixed length header that indicates the length of the body
使用固定长度的Header，Header中指定Body的长度（字节数），将信息的内容放在Body中。例如Header中指定的Body长度是100字节，那么Header之后的100字节就是Body，也就是信息的内容，100字节的Body后面就是下一条信息的Header了。
3、using a delimiter; for example many text-based protocols append a newline (or CR LF pair) after every message
使用分隔符。例如许多文本内容的协议会在每条消息后面加上换行符（CR LF，即"\r\n"），也就是一行一条消息。当然也可以用其他特殊符号作为分隔符，例如逗号、分号等等。
当然除了上面说到的3种方案，还有其他方案。有的协议也可能会同时用到上面多种方案。例如HTTP协议，Header部分用的是CR LF换行来区分每一条Header，而Header中用Content-Length来指定Body字节数。
下面，分别用MINA、Netty、Twisted自带的相关API实现按换行符CR LF来分割消息。
MINA：
MINA可以使用ProtocolCodecFilter来对发送和接收的二进制数据进行加工，如何加工取决于ProtocolCodecFactory或ProtocolEncoder、ProtocolDecoder，加工后在IoHandler中messageReceived事件函数获取的message就不再是IoBuffer了，而是你想要的其他类型，可以是字符串，Java对象。这里可以使用TextLineCodecFactory（ProtocolCodecFactory的一个实现类）实现CR LF分割消息。
public class TcpServer {
public static void main(String[] args) throws IOException {
IoAcceptor acceptor = new NioSocketAcceptor();
// 添加一个Filter，用于接收、发送的内容按照"\r\n"分割
acceptor.getFilterChain().addLast("codec",
new ProtocolCodecFilter(new TextLineCodecFactory(Charset.forName("UTF-8"), "\r\n", "\r\n")));
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 {
// 接收客户端的数据，这里接收到的不再是IoBuffer类型，而是字符串
String line = (String) message;
System.out.println("messageReceived:" + line);
}
@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：
Netty设计上和MINA类似，需要在ChannelPipeline加上一些ChannelHandler用来对原始数据进行处理。这里用LineBasedFrameDecoder将接收到的数据按行分割，StringDecoder再将数据由字节码转成字符串。同样，接收到的数据进过加工后，在channelRead事件函数中，msg参数不再是ByteBuf而是String。
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();
// LineBasedFrameDecoder按行分割消息
pipeline.addLast(new LineBasedFrameDecoder(80));
// 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) {
// msg经过StringDecoder后类型不再是ByteBuf而是String
String line = (String) msg;
System.out.println("channelRead:" + line);
}
@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();
}
}
Twisted：
Twisted的设计和上面两者的设计不太一样，所以实现消息分割也不太一样。处理事件的类TcpServerHandle不再继承Protocol，而是继承Protocol的子类LineOnlyReceiver。接收到新数据的事件方法也不再是dataReceived，而是LineOnlyReceiver提供的lineReceived。看Twisted源码的话可以发现LineOnlyReceiver的内部实际上自己实现了dataReceived，然后将其按行分割，有新的一行数据就调用lineReceived。
# -*- coding:utf-8 –*-
from twisted.protocols.basic import LineOnlyReceiver
from twisted.internet.protocol import Factory
from twisted.internet import reactor
class TcpServerHandle(LineOnlyReceiver):
# 新的连接建立
def connectionMade(self):
print ‘connectionMade‘
# 连接断开
def connectionLost(self, reason):
print ‘connectionLost‘
# 接收到新的一行数据
def lineReceived(self, data):
print ‘lineReceived:‘ + data
factory = Factory()
factory.protocol = TcpServerHandle
reactor.listenTCP(8080, factory)
reactor.run()
下面用一个Java客户端对三个服务器进行测试：
public class TcpClient {
public static void main(String[] args) throws IOException {
Socket socket = null;
OutputStream out = null;
try {
socket = new Socket("localhost", 8080);
out = socket.getOutputStream();
// 请求服务器
String lines = "床前明月光\r\n疑是地上霜\r\n举头望明月\r\n低头思故乡\r\n";
byte[] outputBytes = lines.getBytes("UTF-8");
out.write(outputBytes);
out.flush();
} finally {
// 关闭连接
out.close();
socket.close();
}
}
}
MINA服务器输出结果：
sessionCreated
messageReceived:床前明月光
messageReceived:疑是地上霜
messageReceived:举头望明月
messageReceived:低头思故乡
sessionClosed
Netty服务器输出结果：
channelActive
channelRead:床前明月光
channelRead:疑是地上霜
channelRead:举头望明月
channelRead:低头思故乡
channelInactive
Twisted服务器输出结果：
connectionMade
lineReceived:床前明月光
lineReceived:疑是地上霜
lineReceived:举头望明月
lineReceived:低头思故乡
connectionLost
当然，测试的时候也可以将发送的数据模拟成不按规则分割的情况，下面用一个更变态的客户端来测试：
public class TcpClient {
public static void main(String[] args) throws IOException, InterruptedException {
Socket socket = null;
OutputStream out = null;
try{
socket = new Socket("localhost", 8080);
out = socket.getOutputStream();
String lines = "床前";
byte[] outputBytes = lines.getBytes("UTF-8");
out.write(outputBytes);
out.flush();
Thread.sleep(1000);
lines = "明月";
outputBytes = lines.getBytes("UTF-8");
out.write(outputBytes);
out.flush();
Thread.sleep(1000);
lines = "光\r\n疑是地上霜\r\n举头";
outputBytes = lines.getBytes("UTF-8");
out.write(outputBytes);
out.flush();
Thread.sleep(1000);
lines = "望明月\r\n低头思故乡\r\n";
outputBytes = lines.getBytes("UTF-8");
out.write(outputBytes);
out.flush();
} finally {
// 关闭连接
out.close();
socket.close();
}
}
}
再次分别测试上面三个服务器，结果和上面的输出结果一样，没有任何问题。