Netty為protobuf提供了兩個編碼器（ProtobufEncoder，ProtobufVarint32LengthFieldPrepender），兩個解碼器（ProtobufVarint32FrameDecoder，ProtobufDecoder）。

只需要在childHandler()中設定好以上編解碼器就可以使用protobuf傳輸資料。
伺服器端：

.childHandler(new ChannelInitializer<SocketChannel>() {
    @Override
    protected void initChannel(SocketChannel ch) throws Exception {
        ch.pipeline()
                .addLast(new ProtobufVarint32FrameDecoder())
                .addLast(new ProtobufDecoder(
                        ProtoObject.Req.getDefaultInstance()))
                
                .addLast(new ProtobufVarint32LengthFieldPrepender())
                .addLast(new ProtobufEncoder())
                
                .addLast(new ServerHandler());
    }
})
 

客戶端：

.childHandler(new ChannelInitializer<SocketChannel>() {
    @Override
    protected void initChannel(SocketChannel ch) throws Exception {
        ch.pipeline()
                .addLast(new ProtobufVarint32FrameDecoder())
                .addLast(new ProtobufDecoder(
                        ProtoObject.Resp.getDefaultInstance()))
                
                .addLast(new ProtobufVarint32LengthFieldPrepender())
                .addLast(new ProtobufEncoder())
                
                .addLast(new ClientHandler());
    }
})
 

Netty已經把所有的protobuf的細節給封裝過了，包括半包粘包的處理。看一下Netty是如何傳送和接受protobuf資料的(基於Netty4.0.33)。

public class ProtobufEncoder extends MessageToMessageEncoder<MessageLiteOrBuilder> {
    @Override
    protected void encode(
            ChannelHandlerContext ctx, MessageLiteOrBuilder msg, List<Object> out) throws Exception {
        if (msg instanceof MessageLite) {
            out.add(wrappedBuffer(((MessageLite) msg).toByteArray()));
            return;
        }
        if (msg instanceof MessageLite.Builder) {
            out.add(wrappedBuffer(((MessageLite.Builder) msg).build().toByteArray()));
        }
    }
}
 

encode方法很簡單，實際上它會呼叫protobuf的api，把訊息編碼成protobuf格式的位元組陣列。

ProtobufVarint32LengthFieldPrepender：

public class ProtobufVarint32LengthFieldPrepender extends MessageToByteEncoder<ByteBuf> {

    @Override
    protected void encode(
            ChannelHandlerContext ctx, ByteBuf msg, ByteBuf out) throws Exception {
        int bodyLen = msg.readableBytes();
        int headerLen = CodedOutputStream.computeRawVarint32Size(bodyLen);
        out.ensureWritable(headerLen + bodyLen);

        CodedOutputStream headerOut =
                CodedOutputStream.newInstance(new ByteBufOutputStream(out), headerLen);
        headerOut.writeRawVarint32(bodyLen);
        headerOut.flush();

        out.writeBytes(msg, msg.readerIndex(), bodyLen);
    }
}

它會在原來的資料的前面，追加一個使用Base 128 Varints編碼過的length。例如：

* BEFORE DECODE (300 bytes)       AFTER DECODE (302 bytes)
* +---------------+               +--------+---------------+
* | Protobuf Data |-------------->| Length | Protobuf Data |
* |  (300 bytes)  |               | 0xAC02 |  (300 bytes)  |
* +---------------+               +--------+---------------+

解碼的過程無非就是先讀出length來，根據length讀取出所有的資料來，交給protobuf就能還原訊息出來。ProtobufVarint32FrameDecoder：

public class ProtobufVarint32FrameDecoder extends ByteToMessageDecoder {

    // TODO maxFrameLength + safe skip + fail-fast option
    //      (just like LengthFieldBasedFrameDecoder)

    @Override
    protected void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception {
        in.markReaderIndex();
        final byte[] buf = new byte[5];
        for (int i = 0; i < buf.length; i ++) {
            if (!in.isReadable()) {
                in.resetReaderIndex();
                return;
            }

            buf[i] = in.readByte();
            if (buf[i] >= 0) {
                int length = CodedInputStream.newInstance(buf, 0, i + 1).readRawVarint32();
                if (length < 0) {
                    throw new CorruptedFrameException("negative length: " + length);
                }

                if (in.readableBytes() < length) {
                    in.resetReaderIndex();
                    return;
                } else {
                    out.add(in.readBytes(length));
                    return;
                }
            }
        }

        // Couldn't find the byte whose MSB is off.
        throw new CorruptedFrameException("length wider than 32-bit");
    }
}

ProtobufDecoder：

public class ProtobufDecoder extends MessageToMessageDecoder<ByteBuf> {

    private static final boolean HAS_PARSER;

    static {
        boolean hasParser = false;
        try {
            // MessageLite.getParsetForType() is not available until protobuf 2.5.0.
            MessageLite.class.getDeclaredMethod("getParserForType");
            hasParser = true;
        } catch (Throwable t) {
            // Ignore
        }

        HAS_PARSER = hasParser;
    }

    private final MessageLite prototype;
    private final ExtensionRegistryLite extensionRegistry;

    /**
     * Creates a new instance.
     */
    public ProtobufDecoder(MessageLite prototype) {
        this(prototype, null);
    }

    public ProtobufDecoder(MessageLite prototype, ExtensionRegistry extensionRegistry) {
        this(prototype, (ExtensionRegistryLite) extensionRegistry);
    }

    public ProtobufDecoder(MessageLite prototype, ExtensionRegistryLite extensionRegistry) {
        if (prototype == null) {
            throw new NullPointerException("prototype");
        }
        this.prototype = prototype.getDefaultInstanceForType();
        this.extensionRegistry = extensionRegistry;
    }

    @Override
    protected void decode(ChannelHandlerContext ctx, ByteBuf msg, List<Object> out) throws Exception {
        final byte[] array;
        final int offset;
        final int length = msg.readableBytes();
        if (msg.hasArray()) {
            array = msg.array();
            offset = msg.arrayOffset() + msg.readerIndex();
        } else {
            array = new byte[length];
            msg.getBytes(msg.readerIndex(), array, 0, length);
            offset = 0;
        }

        if (extensionRegistry == null) {
            if (HAS_PARSER) {
                out.add(prototype.getParserForType().parseFrom(array, offset, length));
            } else {
                out.add(prototype.newBuilderForType().mergeFrom(array, offset, length).build());
            }
        } else {
            if (HAS_PARSER) {
                out.add(prototype.getParserForType().parseFrom(array, offset, length, extensionRegistry));
            } else {
                out.add(prototype.newBuilderForType().mergeFrom(array, offset, length, extensionRegistry).build());
            }
        }
    }
}