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/**
 * Cumulate {@link ByteBuf}s by merge them into one {@link ByteBuf}'s, using memory copies.
 */
public static final Cumulator MERGE_CUMULATOR = new Cumulator() {
    // cumulation 已积攒的数据，in 当次数据
    @Override
    public ByteBuf cumulate(ByteBufAllocator alloc, ByteBuf cumulation, ByteBuf in) {
        if (cumulation == in) {
            // when the in buffer is the same as the cumulation it is doubly retained, release it once
            in.release();
            return cumulation;
        }
        if (!cumulation.isReadable() && in.isContiguous()) {
            // If cumulation is empty and input buffer is contiguous, use it directly
            cumulation.release();
            return in;
        }
        try {
            final int required = in.readableBytes();
            if (required > cumulation.maxWritableBytes() ||
                required > cumulation.maxFastWritableBytes() && cumulation.refCnt() > 1 ||
                cumulation.isReadOnly()) {
                // Expand cumulation (by replacing it) under the following conditions:
                // - cumulation cannot be resized to accommodate the additional data
                // - cumulation can be expanded with a reallocation operation to accommodate but the buffer is
                //   assumed to be shared (e.g. refCnt() > 1) and the reallocation may not be safe.
                return expandCumulation(alloc, cumulation, in);
            }
            cumulation.writeBytes(in, in.readerIndex(), required);
            in.readerIndex(in.writerIndex());
            return cumulation;
        } finally {
            // We must release in all cases as otherwise it may produce a leak if writeBytes(...) throw
            // for whatever release (for example because of OutOfMemoryError)
            in.release();
        }
    }
};