Dubbo中编码和解码的解析
原文链接
（这里做的解析不是很详细，等到走完整个流程再来解析）Dubbo中编解码的工作由Codec2接口的实现来处理，回想一下第一次接触到Codec2相关的内容是在服务端暴露服务的时候，根据具体的协议去暴露服务的步骤中，在DubboProtocol的createServer方法中：
private ExchangeServer createServer(URL url) {
。。。
//这里url会添加codec=dubbo
url = url.addParameter(Constants.CODEC_KEY, Version.isCompatibleVersion() ? COMPATIBLE_CODEC_NAME : DubboCodec.NAME);
ExchangeServer server;
try {
server = Exchangers.bind(url, requestHandler);
}
。。。
return server;
}紧接着进入Exchangers.bind(url, requestHandler);：
public static ExchangeServer bind(URL url, ExchangeHandler handler) throws RemotingException {
//如果url中没有codec属性，就会添加codec=exchange
url = url.addParameterIfAbsent(Constants.CODEC_KEY, "exchange");
return getExchanger(url).bind(url, handler);
}然后会继续进入HeaderExchanger的bind方法：
public ExchangeServer bind(URL url, ExchangeHandler handler) throws RemotingException {
return new HeaderExchangeServer(Transporters.bind(url, new DecodeHandler(new HeaderExchangeHandler(handler))));
}在这里会创建一个DecodeHandler实例。继续跟踪Transporters的bind方法，会发现直接返回一个NettyServer实例，在NettyServer的父类AbstractEndpoint构造方法初始的时候，会根据url获取一个ChannelCodec，并将其赋值给codec存放到NettyServer的实例中。
我们先看下getChannelCodec(url);方法：
protected static Codec2 getChannelCodec(URL url) {
//获取codecName，不存在的话，默认为telnet
String codecName = url.getParameter(Constants.CODEC_KEY, "telnet");
//先看下是不是Codec2的实现，是的话就根据SPI扩展机制获得Codec2扩展的实现
//我们这里默认使用的是DubboCountCodec
if (ExtensionLoader.getExtensionLoader(Codec2.class).hasExtension(codecName)) {
return ExtensionLoader.getExtensionLoader(Codec2.class).getExtension(codecName);
} else {
//如果不是Codec2的实现，就去查找Codec的实现
//然后使用CodecAdapter适配器类来转换成Codec2
return new CodecAdapter(ExtensionLoader.getExtensionLoader(Codec.class)
.getExtension(codecName));
}
}这里返回的是Codec2，而Codec这个接口已经被标记为过时。到这里的话，在NettyServer中就会存在一个Codec2的实例了。
在继续往下看到NettyServer中的doOpen()方法，这里是使用Netty的逻辑打开服务并绑定监听服务的地方：
protected void doOpen() throws Throwable {
NettyHelper.setNettyLoggerFactory();
ExecutorService boss = Executors.newCachedThreadPool(new NamedThreadFactory("NettyServerBoss", true));
ExecutorService worker = Executors.newCachedThreadPool(new NamedThreadFactory("NettyServerWorker", true));
ChannelFactory channelFactory = new NioServerSocketChannelFactory(boss, worker, getUrl().getPositiveParameter(Constants.IO_THREADS_KEY, Constants.DEFAULT_IO_THREADS));
bootstrap = new ServerBootstrap(channelFactory);
final NettyHandler nettyHandler = new NettyHandler(getUrl(), this);
channels = nettyHandler.getChannels();
bootstrap.setPipelineFactory(new ChannelPipelineFactory() {
public ChannelPipeline getPipeline() {
//这里的getCodec方法获取到的codec就是在AbstractEndpoint中我们获取到的codec
//NettyCodecAdapter，适配器类
NettyCodecAdapter adapter = new NettyCodecAdapter(getCodec() ,getUrl(), NettyServer.this);
ChannelPipeline pipeline = Channels.pipeline();
pipeline.addLast("decoder", adapter.getDecoder());//SimpleChannelUpstreamHandler
pipeline.addLast("encoder", adapter.getEncoder());//OneToOneEncoder
pipeline.addLast("handler", nettyHandler);
return pipeline;
}
});
// bind
channel = bootstrap.bind(getBindAddress());
}这里就在Netty的pipeline中添加了编解码器。这里涉及到Netty的相关流程，可以先了解下Netty3服务端流程简介。
decoder为解码器，是一个SimpleChannelUpstreamHandler，从Socket到Netty中的时候，需要解码，也就是服务提供端接收到消费者的请求的时候，需要解码。
encoder是编码器，是OneToOneEncoder，这个类实现了ChannelDownstreamHandler，从服务提供端发送给服务消费者的时候，需要编码。
nettyHandler实现了ChannelUpstreamHandler, ChannelDownstreamHandler两个，上下的时候都需要处理。
接收到服务消费者的请求的时候，会先执行decoder，然后执行nettyHandler。
发送给消费者的时候，会先执行nettyHandler，然后执行encoder。
dubbo协议头
协议头是16字节的定长数据：
- 2字节short类型的Magic
1字节的消息标志位
- 5位序列化id
- 1位心跳还是正常请求
- 1位双向还是单向
- 1位请求还是响应
1字节的状态位
- 8字节的消息id
- 4字节数据长度
编码的过程
首先会判断是请求还是响应，代码在ExchangeCodec的encode方法：
public void encode(Channel channel, ChannelBuffer buffer, Object msg) throws IOException {
if (msg instanceof Request) {//Request类型
encodeRequest(channel, buffer, (Request) msg);
} else if (msg instanceof Response) {//Response类型
encodeResponse(channel, buffer, (Response) msg);
} else {//telenet类型的
super.encode(channel, buffer, msg);
}
}服务提供者对响应信息编码
在服务提供者端一般是对响应来做编码，所以这里重点看下encodeResponse。
encodeResponse：
protected void encodeResponse(Channel channel, ChannelBuffer buffer, Response res) throws IOException {
try {
//序列化方式
//也是根据SPI扩展来获取，url中没指定的话默认使用hessian2
Serialization serialization = getSerialization(channel);
//长度为16字节的数组，协议头
byte[] header = new byte[HEADER_LENGTH];
//魔数0xdabb
Bytes.short2bytes(MAGIC, header);
//序列化方式
header[2] = serialization.getContentTypeId();
//心跳消息还是正常消息
if (res.isHeartbeat()) header[2] |= FLAG_EVENT;
//响应状态
byte status = res.getStatus();
header[3] = status;
//设置请求id
Bytes.long2bytes(res.getId(), header, 4);
//buffer为1024字节的ChannelBuffer
//获取buffer的写入位置
int savedWriteIndex = buffer.writerIndex();
//需要再加上协议头的长度之后，才是正确的写入位置
buffer.writerIndex(savedWriteIndex + HEADER_LENGTH);
ChannelBufferOutputStream bos = new ChannelBufferOutputStream(buffer);
ObjectOutput out = serialization.serialize(channel.getUrl(), bos);
// 对响应信息或者错误消息进行编码
if (status == Response.OK) {
if (res.isHeartbeat()) {
//心跳
encodeHeartbeatData(channel, out, res.getResult());
} else {
//正常响应
encodeResponseData(channel, out, res.getResult());
}
}
//错误消息
else out.writeUTF(res.getErrorMessage());
out.flushBuffer();
bos.flush();
bos.close();
//写出去的消息的长度
int len = bos.writtenBytes();
//查看消息长度是否过长
checkPayload(channel, len);
Bytes.int2bytes(len, header, 12);
//重置写入的位置
buffer.writerIndex(savedWriteIndex);
//向buffer中写入消息头
buffer.writeBytes(header); // write header.
//buffer写出去的位置从writerIndex开始，加上header长度，加上数据长度
buffer.writerIndex(savedWriteIndex + HEADER_LENGTH + len);
} catch (Throwable t) {
// 发送失败信息给Consumer，否则Consumer只能等超时了
if (! res.isEvent() && res.getStatus() != Response.BAD_RESPONSE) {
try {
// FIXME 在Codec中打印出错日志？在IoHanndler的caught中统一处理？
logger.warn("Fail to encode response: " + res + ", send bad_response info instead, cause: " + t.getMessage(), t);
Response r = new Response(res.getId(), res.getVersion());
r.setStatus(Response.BAD_RESPONSE);
r.setErrorMessage("Failed to send response: " + res + ", cause: " + StringUtils.toString(t));
channel.send(r);
return;
} catch (RemotingException e) {
logger.warn("Failed to send bad_response info back: " + res + ", cause: " + e.getMessage(), e);
}
}
// 重新抛出收到的异常
if (t instanceof IOException) {
throw (IOException) t;
} else if (t instanceof RuntimeException) {
throw (RuntimeException) t;
} else if (t instanceof Error) {
throw (Error) t;
} else {
throw new RuntimeException(t.getMessage(), t);
}
}
}服务消费者对请求信息编码
消费者端暂先不做解析
解码的过程
服务提供者对请求消息的解码
decode方法一次只会解析一个完整的dubbo协议包，但是每次收到的协议包不一定是完整的，或者有可能是多个协议包。看下代码解析，首先看NettyCodecAdapter的内部类InternalDecoder的messageReceived方法：
public void messageReceived(ChannelHandlerContext ctx, MessageEvent event) throws Exception {
Object o = event.getMessage();
if (! (o instanceof ChannelBuffer)) {
ctx.sendUpstream(event);
return;
}
ChannelBuffer input = (ChannelBuffer) o;
int readable = input.readableBytes();
if (readable <= 0) {
return;
}
com.alibaba.dubbo.remoting.buffer.ChannelBuffer message;
if (buffer.readable()) {
if (buffer instanceof DynamicChannelBuffer) {
buffer.writeBytes(input.toByteBuffer());
message = buffer;
} else {
int size = buffer.readableBytes() + input.readableBytes();
message = com.alibaba.dubbo.remoting.buffer.ChannelBuffers.dynamicBuffer(
size > bufferSize ? size : bufferSize);
message.writeBytes(buffer, buffer.readableBytes());
message.writeBytes(input.toByteBuffer());
}
} else {
message = com.alibaba.dubbo.remoting.buffer.ChannelBuffers.wrappedBuffer(
input.toByteBuffer());
}
NettyChannel channel = NettyChannel.getOrAddChannel(ctx.getChannel(), url, handler);
Object msg;
//读索引
int saveReaderIndex;
try {
do {
saveReaderIndex = message.readerIndex();
try {
//解码
msg = codec.decode(channel, message);
} catch (IOException e) {
buffer = com.alibaba.dubbo.remoting.buffer.ChannelBuffers.EMPTY_BUFFER;
throw e;
}
//不完整的协议包
if (msg == Codec2.DecodeResult.NEED_MORE_INPUT) {
//重置读索引
message.readerIndex(saveReaderIndex);
//跳出循环，之后在finally中把message赋值给buffer保存起来，等到下次接收到数据包的时候会追加到buffer的后面
break;
} else {//有多个协议包，触发messageReceived事件
if (saveReaderIndex == message.readerIndex()) {
buffer = com.alibaba.dubbo.remoting.buffer.ChannelBuffers.EMPTY_BUFFER;
throw new IOException("Decode without read data.");
}
if (msg != null) {
Channels.fireMessageReceived(ctx, msg, event.getRemoteAddress());
}
}
} while (message.readable());
} finally {
if (message.readable()) {
message.discardReadBytes();
buffer = message;
} else {
buffer = com.alibaba.dubbo.remoting.buffer.ChannelBuffers.EMPTY_BUFFER;
}
NettyChannel.removeChannelIfDisconnected(ctx.getChannel());
}
}继续看codec.decode(channel, message);这里是DubboCountCodec的decode方法：
public Object decode(Channel channel, ChannelBuffer buffer) throws IOException {
//当前的读索引记录下来
int save = buffer.readerIndex();
//多消息
MultiMessage result = MultiMessage.create();
do {
//解码消息
Object obj = codec.decode(channel, buffer);
//不是完整的协议包
if (Codec2.DecodeResult.NEED_MORE_INPUT == obj) {
buffer.readerIndex(save);
break;
} else {//多个协议包
result.addMessage(obj);
logMessageLength(obj, buffer.readerIndex() - save);
save = buffer.readerIndex();
}
} while (true);
if (result.isEmpty()) {
return Codec2.DecodeResult.NEED_MORE_INPUT;
}
if (result.size() == 1) {
return result.get(0);
}
return result;
}继续看ExchangeCodec的decode方法：
public Object decode(Channel channel, ChannelBuffer buffer) throws IOException {
//可读字节数
int readable = buffer.readableBytes();
byte[] header = new byte[Math.min(readable, HEADER_LENGTH)];
//协议头
buffer.readBytes(header);
//解码
return decode(channel, buffer, readable, header);
}解码decode：
protected Object decode(Channel channel, ChannelBuffer buffer, int readable, byte[] header) throws IOException {
//检查魔数.
if (readable > 0 && header[0] != MAGIC_HIGH
|| readable > 1 && header[1] != MAGIC_LOW) {
int length = header.length;
if (header.length < readable) {
header = Bytes.copyOf(header, readable);
buffer.readBytes(header, length, readable - length);
}
for (int i = 1; i < header.length - 1; i ++) {
if (header[i] == MAGIC_HIGH && header[i + 1] == MAGIC_LOW) {
buffer.readerIndex(buffer.readerIndex() - header.length + i);
header = Bytes.copyOf(header, i);
break;
}
}
//telenet
return super.decode(channel, buffer, readable, header);
}
//不完整的包
if (readable < HEADER_LENGTH) {
return DecodeResult.NEED_MORE_INPUT;
}
//数据长度
int len = Bytes.bytes2int(header, 12);
checkPayload(channel, len);
int tt = len + HEADER_LENGTH;
if( readable < tt ) {
return DecodeResult.NEED_MORE_INPUT;
}
// limit input stream.
ChannelBufferInputStream is = new ChannelBufferInputStream(buffer, len);
try {
//解码数据
return decodeBody(channel, is, header);
} finally {
if (is.available() > 0) {
try {
StreamUtils.skipUnusedStream(is);
} catch (IOException e) { }
}
}
}decodeBody解析数据部分：
protected Object decodeBody(Channel channel, InputStream is, byte[] header) throws IOException {
byte flag = header[2], proto = (byte) (flag & SERIALIZATION_MASK);
//获取序列化方式
Serialization s = CodecSupport.getSerialization(channel.getUrl(), proto);
//反序列化
ObjectInput in = s.deserialize(channel.getUrl(), is);
//获取请求id
long id = Bytes.bytes2long(header, 4);
//这里是解码响应数据
if ((flag & FLAG_REQUEST) == 0) {
//response的id设为来时候的Request的id，这样才能对上暗号
Response res = new Response(id);
//判断是什么类型请求
if ((flag & FLAG_EVENT) != 0) {
res.setEvent(Response.HEARTBEAT_EVENT);
}
//获取状态
byte status = header[3];
res.setStatus(status);
if (status == Response.OK) {
try {
Object data;
if (res.isHeartbeat()) {
//解码心跳数据
data = decodeHeartbeatData(channel, in);
} else if (res.isEvent()) {
//事件
data = decodeEventData(channel, in);
} else {
//响应
data = decodeResponseData(channel, in, getRequestData(id));
}
res.setResult(data);
} catch (Throwable t) {
res.setStatus(Response.CLIENT_ERROR);
res.setErrorMessage(StringUtils.toString(t));
}
} else {
res.setErrorMessage(in.readUTF());
}
return res;
} else {//这是解码请求数据
// request的id
Request req = new Request(id);
req.setVersion("2.0.0");
req.setTwoWay((flag & FLAG_TWOWAY) != 0);
if ((flag & FLAG_EVENT) != 0) {
req.setEvent(Request.HEARTBEAT_EVENT);
}
try {
Object data;
if (req.isHeartbeat()) {
//心跳
data = decodeHeartbeatData(channel, in);
} else if (req.isEvent()) {
//事件
data = decodeEventData(channel, in);
} else {
//请求
data = decodeRequestData(channel, in);
}
req.setData(data);
} catch (Throwable t) {
// bad request
req.setBroken(true);
req.setData(t);
}
return req;
}
}具体的解码细节交给底层解码器，这里是使用的hessian2。
服务消费者对响应消息的解码
暂先不做解释。