Coding class MessageToByteEncoder
This time, we use ObjectEncoder as an example. When we need to write messages to the network channel, we need to encode the messages to determine whether the message type meets the requirements. Generally, we use type matching to configure the message in the generic type.
public abstract class MessageToByteEncoder<I> extends ChannelOutboundHandlerAdapter
public boolean acceptOutboundMessage(Object msg) throws Exception {
return matcher.match(msg);
}Allocate memory
protected ByteBuf allocateBuffer(ChannelHandlerContext ctx, @SuppressWarnings("unused") I msg,
boolean preferDirect) throws Exception {
if (preferDirect) {
return ctx.alloc().ioBuffer();
} else {
return ctx.alloc().heapBuffer();
}
}
public ByteBuf ioBuffer() {
if (PlatformDependent.hasUnsafe()) {
return directBuffer(DEFAULT_INITIAL_CAPACITY);
}
return heapBuffer(DEFAULT_INITIAL_CAPACITY);
}Encoding messages and writing them to memory
public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
ByteBuf buf = null;
try {
if (acceptOutboundMessage(msg)) {
@SuppressWarnings("unchecked")
I cast = (I) msg;
buf = allocateBuffer(ctx, cast, preferDirect);
try {
encode(ctx, cast, buf);
} finally {
ReferenceCountUtil.release(cast);
}
if (buf.isReadable()) {
ctx.write(buf, promise);
} else {
buf.release();
ctx.write(Unpooled.EMPTY_BUFFER, promise);
}
buf = null;
} else {
ctx.write(msg, promise);
}
} catch (EncoderException e) {
throw e;
} catch (Throwable e) {
throw new EncoderException(e);
} finally {
if (buf != null) {
buf.release();
}
}
}The first four bits need to store the total length of the message. The memory block corresponding to the length behind stores the specific data. Firstly, four bytes are written to represent the empty array of the total length of the message. Then, specific messages are written to calculate the total length, subtract the initial length and the storage length. The first four bits use the length of the message to cover the empty array written before the first four bits use the length of the message. It then determines whether the data has been written in memory, and then executes the write event in the processor context to release the memory.
@Sharable
public class ObjectEncoder extends MessageToByteEncoder<Serializable> {
private static final byte[] LENGTH_PLACEHOLDER = new byte[4];
@Override
protected void encode(ChannelHandlerContext ctx, Serializable msg, ByteBuf out) throws Exception {
int startIdx = out.writerIndex();
ByteBufOutputStream bout = new ByteBufOutputStream(out);
ObjectOutputStream oout = null;
try {
bout.write(LENGTH_PLACEHOLDER);
oout = new CompactObjectOutputStream(bout);
oout.writeObject(msg);
oout.flush();
} finally {
if (oout != null) {
oout.close();
} else {
bout.close();
}
}
int endIdx = out.writerIndex();
out.setInt(startIdx, endIdx - startIdx - 4);
}
}Decoder class ByteToMessageDecoder
For example, the object decoder sets the message length value 4, the offset, and the number of bytes to skip when reading the message entity.
public class ObjectDecoder extends LengthFieldBasedFrameDecoder
public ObjectDecoder(int maxObjectSize, ClassResolver classResolver) {
super(maxObjectSize, 0, 4, 0, 4);
this.classResolver = classResolver;
}Frame Decoder Based on Length Fields
public class LengthFieldBasedFrameDecoder extends ByteToMessageDecoder
public LengthFieldBasedFrameDecoder(
ByteOrder byteOrder, int maxFrameLength, int lengthFieldOffset, int lengthFieldLength,
int lengthAdjustment, int initialBytesToStrip, boolean failFast) {
this.byteOrder = byteOrder;
this.maxFrameLength = maxFrameLength;
this.lengthFieldOffset = lengthFieldOffset;
this.lengthFieldLength = lengthFieldLength;
this.lengthAdjustment = lengthAdjustment;
lengthFieldEndOffset = lengthFieldOffset + lengthFieldLength;
this.initialBytesToStrip = initialBytesToStrip;
this.failFast = failFast;
}When a message needs to be read, it is parsed when the message type is ByteBuf
public abstract class ByteToMessageDecoder extends ChannelInboundHandlerAdapter
public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
if (msg instanceof ByteBuf) {
CodecOutputList out = CodecOutputList.newInstance();
} else {
ctx.fireChannelRead(msg);
}
}Get the data storage class for storing decoded messages
static CodecOutputList newInstance() {
return CODEC_OUTPUT_LISTS_POOL.get().getOrCreate();
}
public CodecOutputList getOrCreate() {
if (count == 0) {
// Return a new CodecOutputList which will not be cached. We use a size of 4 to keep the overhead
// low.
return new CodecOutputList(NOOP_RECYCLER, 4);
}
--count;
int idx = (currentIdx - 1) & mask;
CodecOutputList list = elements[idx];
currentIdx = idx;
return list;
}
private CodecOutputList(CodecOutputListRecycler recycler, int size) {
this.recycler = recycler;
array = new Object[size];
}Determine whether it's the first message
try {
ByteBuf data = (ByteBuf) msg;
first = cumulation == null;
if (first) {
cumulation = data;
} else {
cumulation = cumulator.cumulate(ctx.alloc(), cumulation, data);
}
callDecode(ctx, cumulation, out);
} catch (DecoderException e) {
throw e;
} catch (Exception e) {
throw new DecoderException(e);
} If large messages are read multiple times, multiple sub-messages need to be merged.
public static final Cumulator MERGE_CUMULATOR = new Cumulator() {
@SuppressWarnings("deprecation")
@Override
public ByteBuf cumulate(ByteBufAllocator alloc, ByteBuf cumulation, ByteBuf in) {
final ByteBuf buffer;
if (cumulation.writerIndex() > cumulation.maxCapacity() - in.readableBytes()
|| cumulation.refCnt() > 1 || cumulation instanceof ReadOnlyByteBuf) {
// Expand cumulation (by replace it) when either there is not more room in the buffer
// or if the refCnt is greater then 1 which may happen when the user use slice().retain() or
// duplicate().retain() or if its read-only.
//
// See:
// - https://github.com/netty/netty/issues/2327
// - https://github.com/netty/netty/issues/1764
buffer = expandCumulation(alloc, cumulation, in.readableBytes());
} else {
buffer = cumulation;
}
buffer.writeBytes(in);
in.release();
return buffer;
}
};
Call decoding, determine whether the memory is readable, determine whether there is a message already in the collection, directly activate reading, determine whether the data is parsed, if not parsed, it may be part of the message, need to read again.
protected void callDecode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) {
try {
while (in.isReadable()) {
int outSize = out.size();
if (outSize > 0) {
fireChannelRead(ctx, out, outSize);
out.clear();
// Check if this handler was removed before continuing with decoding.
// If it was removed, it is not safe to continue to operate on the buffer.
//
// See:
// - https://github.com/netty/netty/issues/4635
if (ctx.isRemoved()) {
break;
}
outSize = 0;
}
int oldInputLength = in.readableBytes();
decodeRemovalReentryProtection(ctx, in, out);
// Check if this handler was removed before continuing the loop.
// If it was removed, it is not safe to continue to operate on the buffer.
//
// See https://github.com/netty/netty/issues/1664
if (ctx.isRemoved()) {
break;
}
if (outSize == out.size()) {
if (oldInputLength == in.readableBytes()) {
break;
} else {
continue;
}
}
if (oldInputLength == in.readableBytes()) {
throw new DecoderException(
StringUtil.simpleClassName(getClass()) +
".decode() did not read anything but decoded a message.");
}
if (isSingleDecode()) {
break;
}
}
} catch (DecoderException e) {
throw e;
} catch (Exception cause) {
throw new DecoderException(cause);
}
}
final void decodeRemovalReentryProtection(ChannelHandlerContext ctx, ByteBuf in, List<Object> out)
throws Exception
{
decodeState = STATE_CALLING_CHILD_DECODE;
try {
decode(ctx, in, out);
} finally {
boolean removePending = decodeState == STATE_HANDLER_REMOVED_PENDING;
decodeState = STATE_INIT;
if (removePending) {
handlerRemoved(ctx);
}
}
}Decode the data, read the total length of the message represented by the first few of the settings, and determine whether the readable message has reached a sufficient length. If not, it will be considered as part of the message, return empty, wait for the next complete message, and data will not be added to the message storage set.
protected final void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception {
Object decoded = decode(ctx, in);
if (decoded != null) {
out.add(decoded);
}
}
protected Object decode(ChannelHandlerContext ctx, ByteBuf in) throws Exception {
if (discardingTooLongFrame) {
discardingTooLongFrame(in);
}
if (in.readableBytes() < lengthFieldEndOffset) {
return null;
}
int actualLengthFieldOffset = in.readerIndex() + lengthFieldOffset;
long frameLength = getUnadjustedFrameLength(in, actualLengthFieldOffset, lengthFieldLength, byteOrder);
if (frameLength < 0) {
failOnNegativeLengthField(in, frameLength, lengthFieldEndOffset);
}
frameLength += lengthAdjustment + lengthFieldEndOffset;
if (frameLength < lengthFieldEndOffset) {
failOnFrameLengthLessThanLengthFieldEndOffset(in, frameLength, lengthFieldEndOffset);
}
if (frameLength > maxFrameLength) {
exceededFrameLength(in, frameLength);
return null;
}
// never overflows because it's less than maxFrameLength
int frameLengthInt = (int) frameLength;
if (in.readableBytes() < frameLengthInt) {
return null;
}
if (initialBytesToStrip > frameLengthInt) {
failOnFrameLengthLessThanInitialBytesToStrip(in, frameLength, initialBytesToStrip);
}
in.skipBytes(initialBytesToStrip);
// extract frame
int readerIndex = in.readerIndex();
int actualFrameLength = frameLengthInt - initialBytesToStrip;
ByteBuf frame = extractFrame(ctx, in, readerIndex, actualFrameLength);
in.readerIndex(readerIndex + actualFrameLength);
return frame;
}When the complete message memory block is read and parsed, the object is parsed out by deserialization and added to the message set.
protected Object decode(ChannelHandlerContext ctx, ByteBuf in) throws Exception {
ByteBuf frame = (ByteBuf) super.decode(ctx, in);
if (frame == null) {
return null;
}
ObjectInputStream ois = new CompactObjectInputStream(new ByteBufInputStream(frame, true), classResolver);
try {
return ois.readObject();
} finally {
ois.close();
}
}When the complete message is parsed and read, the memory block is unreadable, the memory block is released, and the number of reads is reset. The default maximum number of reads is 16.
finally {
if (cumulation != null && !cumulation.isReadable()) {
numReads = 0;
cumulation.release();
cumulation = null;
} else if (++ numReads >= discardAfterReads) {
// We did enough reads already try to discard some bytes so we not risk to see a OOME.
// See https://github.com/netty/netty/issues/4275
numReads = 0;
discardSomeReadBytes();
}
int size = out.size();
decodeWasNull = !out.insertSinceRecycled();
fireChannelRead(ctx, out, size);
out.recycle();
}Subsequent processor read events are activated only when there is a message
static void fireChannelRead(ChannelHandlerContext ctx, CodecOutputList msgs, int numElements) {
for (int i = 0; i < numElements; i ++) {
ctx.fireChannelRead(msgs.getUnsafe(i));
}
}