上一节我们说了LineBasedframeDecoder来解决粘包拆包的问题，TCP以流的方式进行数据传输，上层应用协议为了对消息进行区分，一般采用如下4种方式：
- 消息长度固定，累计读取到消息长度总和为定长Len的报文之后即认为是读取到了一个完整的消息。计数器归位，重新读取。
- 将回车换行符作为消息结束符。
- 将特殊的分隔符作为消息分隔符，回车换行符是他的一种。
- 通过在消息头定义长度字段来标识消息总长度。
LineBasedframeDecoder属于第二种，今天我们要说的DelimiterBasedFrameDecoder和FixedLengthFrameDecoder属于第三种和第一种。DelimiterBasedFrameDecoder用来解决以特殊符号作为消息结束符的粘包问题，FixedLengthFrameDecoder用来解决定长消息的粘包问题。下面首先来用DelimiterBasedFrameDecoder来写一个例子，我们看一下效果然后接着分析用法。
1. DelimiterBasedFrameDecoder使用
服务端：
public class HelloWordServer {
private int port;
public HelloWordServer(int port) {
this.port = port;
}
public void start(){
EventLoopGroup bossGroup = new NioEventLoopGroup();
EventLoopGroup workGroup = new NioEventLoopGroup();
ServerBootstrap server = new ServerBootstrap().group(bossGroup,workGroup)
.channel(NioServerSocketChannel.class)
.childHandler(new ServerChannelInitializer());
try {
ChannelFuture future = server.bind(port).sync();
future.channel().closeFuture().sync();
} catch (InterruptedException e) {
e.printStackTrace();
}finally {
bossGroup.shutdownGracefully();
workGroup.shutdownGracefully();
}
}
public static void main(String[] args) {
HelloWordServer server = new HelloWordServer(7788);
server.start();
}
}服务端ServerChannelInitializer：
public class ServerChannelInitializer extends ChannelInitializer<SocketChannel> {
@Override
protected void initChannel(SocketChannel socketChannel) throws Exception {
ChannelPipeline pipeline = socketChannel.pipeline();
ByteBuf delimiter = Unpooled.copiedBuffer(" ".getBytes());
pipeline.addLast("framer", new DelimiterBasedFrameDecoder(2048,delimiter));
// 字符串解码 和 编码
pipeline.addLast("decoder", new StringDecoder());
pipeline.addLast("encoder", new StringEncoder());
// 自己的逻辑Handler
pipeline.addLast("handler", new ServerHandler());
}
}服务端handler：
public class ServerHandler extends ChannelInboundHandlerAdapter {
private int counter;
@Override
public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
String body = (String)msg;
System.out.println("server receive order : " + body + ";the counter is: " + ++counter);
}
@Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
super.exceptionCaught(ctx, cause);
}
}客户端：
public class HelloWorldClient {
private int port;
private String address;
public HelloWorldClient(int port,String address) {
this.port = port;
this.address = address;
}
public void start(){
EventLoopGroup group = new NioEventLoopGroup();
Bootstrap bootstrap = new Bootstrap();
bootstrap.group(group)
.channel(NioSocketChannel.class)
.handler(new ClientChannelInitializer());
try {
ChannelFuture future = bootstrap.connect(address,port).sync();
future.channel().closeFuture().sync();
} catch (Exception e) {
e.printStackTrace();
}finally {
group.shutdownGracefully();
}
}
public static void main(String[] args) {
HelloWorldClient client = new HelloWorldClient(7788,"127.0.0.1");
client.start();
}
}客户端ClientChannelInitializer：
public class ClientChannelInitializer extends ChannelInitializer<SocketChannel> {
protected void initChannel(SocketChannel socketChannel) throws Exception {
ChannelPipeline pipeline = socketChannel.pipeline();
/*
* 这个地方的 必须和服务端对应上。否则无法正常解码和编码
*
*
*/
ByteBuf delimiter = Unpooled.copiedBuffer(" ".getBytes());
pipeline.addLast("framer", new DelimiterBasedFrameDecoder(2048,delimiter));
pipeline.addLast("decoder", new StringDecoder());
pipeline.addLast("encoder", new StringEncoder());
// 客户端的逻辑
pipeline.addLast("handler", new ClientHandler());
}
}客户端handler：
public class ClientHandler extends ChannelInboundHandlerAdapter {
private byte[] req;
private int counter;
public ClientHandler() {
req = ("Unless required by applicable law or agreed to in writing, software " +
" distributed under the License is distributed on an "AS IS" BASIS, " +
" WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. " +
" See the License for the specific language governing permissions and " +
" limitations under the License.This connector uses the BIO implementation that requires the JSSE " +
" style configuration. When using the APR/native implementation, the " +
" penSSL style configuration is required as described in the APR/native " +
" documentation.An Engine represents the entry point (within Catalina) that processes " +
" every request. The Engine implementation for Tomcat stand alone " +
" analyzes the HTTP headers included with the request, and passes them " +
" on to the appropriate Host (virtual host)# Unless required by applicable law or agreed to in writing, software " +
"# distributed under the License is distributed on an "AS IS" BASIS, " +
"# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. " +
"# See the License for the specific language governing permissions and " +
"# limitations under the License.# For example, set the org.apache.catalina.util.LifecycleBase logger to log " +
"# each component that extends LifecycleBase changing state: " +
"#org.apache.catalina.util.LifecycleBase.level = FINE "
).getBytes();
}
@Override
public void channelActive(ChannelHandlerContext ctx) throws Exception {
ByteBuf message;
message = Unpooled.buffer(req.length);
message.writeBytes(req);
ctx.writeAndFlush(message);
}
@Override
public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
String buf = (String)msg;
System.out.println("Now is : " + buf + " ; the counter is : "+ (++counter));
}
@Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
ctx.close();
}
}输出如下：
server receive order : Unless required by applicable law or agreed to in writing, software;the counter is: 1
server receive order : distributed under the License is distributed on an "AS IS" BASIS,;the counter is: 2
server receive order : WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.;the counter is: 3
server receive order : See the License for the specific language governing permissions and;the counter is: 4
server receive order : limitations under the License.This connector uses the BIO implementation that requires the JSSE;the counter is: 5
server receive order : style configuration. When using the APR/native implementation, the;the counter is: 6
server receive order : penSSL style configuration is required as described in the APR/native;the counter is: 7
server receive order : documentation.An Engine represents the entry point (within Catalina) that processes;the counter is: 8
server receive order : every request. The Engine implementation for Tomcat stand alone;the counter is: 9
server receive order : analyzes the HTTP headers included with the request, and passes them;the counter is: 10
server receive order : on to the appropriate Host (virtual host)# Unless required by applicable law or agreed to in writing, software;the counter is: 11
server receive order : # distributed under the License is distributed on an "AS IS" BASIS,;the counter is: 12
server receive order : # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.;the counter is: 13
server receive order : # See the License for the specific language governing permissions and;the counter is: 14
server receive order : # limitations under the License.# For example, set the org.apache.catalina.util.LifecycleBase logger to log;the counter is: 15
server receive order : # each component that extends LifecycleBase changing state:;the counter is: 16
server receive order : #org.apache.catalina.util.LifecycleBase.level = FINE;the counter is: 17启动服务端和客户端，我们能看到服务端接收客户端发过来的消息一共分17次接收。那么为什么是17次呢？而且我们并没有使用在上一篇中解决拆包和粘包问题的LineBasedFrameDecoder，并且这次我们的消息每一行的末尾也换成了” ”。下面就来讲解一下DelimiterBasedFrameDecoder的使用。
DelimiterBasedFrameDecoder是将特殊的字符作为消息的分隔符，本例中用到的是” ”。而LineBasedFrameDecoder是默认将换行符” ”作为消息分隔符。首先我们注意到在ServerChannelInitializer中我们在添加解码器时跟以前有点不一样：
ByteBuf delimiter = Unpooled.copiedBuffer(" ".getBytes());
pipeline.addLast("framer", new DelimiterBasedFrameDecoder(2048, delimiter));这里我们添加DelimiterBasedFrameDecoder解码器并且手动指定消息分隔符为：” ”。我们可以看一下DelimiterBasedFrameDecoder的构造方法：
public DelimiterBasedFrameDecoder(int maxFrameLength, boolean stripDelimiter, ByteBuf delimiter) {
this(maxFrameLength, stripDelimiter, true, delimiter);
}maxFrameLength：解码的帧的最大长度
stripDelimiter：解码时是否去掉分隔符
failFast：为true，当frame长度超过maxFrameLength时立即报TooLongFrameException异常，为false，读取完整个帧再报异常
delimiter：分隔符
这个时候大家应该明白了为什么服务端分17次收到消息。我们在消息的每一行都加了一个” ”,自然解码器在度消息时遇到” ”就会认为这是一条消息的结束。用这种方式我们可以把” ”换成任何我们自定义的字符对象。换成” ”也是可以的。
2. FixedLengthFrameDecoder使用
FixedLengthFrameDecoder是固定长度解码器，它能够按照指定的长度对消息进行自动解码。使用它也没有什么特别费力的事情，在ServerChannelInitializer类中添加：
pipeline.addLast(new FixedLengthFrameDecoder(23));//参数为一次接受的数据长度
即可，同时也别忘了把刚才使用的DelimiterBasedFrameDecoder注释掉啊，不然达不到效果。