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12.1 客户端请求编码

以dubbo使用netty4为通信框架来进行分析。

客户端请求编码总体流程如下：

 1 NettyCodecAdapter$InternalEncoder.encode(ChannelHandlerContext ctx, Channel ch, Object msg)
 2 -->new NettyBackedChannelBuffer(ByteBuf buffer) // 创建一个buffer
 3 -->NettyChannel.getOrAddChannel(io.netty.channel.Channel ch, URL url, ChannelHandler handler)
 4 -->DubboCountCodec.encode(Channel channel, ChannelBuffer buffer, Object msg)
 5   -->ExchangeCodec.encode(Channel channel, ChannelBuffer buffer, Object msg)
 6       -->encodeRequest(Channel channel, ChannelBuffer buffer, Request req)
 7         -->getSerialization(Channel channel)   //获取Hessian2Serialization序列化实例
 8           -->CodecSupport.getSerialization(URL url)
 9             -->ExtensionLoader.getExtensionLoader(Serialization.class).getExtension(url.getParameter("serialization", "hessian2"))
10         <!-- 构造一个16字节的byte[16] header -->
11         -->byte[] header = new byte[16]
12         -->Bytes.short2bytes(MAGIC, header)  //设置前两个字节为魔数[-38, -69, 0, ..., 0]
13         <!-- 第三个字节：表示消息是req，序列化协议ID，twoway/event -->
14         -->header[2] = (byte) (FLAG_REQUEST | serialization.getContentTypeId());
15          if (req.isTwoWay()) header[2] |= FLAG_TWOWAY;
16          if (req.isEvent()) header[2] |= FLAG_EVENT;
17       <!-- 设置第5~12个字节（long是64bit，即8byte）：requestID -->
18       -->Bytes.long2bytes(req.getId(), header, 4);
19       <!-- 下面序列化请求体数据 -->
20       -->new Hessian2ObjectOutput(out)
21       -->DubboCodec.encodeRequestData(Channel channel, ObjectOutput out, Object data)
22       -->Bytes.int2bytes(len, header, 12); // 设置第13~16个字节（int是32位，4个字节）：消息体长度
23       -->buffer.writeBytes(header); // 将header写入buffer的前16位

总体流程很简单：

创建一个buffer
创建一个16位的byte[16] header，将魔数、请求标志、序列化协议ID、twoway/event标志、requestID、请求体长度写入header
之后序列化请求体，从buffer的第17位向后写入序列化后的请求体字节数组
最后，将header中的内容写入buffer的前16位
最后发送buffer

首先来看一下netty编解码的入口：com.alibaba.dubbo.remoting.transport.netty4：

 1     @Override
 2     protected void doOpen() throws Throwable {
 3         NettyHelper.setNettyLoggerFactory();
 4         final NettyClientHandler nettyClientHandler = new NettyClientHandler(getUrl(), this);
 5         bootstrap = new Bootstrap();
 6         bootstrap.group(nioEventLoopGroup)
 7                 .option(ChannelOption.SO_KEEPALIVE, true)
 8                 .option(ChannelOption.TCP_NODELAY, true)
 9                 .option(ChannelOption.ALLOCATOR, PooledByteBufAllocator.DEFAULT)
10                 //.option(ChannelOption.CONNECT_TIMEOUT_MILLIS, getTimeout())
11                 .channel(NioSocketChannel.class);
12 
13         if (getTimeout() < 3000) {
14             bootstrap.option(ChannelOption.CONNECT_TIMEOUT_MILLIS, 3000);
15         } else {
16             bootstrap.option(ChannelOption.CONNECT_TIMEOUT_MILLIS, getTimeout());
17         }
18 
19         bootstrap.handler(new ChannelInitializer() {
20 
21             protected void initChannel(Channel ch) throws Exception {
22                 NettyCodecAdapter adapter = new NettyCodecAdapter(getCodec(), getUrl(), NettyClient.this);
23                 ch.pipeline()//.addLast("logging",new LoggingHandler(LogLevel.INFO))//for debug
24                         .addLast("decoder", adapter.getDecoder())
25                         .addLast("encoder", adapter.getEncoder())
26                         .addLast("handler", nettyClientHandler);
27             }
28         });
29     }

NettyCodecAdapter：

 1 final class NettyCodecAdapter {
 2     private final ChannelHandler encoder = new InternalEncoder();
 3     private final ChannelHandler decoder = new InternalDecoder();
 4     private final Codec2 codec;
 5     private final URL url;
 6     private final com.alibaba.dubbo.remoting.ChannelHandler handler;
 7 
 8     public NettyCodecAdapter(Codec2 codec, URL url, com.alibaba.dubbo.remoting.ChannelHandler handler) {
 9         this.codec = codec;
10         this.url = url;
11         this.handler = handler;
12     }
13 
14     public ChannelHandler getEncoder() {
15         return encoder;
16     }
17 
18     public ChannelHandler getDecoder() {
19         return decoder;
20     }
21 
22     private class InternalEncoder extends MessageToByteEncoder {
23         protected void encode(ChannelHandlerContext ctx, Object msg, ByteBuf out) throws Exception {
24             com.alibaba.dubbo.remoting.buffer.ChannelBuffer buffer = new NettyBackedChannelBuffer(out);
25             Channel ch = ctx.channel();
26             NettyChannel channel = NettyChannel.getOrAddChannel(ch, url, handler);
27             try {
28                 codec.encode(channel, buffer, msg);
29             } finally {
30                 NettyChannel.removeChannelIfDisconnected(ch);
31             }
32         }
33     }
34 
35     private class InternalDecoder extends ByteToMessageDecoder {
36         protected void decode(ChannelHandlerContext ctx, ByteBuf input, List<Object> out) throws Exception {
37             ChannelBuffer message = new NettyBackedChannelBuffer(input);
38             NettyChannel channel = NettyChannel.getOrAddChannel(ctx.channel(), url, handler);
39             Object msg;
40             int saveReaderIndex;
41 
42             try {
43                 // decode object.
44                 do {
45                     saveReaderIndex = message.readerIndex();
46                     try {
47                         msg = codec.decode(channel, message);
48                     } catch (IOException e) {
49                         throw e;
50                     }
51                     if (msg == Codec2.DecodeResult.NEED_MORE_INPUT) {
52                         message.readerIndex(saveReaderIndex);
53                         break;
54                     } else {
55                         //is it possible to go here ?
56                         if (saveReaderIndex == message.readerIndex()) {
57                             throw new IOException("Decode without read data.");
58                         }
59                         if (msg != null) {
60                             out.add(msg);
61                         }
62                     }
63                 } while (message.readable());
64             } finally {
65                 NettyChannel.removeChannelIfDisconnected(ctx.channel());
66             }
67         }
68     }
69 }

一、创建ChannelBuffer

1 com.alibaba.dubbo.remoting.buffer.ChannelBuffer buffer = new NettyBackedChannelBuffer(out);

这里的out是：

1 ByteBuf buffer = SimpleLeakAwareByteBuf
2 -->ByteBuf buf = PooledUnsafeDirectByteBuf

NettyBackedChannelBuffer：

1     private ByteBuf buffer;
2 
3     public NettyBackedChannelBuffer(ByteBuf buffer) {
4         Assert.notNull(buffer, "buffer == null");
5         this.buffer = buffer;
6     }

最终的buffer：

1 NettyBackedChannelBuffer
2 -->ByteBuf buffer = SimpleLeakAwareByteBuf
3   -->ByteBuf buf = PooledUnsafeDirectByteBuf

二、获取NettyChannel

之后从获取io.netty.channel实例，然后包装在NettyChannel中。

1 NettyChannel channel = NettyChannel.getOrAddChannel(ch, url, handler);

 1     private static final ConcurrentMap<Channel, NettyChannel> channelMap = new ConcurrentHashMap<Channel, NettyChannel>();
 2     private final Channel channel;
 3 
 4     private NettyChannel(Channel channel, URL url, ChannelHandler handler) {
 5         super(url, handler);
 6         if (channel == null) {
 7             throw new IllegalArgumentException("netty channel == null;");
 8         }
 9         this.channel = channel;
10     }
11 
12     static NettyChannel getOrAddChannel(Channel ch, URL url, ChannelHandler handler) {
13         if (ch == null) {
14             return null;
15         }
16         NettyChannel ret = channelMap.get(ch);
17         if (ret == null) {
18             NettyChannel nettyChannel = new NettyChannel(ch, url, handler);
19             if (ch.isActive()) {
20                 ret = channelMap.putIfAbsent(ch, nettyChannel);
21             }
22             if (ret == null) {
23                 ret = nettyChannel;
24             }
25         }
26         return ret;
27     }

首先从缓存ConcurrentMap<Channel, NettyChannel> channelMap中获取key=io.netty.channel的NettyChannel，有则返回，没有则新建并返回。

最终获取到的NettyChannel实例如下：

1 -->Channel channel = NioSocketChannel
2 -->ChannelHandler handler = NettyClient
3 -->URL url = dubbo://10.10.10.10:20880/com.alibaba.dubbo.demo.DemoService?anyhost=true&application=demo-consumer&check=false&codec=dubbo&default.client=netty4&default.server=netty4&dubbo=2.0.0&generic=false&heartbeat=60000&interface=com.alibaba.dubbo.demo.DemoService&methods=sayHello&pid=2204&register.ip=10.10.10.10&remote.timestamp=1514958356359&side=consumer&timeout=6000000&timestamp=1514959413199

三、进行编码

1 codec.encode(channel, buffer, msg)

这里的codec是：

1 Codec2 codec = 
2 DubboCountCodec
3 -->DubboCodec codec = new DubboCodec()

DubboCountCodec

1     private DubboCodec codec = new DubboCodec();
2 
3     public void encode(Channel channel, ChannelBuffer buffer, Object msg) throws IOException {
4         codec.encode(channel, buffer, msg);
5     }

入参：

channel：上述的NettyChannel对象
buffer：上述的NettyBackedChannelBuffer对象
msg：Request对象，其属性如下：

 1 long mId = 0
 2 String mVersion = "2.0.0"
 3 boolean mTwoWay = true
 4 boolean mEvent = false
 5 boolean mBroken = false
 6 Object mData = RpcInvocation对象
 7 -->String methodName = "sayHello"
 8 -->Class<?>[] parameterTypes = [java.lang.String]
 9 -->Object[] arguments = ["world"]
10 -->Map<String, String> attachments = {
11      "path" -> "com.alibaba.dubbo.demo.DemoService"
12      "interface" -> "com.alibaba.dubbo.demo.DemoService"
13      "version" -> "0.0.0"
14      "timeout" -> "6000000"
15 }
16 -->Invoker<?> invoker = DubboInvoker对象

之后调用DubboCodec.encode(Channel channel, ChannelBuffer buffer, Object msg)，该方法位于其父类ExchangeCodec中。

 1     public void encode(Channel channel, ChannelBuffer buffer, Object msg) throws IOException {
 2         if (msg instanceof Request) {
 3             encodeRequest(channel, buffer, (Request) msg);
 4         } else if (msg instanceof Response) {
 5             encodeResponse(channel, buffer, (Response) msg);
 6         } else {
 7             super.encode(channel, buffer, msg);
 8         }
 9     }
10 
11     protected void encodeRequest(Channel channel, ChannelBuffer buffer, Request req) throws IOException {
12         Serialization serialization = getSerialization(channel);
13         // header.
14         byte[] header = new byte[HEADER_LENGTH];
15         // set magic number.
16         Bytes.short2bytes(MAGIC, header);
17 
18         // set request and serialization flag.
19         header[2] = (byte) (FLAG_REQUEST | serialization.getContentTypeId());
20 
21         if (req.isTwoWay()) header[2] |= FLAG_TWOWAY;
22         if (req.isEvent()) header[2] |= FLAG_EVENT;
23 
24         // set request id.
25         Bytes.long2bytes(req.getId(), header, 4);
26 
27         // encode request data.
28         int savedWriteIndex = buffer.writerIndex();
29         buffer.writerIndex(savedWriteIndex + HEADER_LENGTH);//设置writerIndex为0+16，先输入请求体的字节
30         ChannelBufferOutputStream bos = new ChannelBufferOutputStream(buffer);
31         ObjectOutput out = serialization.serialize(channel.getUrl(), bos);
32         if (req.isEvent()) {
33             encodeEventData(channel, out, req.getData());
34         } else {
35             encodeRequestData(channel, out, req.getData());
36         }
37         out.flushBuffer();
38         bos.flush();
39         bos.close();
40         int len = bos.writtenBytes();
41         checkPayload(channel, len);
42         Bytes.int2bytes(len, header, 12);
43 
44         // write
45         buffer.writerIndex(savedWriteIndex);
46         buffer.writeBytes(header); // write header.
47         buffer.writerIndex(savedWriteIndex + HEADER_LENGTH + len);
48     }

1 首先使用spi获取序列化协议

1 Serialization serialization = getSerialization(channel);

getSerialization位于ExchangeCodec的父类AbstractCodec中。

1     protected Serialization getSerialization(Channel channel) {
2         return CodecSupport.getSerialization(channel.getUrl());
3     }

1     public static Serialization getSerialization(URL url) {
2         return ExtensionLoader.getExtensionLoader(Serialization.class).getExtension(
3                 url.getParameter("serialization", "hessian2"));
4     }

最终获取到的Serialization serialization = Hessian2Serialization对象：

 1 public class Hessian2Serialization implements Serialization {
 2     public static final byte ID = 2;
 3 
 4     public byte getContentTypeId() {
 5         return ID;
 6     }
 7 
 8     public String getContentType() {
 9         return "x-application/hessian2";
10     }
11 
12     public ObjectOutput serialize(URL url, OutputStream out) throws IOException {
13         return new Hessian2ObjectOutput(out);
14     }
15 
16     public ObjectInput deserialize(URL url, InputStream is) throws IOException {
17         return new Hessian2ObjectInput(is);
18     }
19 }

注意：hessian2序列化方式的id是2，该序列化方式ID会写在协议头里传给服务端，服务端根据序列化方式ID获取对应的序列化方式来反序列化请求体。

2 创建16字节header字节数组

1 byte[] header = new byte[16];

然后填充第1~2个字节为魔数；填充第3个字节为requestFlag、序列化方式ID（这里是2）、twowayFlag或eventFlag；填充第5~12个字节为requestID（long==64bit==8byte）

 1         // set magic number.
 2         Bytes.short2bytes(MAGIC, header);
 3 
 4         // set request and serialization flag.
 5         header[2] = (byte) (FLAG_REQUEST | serialization.getContentTypeId());
 6 
 7         if (req.isTwoWay()) header[2] |= FLAG_TWOWAY;
 8         if (req.isEvent()) header[2] |= FLAG_EVENT;
 9 
10         // set request id.
11         Bytes.long2bytes(req.getId(), header, 4);

3 序列化请求体

首先设置buffer的writerIndex：

1         int savedWriteIndex = buffer.writerIndex();
2         buffer.writerIndex(savedWriteIndex + HEADER_LENGTH);//设置writerIndex为0+16，先输入请求体的字节

首先存储了buffer当前的writeIndex（可写位置），从该位置开始到“该位置+15”这一段我们会写入header字节数组（例如，[0,15]），从“该位置+16”开始向后写入请求体字节数组（例如，[16, x)）。

然后就是设置buffer的writerIndex为当前位置+16，因为接下来我们要先序列化请求体，然后将请求体写入buffer，最后才会将header写入buffer。

序列化请求体：

 1         ChannelBufferOutputStream bos = new ChannelBufferOutputStream(buffer);
 2         ObjectOutput out = serialization.serialize(channel.getUrl(), bos);
 3         if (req.isEvent()) {
 4             encodeEventData(channel, out, req.getData());
 5         } else {
 6             encodeRequestData(channel, out, req.getData());
 7         }
 8         out.flushBuffer();
 9         bos.flush();
10         bos.close();

首先新建一个ChannelBufferOutputStream对象（该对象继承了java.io.OutputStream抽象类）：

 1     private final ChannelBuffer buffer;
 2     private final int startIndex;
 3 
 4     public ChannelBufferOutputStream(ChannelBuffer buffer) {
 5         if (buffer == null) {
 6             throw new NullPointerException("buffer");
 7         }
 8         this.buffer = buffer;
 9         startIndex = buffer.writerIndex();
10     }

buffer为上述的NettyBackedChannelBuffer对象；startIndex == 16

然后获取ObjectOutput对象：

1     public ObjectOutput serialize(URL url, OutputStream out) throws IOException {
2         return new Hessian2ObjectOutput(out);
3     }

1     private final Hessian2Output mH2o;
2 
3     public Hessian2ObjectOutput(OutputStream os) {
4         mH2o = new Hessian2Output(os);
5         mH2o.setSerializerFactory(Hessian2SerializerFactory.SERIALIZER_FACTORY);
6     }

1     public final static int SIZE = 4096;
2     private final byte[] _buffer = new byte[SIZE];
3     protected OutputStream _os;
4 
5     public Hessian2Output(OutputStream os) {
6         _os = os;
7     }

最终得到的ObjectOutput对象：

1 Hessian2ObjectOutput
2 -->Hessian2Output mH2o
3    -->byte[] _buffer = new byte[4096]
4    -->OutputStream _os = 上述的ChannelBufferOutputStream对象
5    -->SerializerFactory _serializerFactory = Hessian2SerializerFactory实例

最后执行DubboCodec.encodeRequestData(Channel channel, ObjectOutput out, Object data)，该方法是真正的进行请求体序列化的地方。

 1     @Override
 2     protected void encodeRequestData(Channel channel, ObjectOutput out, Object data) throws IOException {
 3         RpcInvocation inv = (RpcInvocation) data;
 4 
 5         out.writeUTF(inv.getAttachment(Constants.DUBBO_VERSION_KEY, DUBBO_VERSION));
 6         out.writeUTF(inv.getAttachment(Constants.PATH_KEY));
 7         out.writeUTF(inv.getAttachment(Constants.VERSION_KEY));
 8 
 9         out.writeUTF(inv.getMethodName());
10         out.writeUTF(ReflectUtils.getDesc(inv.getParameterTypes()));
11         Object[] args = inv.getArguments();
12         if (args != null)
13             for (int i = 0; i < args.length; i++) {
14                 out.writeObject(encodeInvocationArgument(channel, inv, i));
15             }
16         out.writeObject(inv.getAttachments());
17     }

其中，channel是上述的NettyChannel实例；out是上述的Hessian2ObjectOutput实例；data是Request对象中的data属性

 1 Object mData = RpcInvocation对象
 2 -->String methodName = "sayHello"
 3 -->Class<?>[] parameterTypes = [java.lang.String]
 4 -->Object[] arguments = ["world"]
 5 -->Map<String, String> attachments = {
 6      "path" -> "com.alibaba.dubbo.demo.DemoService"
 7      "interface" -> "com.alibaba.dubbo.demo.DemoService"
 8      "version" -> "0.0.0"
 9      "timeout" -> "6000000"
10 }
11 -->Invoker<?> invoker = DubboInvoker对象

从DubboCodec.encodeRequestData方法中，我们可以看到只会序列化Request请求体中的RpcInvocation对象的：

methodName：方法名
parameterTypes：参数类型
arguments：参数值
attachments：附加参数

其中附加参数中的"dubbo"、"path"、"version"还会单独使用out.writeUTF进行序列化。

首先来看一下：

1 Hessian2ObjectOutput.writeUTF(String v)
2 -->Hessian2Output.writeString(String value)
3    -->printString(String v, int strOffset, int length)

通过这个方法，我们将传入的v存储在ObjectOutput对象的byte[] _buffer = new byte[4096]数组中。

 1     Hessian2Output：
 2     /**
 3      * Writes any object to the output stream.
 4      */
 5     public void writeObject(Object object)
 6             throws IOException {
 7         if (object == null) {
 8             writeNull();
 9             return;
10         }
11 
12         Serializer serializer = findSerializerFactory().getSerializer(object.getClass());
13         serializer.writeObject(object, this);
14     }
15 
16     public final SerializerFactory findSerializerFactory() {
17         SerializerFactory factory = _serializerFactory;
18         if (factory == null)
19             _serializerFactory = factory = new SerializerFactory();
20         return factory;
21     }
22 
23     SerializerFactory：
24     private static HashMap _staticSerializerMap;
25     private HashMap _cachedSerializerMap;
26     /**
27      * Returns the serializer for a class.
28      * @param cl the class of the object that needs to be serialized.
29      * @return a serializer object for the serialization.
30      */
31     public Serializer getSerializer(Class cl)
32             throws HessianProtocolException {
33         Serializer serializer;
34 
35         serializer = (Serializer) _staticSerializerMap.get(cl);
36         if (serializer != null)
37             return serializer;
38 
39         if (_cachedSerializerMap != null) {
40             synchronized (_cachedSerializerMap) {
41                 serializer = (Serializer) _cachedSerializerMap.get(cl);
42             }
43 
44             if (serializer != null)
45                 return serializer;
46         }
47         
48         ......
49 
50         if (serializer != null) {
51 
52         } 
53         .......
54         else if (Map.class.isAssignableFrom(cl)) {
55             if (_mapSerializer == null)
56                 _mapSerializer = new MapSerializer();
57 
58             serializer = _mapSerializer;
59         } 
60         ......
61         if (serializer == null)
62             serializer = getDefaultSerializer(cl);
63 
64         if (_cachedSerializerMap == null)
65             _cachedSerializerMap = new HashMap(8);
66 
67         synchronized (_cachedSerializerMap) {
68             _cachedSerializerMap.put(cl, serializer);
69         }
70 
71         return serializer;
72     }

out.writeObject(Object object)：

首先获取_serializerFactory工厂，这里是Hessian2SerializerFactory实例。其getSerializer(Class cl)方法位于其父类SerializerFactory中：获取序列化器的逻辑是：首先从_staticSerializerMap中获取相关类型的序列化器（_staticSerializerMap中启动时就缓存好一堆类型的序列化器：具体见com.alibaba.com.caucho.hessian.io.SerializerFactory），如果有返回，否则从_cachedSerializerMap缓存中获取相关的类加载器，如果没有，根据类型先创建序列化器（new MapSerializer()，当然还有getDefaultSerializer(cl)来兜底），最后放入缓存_cachedSerializerMap中。最后返回创建好的类加载器。

最后调用MapSerializer.writeObject(Object obj, AbstractHessianOutput out)进行序列化。

DubboCodec.encodeRequestData执行完毕之后，我们将所有的信息写入了ObjectOutput对象的byte[] _buffer = new byte[4096]数组中。

注意：

如果在将数据写入到_buffer的过程中，字节量超出了4096，会先执行一把Hessian2ObjectOutput.flushBuffer()将_buffer中的数据拷贝到PooledUnsafeDirectByteBuf中，之后再往_buffer中写入字节

最后执行Hessian2ObjectOutput.flushBuffer()

 1 Hessian2ObjectOutput
 2     public void flushBuffer() throws IOException {
 3         mH2o.flushBuffer();
 4     }
 5 
 6 Hessian2Output
 7     public final void flushBuffer()
 8             throws IOException {
 9         int offset = _offset;
10 
11         if (!_isStreaming && offset > 0) {
12             _offset = 0;
13             _os.write(_buffer, 0, offset);
14         } else if (_isStreaming && offset > 3) {
15             int len = offset - 3;
16             _buffer[0] = 'p';
17             _buffer[1] = (byte) (len >> 8);
18             _buffer[2] = (byte) len;
19             _offset = 3;
20             _os.write(_buffer, 0, offset);
21         }
22     }

此处执行ChannelBufferOutputStream.write(byte[] b, int off, int len)

1     @Override
2     public void write(byte[] b, int off, int len) throws IOException {
3         if (len == 0) {
4             return;
5         }
6         buffer.writeBytes(b, off, len);
7     }

 1 ChannelBuffer:
 2     /**
 3      * Transfers the specified source array's data to this buffer starting at
 4      * the current {@code writerIndex} and increases the {@code writerIndex} by
 5      * the number of the transferred bytes (= {@code length}).
 6      *
 7      * @param index  the first index of the source
 8      * @param length the number of bytes to transfer
 9      */
10     void writeBytes(byte[] src, int index, int length);

就是将ObjectOutput对象的byte[] _buffer = new byte[4096]数组中的数据转移到buf中。（具体方法见：unsafe.copyMemory(Object srcBase, long srcOffset, Object destBase, long destOffset,long bytes)）

1 NettyBackedChannelBuffer
2 -->ByteBuf buffer = SimpleLeakAwareByteBuf
3   -->ByteBuf buf = PooledUnsafeDirectByteBuf

4 将header写入buffer

1         int len = bos.writtenBytes();//计算请求体长度
2         checkPayload(channel, len);
3         Bytes.int2bytes(len, header, 12);//将请求体长度写入header的第13~16个字节（int=4byte）
4 
5         // write
6         buffer.writerIndex(savedWriteIndex);//设置buffer的writerIndex为该次写入的开始位置
7         buffer.writeBytes(header); // 将header数组写入buffer
8         buffer.writerIndex(savedWriteIndex + HEADER_LENGTH + len);//设置buffer的writerIndex，为下一次写入做准备

到此为止，整个编码就结束了。之后存储了<header><body>数据的ByteBuf由netty自己来进行网络传输。

来看一下请求编码的byte[] header的最终结构:

1~2 byte：魔数
3 byte：requestFlag、序列化方式ID、twowayFlag或eventFlag
5~12 byte ：requestID
13~16：请求体长度

这里有一个小插曲：

 1     protected static void checkPayload(Channel channel, long size) throws IOException {
 2         int payload = Constants.DEFAULT_PAYLOAD;
 3         if (channel != null && channel.getUrl() != null) {
 4             payload = channel.getUrl().getParameter(Constants.PAYLOAD_KEY, Constants.DEFAULT_PAYLOAD);//8M
 5         }
 6         if (payload > 0 && size > payload) {
 7             ExceedPayloadLimitException e = new ExceedPayloadLimitException("Data length too large: " + size + ", max payload: " + payload + ", channel: " + channel);
 8             logger.error(e);
 9             throw e;
10         }
11     }

dubbo限制了如果传输的请求体长度大于8M，将会直接抛出异常。

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原文地址：https://www.cnblogs.com/java-zhao/p/8179098.html