上一篇随笔中已经介绍了解码核心工作流程，里面有个数据积累器的存在（Cumulator），其实解码中有两种Cumulator，那他们的区别是什么呢？
还是先打开ByteToMessageDecoder的channelRead();
点进去查看cumulate()实现
又是一个抽象方法，看实现不难发现它有两种实现方式
两种实现分别为：MERGE_CUMULATOR（默认的）：采用的是内存复制，先扩容空间，再追加数据
public static final Cumulator MERGE_CUMULATOR = new Cumulator() {
@Override
public ByteBuf cumulate(ByteBufAllocator alloc, ByteBuf cumulation, ByteBuf in) {
try {
final ByteBuf buffer;
if (cumulation.writerIndex() > cumulation.maxCapacity() - in.readableBytes()
|| cumulation.refCnt() > 1 || cumulation.isReadOnly()) {
// 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);
return buffer;
} finally {
// We must release in in all cases as otherwise it may produce a leak if writeBytes(...) throw
// for whatever release (for example because of OutOfMemoryError)
in.release();
}
}
};
第二种 ：COMPOSITE_CUMULATOR ：不是复制而是组合，先扩容，如果数据够了的话就直接把数据给组合起来，避免了内存复制
public static final Cumulator COMPOSITE_CUMULATOR = new Cumulator() {
@Override
public ByteBuf cumulate(ByteBufAllocator alloc, ByteBuf cumulation, ByteBuf in) {
ByteBuf buffer;
try {
if (cumulation.refCnt() > 1) {
// Expand cumulation (by replace it) when the refCnt is greater then 1 which may happen when the
// user use slice().retain() or duplicate().retain().
//
// See:
// - https://github.com/netty/netty/issues/2327
// - https://github.com/netty/netty/issues/1764
buffer = expandCumulation(alloc, cumulation, in.readableBytes());
buffer.writeBytes(in);
} else {
CompositeByteBuf composite;
if (cumulation instanceof CompositeByteBuf) {
composite = (CompositeByteBuf) cumulation;
} else {
composite = alloc.compositeBuffer(Integer.MAX_VALUE);
composite.addComponent(true, cumulation);
}
composite.addComponent(true, in);
in = null;
buffer = composite;
}
return buffer;
} finally {
if (in != null) {
// We must release if the ownership was not transferred as otherwise it may produce a leak if
// writeBytes(...) throw for whatever release (for example because of OutOfMemoryError).
in.release();
}
}
}
};
