上一篇隨筆中已經(jīng)介紹了解碼核心工作流程，里面有個(gè)數(shù)據(jù)積累器的存在（Cumulator），其實(shí)解碼中有兩種Cumulator，那他們的區(qū)別是什么呢？

還是先打開ByteToMessageDecoder的channelRead();

點(diǎn)進(jìn)去查看cumulate()實(shí)現(xiàn)

又是一個(gè)抽象方法，看實(shí)現(xiàn)不難發(fā)現(xiàn)它有兩種實(shí)現(xiàn)方式

兩種實(shí)現(xiàn)分別為：MERGE_CUMULATOR（默認(rèn)的）：采用的是內(nèi)存復(fù)制，先擴(kuò)容空間，再追加數(shù)據(jù)

 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();
            }
        }
    };

第二種 ：COMPOSITE_CUMULATOR ：不是復(fù)制而是組合，先擴(kuò)容，如果數(shù)據(jù)夠了的話就直接把數(shù)據(jù)給組合起來，避免了內(nèi)存復(fù)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();
                }
            }
        }
    };

我只想做的更好，僅此而已

總結(jié)

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