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
Mina、Netty、Twisted一起学（三）：TCP消息固定大小的前缀(Header),布布扣,bubuko.com
Mina、Netty、Twisted一起学（三）：TCP消息固定大小的前缀(Header)
原文地址：http://blog.csdn.net/xiao__gui/article/details/38752105
